A to Z Chemistry Dictionary – Comprehensive Glossary of Chemistry Definitions

Chemistry Dictionary - Glossary of Chemistry Definitions

This comprehensive chemistry dictionary or glossary offers definitions for terms which are commonly used in chemistry and chemical engineering. This page contains the chemistry definitions starting with the letter A. Click the letter to go to the page containing glossary terms beginning with that letter.


α-carbon – An alpha carbon is the carbon in a molecule bonded to an atom or moiety of interest. α-carbon is the most common notation for the alpha carbon.

α-hydrogen – An alpha hydrogen is a hydrogen atom bonded to the α-carbon in a molecule. α-hydrogen is the most common notation for alpha hydrogen.

Abegg’s rule – Abegg’s rule states the sum of the absolute values of the maximum positive and negative valence of an atom is often equal to eight.

abhesive – An abhesive is a material that prevents two surfaces from sticking together.
Antonym: adhesive
Example: Teflon is an abhesive material used to make frying pans non-stick.

ab initioAb initio is Latin for “from first principles”. Ab initio refers to predictions or calculations which are based entirely on theory as opposed to experimentation.

ablation – Ablation is the process of transferring heat by removing material by melting, vaporization or other erosive processes.

abrasive – An abrasive is a material that is used to polish surfaces or smooth rough edges. Most abrasives are very hard, brittle, and heat-resistant.
Examples: Diamond, corundum, sandpaper are all abrasive materials.

absolute alcohol – Absolute alcohol is a common name for the chemical compound ethanol. Ethanol is a colorless liquid with molecular formula C2H2OH. It is the alcohol found in alcoholic beverages.
Also known as: ethanol, ethyl alcohol, pure alcohol, grain alcohol
Alternate Spellings: EtOH

absolute error – Absolute error or absolute uncertainty is the uncertainty in a measurement, which is expressed using the relevant units. Also, absolute error may be used to express the inaccuracy in a measurement.
Examples: If a measurement is recorded to be 1.12 and the true value is known to be 1.00 then the absolute error is 1.12 – 1.00 = 0.12. If the mass of an object is measured three times with values recorded to be 1.00 g, 0.95 g, and 1.05 g, then the absolute error could be expressed as ± 0.05 g.
Also known as: Absolute uncertainty

absolute pressure – Absolute pressure is a measurement of pressure equal to the gauge pressure plus atmospheric pressure.

absolute temperature – Absolute temperature is the temperature measured using the Kelvin temperature scale where zero is absolute zero.

absolute uncertainty – Absolute uncertainty is another term for absolute error. See definition above.

absolute vacuum – An absolute vacuum is a volume of space containing no matter.
Also known as: perfect vacuum

absolute zero – Absolute zero the lowest possible temperature at which matter can exist, 0 K or -273.15°C.

absorbance – Absorbance is a measure of the quantity of light absorbed by a sample.
Also known as: Optical Density, Extinction, Decadic Absorbance

absorbed dose – Absorbed dose is the amount of energy from radiation deposited or absorbed by an object per unit mass. The SI unit of absorbed dose is the Gray (Gy) or J/kg.

absorption – Absorption is the process by which atoms, molecules, or ions enter a bulk phase (liquid, gas, solid). Absorption differs from adsorption since the atoms/molecules/ions are taken up by the volume, not by a surface.

absorption cross section – Another term for adsorptivity. See definition below.

absorption spectroscopy – Absorption spectroscopy is a laboratory technique used to determine the structure and concentration of a sample based on the amount and wavelengths of light that it absorbs.

absorption spectrum – An absorption spectrum is a graph depicting the absorption of radiation by a material over a range of wavelengths.

absorptivity – Adsorptivity is the absorption cross section or extinction coefficient. Absorptivity varies with wavelength and is defined as the absorbance of a solution per unit path length and concentration:
a = A/(bc)
where a is absorptivity, A is absorbance, b is path length, and c is the concentration.

abstraction – An abstraction is a chemical reaction where an atom or ion is removed from one molecule by a radical. Hydrogen abstraction is different from deprotonization. In abstraction, the hydrogen atom supplies an electron to the bonding where deprotonization, the base supplies both electrons in the bond.
Example: Hydrogen abstraction of acetone by chlorine radical.
CH4 + Cl → H3C + HCl

abundance ratio – Abundance ratio refers to the ratio of the number of atoms of one isotope to the number of atoms of another isotope in the same sample.

AC or A/C – AC is an acronym for alternating current. AC or alternating current is an electric current that reverses its direction at regular intervals.

accelerator – An accelerator is a substance that speeds up (accelerates) a chemical reaction. The term often is applied to polymerization. An accelerator can speed the vulcanization of rubber or cause it to occur at a lower temperature than normal. More generally, an accelerator can speed cross-linking of polymer subunits or cause polymerization to occur at a lower temperature than normal.

accuracy – Accuracy refers to the correctness of a single measurement. Accuracy is determined by comparing the measurement against the true or accepted value.

acetal – An acetal is an organic molecule where two separate oxygen atoms are single bonded to a central carbon atom. Acetals have the general structure of R2C(OR’)2.
An older definition of acetal had one at least one R group as a derivative of an aldehyde where R = H, but an acetal can contain derivatives of ketones where neither R group is a hydrogen. This type of acetal is called a ketal.
Acetals that contain different R’ groups are called mixed acetals.
Acetal is also a common name for the compound 1,1-diethoxyethane.

acetate – Acetate can have several meanings in chemistry.

  1. Acetate refers to the ion formed by removing the acidic hydrogen from acetic acid. The formula for this ion is CH3COO.
  2. Acetate is a general name for a compound containing the acetate ion.
  3. Acetate is a fiber made from cellulose acetate.

achiral – Achiral literally means “not chiral”. Achiral refers to an object which can be superimposed on its mirror image.
Also known as: amphichiral
Example: Methane is an achiral molecule.

acid – An acid is a chemical species that donates protons or hydrogen ions and/or accepts electrons.

acid anhydride – An acid anhydride is a nonmetal oxide which reacts with water to form an acidic solution.
In organic chemistry, an acid anhydride is a functional group consisting of two acyl groups joined together by an oxygen atom.
Acid anhydride also refers to compounds containing the acid anhydride functional group. Acid anhydrides are named from the acids that created them. The ‘acid’ part of the name is replaced with ‘anhydride’. For example, the acid anhydride formed from acetic acid would be acetic anhydride.

acid-base indicator – An acid-base indicator is either a weak acid or weak base that exhibits a color change as the concentration of hydrogen (H+) or hydroxide (OH) ions changes in an aqueous solution.
Also known as: pH indicator
Examples: Thymol Blue, Phenol Red, and Methyl Orange are all common acid-base indicators. Red cabbage can also be used as an acid-base indicator.

acid-base titration – An acid-base titration is a procedure which is used to determine the concentration of an acid or base. A measured volume of an acid or base of known concentration is reacted with a sample to the equivalence point.

acid dissociation constant – Ka – The acid dissociation constant is the equilibrium constant of the dissociation reaction of an acid and is denoted by Ka.
Example: The acid dissociation constant, Ka of the acid HB:
HB(aq) ↔ H+(aq) + B(aq)
Ka = [H+][B] / [HB]

acid catalysis -An acid catalyst is a chemical reaction that requires the presence of an acid to act as a catalyst in order to proceed. The acid catalyst typically acts as a supply of protons to activate bonding sites in a molecule to induce a reaction.

acidic solution – An acidic solution is any aqueous solution which has a pH < 7.0 ([H+] > 1.0 x 10-7 M).

acid promoted – Acid promoted refers to a chemical reaction that needs an acid to proceed but does not act as a catalyst for the reaction. Reactions, where the acid acts as a catalyst, are acid catalysis reactions.

acidulant – Acidulant refers to a food additive that lowers the pH to give a tart or bitter taste.

acology – Acology is the study of medical remedies.

acrylic fiber – Acrylic fiber refers to a polymer that contains at least 85% by mass acrylonitrile (CH2CHCN) monomers.
Also known as: acrylic

actinic – Actinic refers to light capable of initiating a chemical reaction.
Example: Ultraviolet light is actinic since it can cause photosynthesis.

actinides – The actinides are a group of elements considered to be elements 90 (thorium) through 103 (lawrencium).
Examples: Thorium, uranium, and plutonium are just a few of the actinides.

actinium – Actinium is the name for the actinide element with atomic number 89 and is represented by the symbol Ac.

activated carbon or activated charcoal – Activated carbon or charcoal is a processed form of carbon treated to be more porous. It is often used as an absorbent material to remove impurities in water.

activated complex – An activated complex is an intermediate state that is formed during the conversion of reactants into products. An activated complex is the structure that results at the maximum energy point along the reaction path. The activation energy of a chemical reaction is the difference between the energy of the activated complex and the energy of the reactants.

activation energy – Ea – Activation energy is the minimum amount of energy required to initiate a reaction and is denoted by Ea.

active ingredient – An active ingredient is a chemical or substance that has a biological effect. The term is applied to drugs, pesticides, herbicides, and herbal medicine. The other ingredients are termed excipients or inert ingredients. Excipients are either biologically nonreactive or else do not affect the biochemical process that is targeted by the product. A formulation may contain more than one active ingredient.
Also known as: AI, Active Pharmaceutical Ingredient (API), bulk active, active substance, pharmacon, pharmakon

active transport – Active transport is the movement of molecules or ions against a concentration gradient from an area of lower to higher concentration.

activity series – The activity series of metals is a list of metals ranked in order of decreasing reactivity to displace hydrogen gas from water and acid solutions. It can also be used to predict which metals will displace other metals in aqueous solutions.
Also known as: Reactivity series of metals

actual yield – Actual yield is the quantity of a product that is obtained from a chemical reaction. The amount of product actually produced by the reaction as opposed to the calculated or theoretical yield.

acute health effect – The acute health effect is the effect caused by the initial exposure of a hazardous chemical on a human or animal body. When a hazardous material’s acute health effect is listed, the effects are generally severe and dangerous adverse effects, but subside after the exposure stops.

acylating agent – An acylating agent is a compound that supplies the acyl group (RCO-) in an acylation reaction.

acylation – Acylation is a chemical reaction that adds an acyl group (RCO-) to a molecule.
Also known as: alkanoylation

acyl group – An acyl group is a functional group with formula RCO- where R is bound to the carbon atom with a single bond. Acyl groups are formed when one or more hydroxyl groups are removed from an oxoacid.
Examples: Esters, ketones, aldehydes, and amides all contain the acyl group

acyl halide – An acyl halide is a functional group with formula R-COX where X is a halogen atom.
Acyl halide also refers to organic compounds containing the acyl halide functional group. Acyl halides are formed by substituting a hydroxyl group of an oxoacid with a halogen atom.

addition compound – An addition compound is a compound containing two or more simple compounds combined into an orderly defined crystal matrix. The two simple compounds are separated by a dot (·) in their formula.
Example: One of the most common addition compounds are hydrates where a salt is crystallized with water. Copper sulfate pentahydrate is copper sulfate (CuSO4) combined with five water (H2O) molecules to form blue crystals of CuSO4·5H2O.

addition polymer – An addition polymer is a polymer produced through the reaction of a monomer adding to itself. No further product is formed. The monomer is most commonly a derivative of ethylene.

addition reaction – An addition reaction is a process or reaction where a small molecule (e.g., H2) is inserted directly into a double or triple carbon bond.

adenosine triphosphate – ATP is the acronym for the molecule adenosine triphosphate. The empirical formula of ATP is C10H16N5O13P3.
ATP is a nucleoside triphosphate made by bonding three phosphate groups to adenosine (adenine ring plus a ribose sugar). This organic compound often is termed the ‘energy currency’ of cellular metabolism because hydrolysis of the phosphate bonds releases considerable energy. In addition to its function for intracellular energy transport, ATP serves as a substrate for enzymes that produce cyclic AMP (adenosine monophosphate) and phosphorylate lipids and proteins.

adhesion – Adhesion is attraction between dissimilar atoms or molecules.

adhesive – An adhesive is a material which bonds together the surfaces of two other materials.
Examples: Glues and cements are common adhesives.

adsorption – Adsorption is the adhesion of a chemical species onto the surface of particles. Adsorption is a different process from absorption, in which a substance diffuses into a liquid or solid to form a solution.

adulterant – An adulterant is a chemical which acts as a contaminant when combined with other substances. Adulterants are added to pure substances to extend the quantity while reducing the quality.
Example: When water is added to alcohol, the water is an adulterant.

aeration – Aeration is a process where air is forced into a substance.
Example: Aquarium bubblers increase the oxygen content of the water by aeration.

aerosol – An aerosol is a dispersion of a liquid in a gas or a solid in a gas.

aether – Aether was the fifth element in alchemical chemistry. Aether was also considered the medium that carried light waves in space in 18th and 19th Century science.
Alternate Spellings: Æther, ether

air – Air is the general name for the mixture of gases that make up the Earth’s atmosphere.
Air is also an early chemical term for a type of gas. Many individual airs made up the air we breathe. Vital air was later determined to be oxygen, the phlogisticated air became nitrogen.

ala – Ala is an abbreviation for the amino acid alanine. Alanine is also abbreviated as A.

alchemy -Several definitions of alchemy exist. Originally, alchemy was an ancient tradition of sacred chemistry used to discern the spiritual and temporal nature of reality, its structure, laws, and functions.

alcohol – An alcohol is a substance containing an OH group attached to a hydrocarbon group.
Examples: Ethyl alcohol (ethanol): C2H5OH; butyl alcohol (butanol): C4H9OH are both alcohols.

alcoholate – Alcoholate can refer to either alkoxide anions or the salts formed where alcohol replaces the water of crystallization in hydrated crystals.

alcohol dehydrogenase – Alcohol dehydrogenase is an enzyme that facilitates reactions to oxidizes alcohol to an aldehyde or ketone in conjunction with NADH. In reverse, alcohol dehydrogenases assist to reduce aldehydes and ketones to alcohols.

alcoholysis – Alcoholysis reactions are substitution chemical reactions where an alcohol acts as a reactant that is incorporated into part of the product molecule. Alcoholysis reactions have the form:
Alcohol + R-LG → R-Alcohol + LG
where LG is a leaving group.

aldehyde – An aldehyde is an organic compound containing the -CHO functional group at the end of a hydrocarbon chain. Aldehyde also refers to the aldehyde functional group R-CHO appearing at the endpoints of molecules.

aldoheptose – Aldoheptose is a heptose carbohydrate with an aldehyde functional group at the first carbon.

aldohexose – Aldohexose is a hexose carbohydrate with an aldehyde functional group at the first carbon.

aldoxime – Aldoxime refers to an oxime where one R group is a hydrogen.

aldopentose – Aldopentose is a pentose carbohydrate with an aldehyde functional group at the first carbon.

aldose – An aldose is a molecule made up of a monosaccharide bonded to an aldehyde chain. Aldose molecules have a general chemical formula of Cn(H2O)n.

aldotetrose – An aldotetrose is a tetrose carbohydrate with an aldehyde functional group at the first carbon.

algaecide or algicide – Algaecides are substances used to control or kill algae.
Example: Copper sulfate is used as an algaecide in aquariums and ornamental ponds. This is why many public fountains have the vivid blue water.

aliphatic – Aliphatic refers to organic molecules or functional groups where the carbon bonds are not aromatic.
Examples: All simple alkane chains are aliphatic.

aliphatic amino acid – An aliphatic amino acid is an amino acid containing an aliphatic side chain functional group. Aliphatic amino acids are non-polar and hydrophobic.
Examples: Alanine, isoleucine, leucine, proline, and valine, are all aliphatic amino acids. Methionine is sometimes considered an aliphatic amino acid even though the side chain contains a sulfur atom because it is fairly nonreactive like the true aliphatic amino acids.

aliphatic compound – An aliphatic compound is a compound containing carbon and hydrogen joined together in straight chains, branched trains or non-aromatic rings.
Also known as: aliphatic hydrocarbon

aliphatic group – a functional group where the group is made up of carbon and hydrogen atoms and not aromatic. Example: The propyl functional group is an aliphatic group.

aliphatic hydrocarbon – Another term for aliphatic compound. See definition above.

alkali metal – An alkali metal is any of the elements found in Group IA (or Group 1) of the periodic table. Alkali metals are very reactive chemical species which readily lose their one valence electron to form ionic compounds with nonmetals.
Examples: Lithium, potassium, and cesium are a few of the alkali metal elements.

alkaline – Alkaline refers to an aqueous solution having a pH greater than 7 or having an [OH] greater than 10-7.
Also known as: basic

alkaline earth metal – an element belonging to group 2 of the periodic table. The alkaline earth metals, as a group, share characteristic properties.
Also known as: alkaline earths
Examples: Beryllium, magnesium, calcium, barium are all alkaline earths.

alkalinity – Alkalinity is a quantitative measurement of the ability of an aqueous solution to neutralize an acid. Alkalinity is denoted by AT and calculated by adding all the stoichiometric amounts of each base in the solution.

alkaloid – Alkaloids are a class of organic compounds with at least one nitrogen in a heterocyclic ring. Alkaloids often have pharmacological effects on humans.
Examples of alkaloids: caffeine, nicotine, morphine, cocaine, theobromine

alkane – An alkane is a hydrocarbon containing only single carbon-carbon bonds.

alkanoylation – Alkanoylation refers to a chemical reaction that adds an acyl group (RCO-) to a molecule. Also known as: acylation

alkene – An alkene is a hydrocarbon containing a double carbon-carbon bond.
Example: H2C=CH2 (ethene or ethylene)

alkenyl group – An alkenyl group is a hydrocarbon group formed when a hydrogen atom is removed from an alkene group. Alkenyl compounds are named by replacing the -e from the parent alkene’s name with -yl.
Example: H2C=CH- (ethenyl or commonly known as vinyl). The parent alkene was H2C=CH2, ethene.

alkoxide – An alkoxide is an organic functional group formed when a hydrogen atom is removed from an hydroxyl group of an alcohol when reacted with a metal. Alkoxides have the formula RO- where R is the organic substituent from the alcohol. Alkoxides are strong bases.
Example: Sodium reacting with methanol (CH3OH) reacts to form the alkoxide sodium methoxide (CH3NaO).

alkoxy group – An alkoxy group is a functional group containing an alkyl group bonded to an oxygen atom. Alkoxy groups have the general formula: R-O. An alkoxy group bonded to a hydrogen atom is an alcohol. An alkoxy group bonded to another alkyl group is an ether.
Also known as: alkyloxy group
Example: The simplest alkoxy group is the methoxy group: CH3O-.

alkylammonium salt – An alkylammonium salt is an ammonium salt where the ammonium cation has the general structure NRxH4-x where x = 1-4.
Example: Dimethylamine hydrochloride is an alkylammonium salt. It is also known by the name dimethyl ammonium chloride.

alkylate – An alkylate is the product formed by the reaction of an alkane and an alkyne.

alkylation – Alkylation is the process where an alkyl group is introduced into a molecule.

alkyl group – The alkyl group is a hydrocarbon group, such as CH3– or C3H7-.

alkyne – An alkyne is a hydrocarbon containing a triple carbon-carbon bond.
Example: Acetylene (H-C≡C-H) is the simplest alkyne.

allotrope – The term allotrope refers to one or more forms of an elementary substance.
Examples: Graphite and diamond are both allotropes of carbon. O2 and ozone, O3, are allotropes of oxygen.

alloy – An alloy is a substance made by melting two or more elements together, at least one of them a metal. An alloy crystallizes upon cooling into a solid solution, mixture, or intermetallic compound.

alpha carbon – An alpha carbon the carbon in a molecule bonded to an atom or moiety of interest. α-carbon is the most common notation for the alpha carbon.

alpha decay – Alpha decay is the spontaneous radioactive decay where an alpha particle is produced. An atom that undergoes alpha decay will reduce its atomic mass by 4 and become the element two atomic numbers less. The general reaction for alpha decay is
ZXAZ-4YA-2 + 4He2
where X is the parent atom, Y is the daughter atom, Z is the atomic mass of X, A is the atomic number of X.
Example: 238U92 decays by alpha decay into 234Th90.

alpha hydrogen – An alpha hydrogen is a hydrogen atom bonded to the α-carbon in a molecule. α-hydrogen is the most common notation for alpha hydrogen.

alpha particle – An alpha particle is a He2+ ion or the helium nucleus. This particle is commonly denoted by the Greek letter α.

alpha radiation – Alpha radiation is ionizing radiation resulting from the decay of radioisotopes where an alpha particle is emitted. This radiation is denoted by the Greek letter α.
Example: When Uranium-238 decays into Thorium-234, an alpha particle is produced in the form of alpha radiation.

alternating copolymer – Alternating copolymer refers to a type of polymer consisting of two different repeating mer units in which the mer units alternate positions within the chain of the molecule.

aluminum or aluminiumAluminum the name for the metal element with atomic number 13 and is represented by the symbol Al.

amalgam – An amalgam is any alloy of mercury and one or more other metals.

americium – Americium is the name for the actinide element with atomic number 95 and is represented by the symbol Am.

amide – An amide is a functional group containing a carbonyl group linked to a nitrogen atom.
Amide also refers to any compound containing the amide functional group. Amides are derived from carboxylic acid and an amine.
Amide is also the name for the inorganic anion NH2. It is the conjugate base of ammonia (NH3).

amidogen – Amidogen refers to a radical composed of a nitrogen and two hydrogen atoms (NH2)
Also known as: amino radical (preferred IUPAC name), amido, azanyl

amine – An amine is a compound in which one or more of the hydrogen atoms in ammonia have been replaced by an organic functional group. Amines are generally weak bases. Further, most amines are organic bases. Amines have the prefix amino- or the suffix -amine included in their name.

amine functional group – An amine functional group is a functional group containing three substituents around a central nitrogen atom containing a lone pair of electrons. Amines are further classified by the number of substituents replaced by hydrogen. Primary amines have two substituents replaced by hydrogen. Secondary amines have one substituent replaced by hydrogen. Tertiary amines have no hydrogen substituents. Ammonia is formed when all three substituents are hydrogen.

amino acid – An amino acid is a type of organic acid that contains an acid functional group and an amine functional group on adjacent carbon atoms. Amino acids are considered to be the building blocks of proteins.

aminolysis – Aminolysis is a substitution chemical reaction which has an amine as a reactant that is incorporated into part of the product molecule. Aminolysis reactions have the form:
Amine + R-LG → R-Amine + LG
where LG is a leaving group.

ammeter – An ammeter (or ampmeter) is an instrument used to measure current.

ammonium – Ammonium is a cation with formula NH4+. It is the conjugate acid of ammonia. Ammonium is also added into the name of any molecule where a nitrogen atom has four single bonds and positive formal charge.

ammonium salt – An ammonium salt is a salt containing an ammonium cation and any anion. Ammonium nitrate and ammonium chloride are both ammonium salts.

amorphography – Amorphography is the science concerned with the classification and characterization of amorphous solids.

amorphous – Amorphous refers to a solid which does not exhibit a crystalline structure. While there may be some local ordering of the atoms or molecules in an amorphous solid, no long-term ordering is present.
Examples: Window glass and polystyrene are amorphous solids.

ampere – The ampere is the base SI unit of electrical current. The ampere is defined as the amount of electrical current required to maintain a force of 2 x 10-7 newtons per meter between two infinitely long parallel wires of negligible cross section held one meter apart.
The symbol for ampere is a capital letter A. The contraction “amp” is also often used.

amphetamine – An amphetamine is a psychostimulant drug, based on the chemical formula C9H13N. Amphetamine is a contraction of alpha-methyl-phenylethylamine, which has the systematic name 1-phenylpropan-2-amine. It belongs to the phenethylamine class of molecules. Derivatives of amphetamines, such as dextroamphetamine or the sulfate or phosphate of amphetamine, also are considered to be amphetamines. Pure amphetamine is a pure colorless liquid.

amphiprotic – Amphiprotic describes a substance that can both accept and donate a proton or H+. An amphiprotic molecule has characteristics of both and acid and a base and can act as either. It is an example of a type of amphoteric molecule.

amphoteric – An amphoteric substance is one which can act as either an acid or a base.
Examples: Metal oxides or hydroxides are amphoteric. Amphiprotic molecules, such as amino acids, are amphoteric.

amphoteric oxide – An amphoteric oxide is an oxide that can act as either an acid or base in a reaction.
Examples: Al2O3 is an amphoteric oxide. When reacted with HCl, it acts as a base to form the salt AlCl3. When reacted with NaOH, it acts as an acid to form NaAlO2.
Typically, oxides of medium electronegativity are amphoteric.

amplitude – Amplitude refers to the magnitude of change in an oscillating system. Peak to peak amplitude refers to the total change between maximum and minimum values of the oscillating system. Semi-amplitude is half the peak to peak amplitude. In general use, the term amplitude refers to the semi-amplitude.

amu – An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses.
Also known as: unified atomic mass unit (u), Dalton (Da) or universal mass unit

anaerobic – Anaerobic means “without oxygen”.

analyte – Analyte is the substance being analyzed in an analytical procedure.

analytical chemistry – Analytical chemistry is the chemistry discipline concerned with the chemical composition of materials. Analytical chemistry also is concerned with developing the tools used to examine chemical compositions.

angstrom – An angstrom is a unit of measurement for very small distances. The symbol for angstrom is Å.
1 Å = 10-10 meters.
Alternate Spellings: Åstrom
Example: The diameter of an atom is on the order of 1 angstrom.

angular momentum quantum number – The angular momentum quantum number, ℓ, is the quantum number associated with the angular momentum of an atomic electron. The angular momentum quantum number determines the shape of the electron’s orbital.
Also known as: azimuthal quantum number, second quantum number
Example: A p orbital is associated with an angular momentum quantum number equal to 1.

anhydrous – Anhydrous literally means ‘no water’. Substances without water are anhydrous.
Example: Table salt is anhydrous sodium chloride.
Anhydrous can also refer to the gaseous form of some concentrated solutions such as ammonia to distinguish it from the aqueous solution form.

anion – An ionic species having a negative charge.
Examples: free chloride in an aqueous table salt (NaCl) solution and singlet oxygen are both anions.

androgen – Androgen is the name given to any natural or synthetic compound that stimulates or controls male sex characteristics. Androgens typically are steroid hormones. Androgens are precursor molecules to estrogens, the female sex hormones.
Examples: testosterone, dihydroxytestosterone

anisotropy – Anisotropy refers to a material exhibiting different values of a property in different crystallographic directions.

annealing – Annealing is a term used to denote any heat treatment in which the microstructure and therefore the properties of a material are altered. Annealing typically refers to heat treatment in which a cold-worked metal is softened by allowing it to recrystallize.

anode – An anode is the electrode where oxidation reactions take place. Anodes are positively charged and attract anions and expel electrons.

anodize – Anodize refers to coating a metal with a protective layer by means of electrolysis.

anti addition – Anti addition is an addition reaction that adds two substituents to opposite sides of a double or triple bond such that the bond order of the bond decreases but the number of substituents increases.

antiaromaticity – Antiaromaticity refers to a planar ring molecules with 4n conjugated delocalized π-electrons in the rings where n is an integer. Antiaromatic molecules are unstable and highly reactive. Antiaromaticity differs from aromaticity by the number of π-electrons. Antiaromaticity has 4n, aromaticity has 4n+2.

antibonding orbital – An antibonding orbital is a molecular orbital containing an electron outside the region between the two nuclei. As two atoms approach each other, their electron orbitals begin to overlap. This overlap forms a molecular bond between the two atoms with its own molecular orbital shape. These orbitals follow the Pauli exclusion principle in the same way as atomic orbitals. No two electrons in an orbital can have the same quantum state. If the original atoms contain electrons where a bond would violate the rules, the electron will populate the higher energy antibonding orbital. Antibonding orbitals are denoted by an asterisk symbol next to the associated type of molecular orbital. σ* is the antibonding orbital associated with sigma orbitals and π* orbitals are antibonding pi orbitals. When speaking of these orbitals, the word ‘star’ is often added to the end of the orbital name: σ* = sigma-star.
Example: H2 is a diatomic molecule containing three electrons. One of the electrons is found in an antibonding orbital. Hydrogen atoms have a single 1s electron. The 1s orbital has room for 2 electrons, a spin “up” electron and a spin “down” electron. If a hydrogen atom contains an extra electron, forming a H ion, the 1s orbital is filled.
If a H atom and H ion approach each other, a sigma bond will form between the two atoms. Each atom will contribute an electron to the bond filling the lower energy σ bond. The extra electron will fill a higher energy state to avoid interacting with the other two electrons. This higher energy orbital is called the antibonding orbital. In this case, the orbital is a σ* antibonding orbital.

antichlor – An antichlor is a substance that removes excess chlorine to stop a bleaching reaction.
Example: Sodium bisulfate and other trisulfates are antichlors.

antiferromagnetism – Antiferromagnetism refer to a phenomenon exhibited by some materials in which complete magnetic moment cancellation occurs as a result of the antiparallel coupling of adjacent atoms or ions. The macroscopic solid of an antiferromagnetic material has no net magnetic moment.

anti-Markovnikov addition – Anti-Markovnikov addition is an addition reaction between an electrophile compound HX and either an alkene or alkyne where the hydrogen atom of HX bonds to the carbon atom with the least number of hydrogen atoms in the initial alkene double bond or alkyne triple bond and the X bonds to the other carbon atom.

antimony – Antimony is the name for the metalloid element with atomic number 51 and is represented by the symbol Sb.

antioxidant – An antioxidant is defined as an enzyme or other organic molecules that can counteract the damaging effects of oxygen in tissues. Although the term technically applies to molecules reacting with oxygen, it is often applied to molecules that protect from any free radical (molecules with unpaired electron).
Examples: beta-carotene, lycopene, vitamin E

antiperiplanar – Antiperiplanar refers to a periplanar conformation where the dihedral angle between two atoms or groups of atoms is between ±150° and 180°.

aprotic solvent – An aprotic solvent is a solvent that does not donate hydrogen (or proton).
Example: Acetone is an aprotic solvent.

aqueous – Aqueous is a term used to describe a system which involves water. The word aqueous is also applied to a solution or mixture in which water is the solvent. When a chemical species has been dissolved in water, this is denoted by writing (aq) after the chemical name.

aqueous solution – An aqueous solution is any solution in which water (H2O) is the solvent.
Examples: Cola, saltwater, and rain are all aqueous solutions.

aqua fortis – Aqua fortis is an old name for nitric acid (HNO3).
Also Known As: acid of nitre, acid of spirit, spirit of nitre

aqua regia – Aqua regia is a mixture of hydrochloric acid (HCl) and nitric acid (HNO3) at a ratio of either 3:1 or 4:1. Aqua regia is useful to dissolve gold.
Also known as: royal water

aqua vitae – Aqua vitae or aqua vita is an old word for a concentrated solution of ethanol in water or strong spirits.

arene – An arene is a aromatic hydrocarbon molecule. Benzene is a simple arene. Arenes are also called simply aromatic hydrocarbons or aryl hydrocarbons.

arg – Arg is an abbreviation for the amino acid arginine. Arginine is also abbreviated as R.

argentum – Argentum is the Latin name for the element silver. Silver’s symbol Ag comes from argentum.

argon Argon is the name for the noble gas element with atomic number 18 and is represented by the symbol Ar.

aromatic compound – An aromatic compound is an organic molecule containing a benzene ring.

Arrhenius acid – An Arrhenius acid is a substance that when added to water increases the number of H+ ions in the water. The H+ ion is also associated with the water molecule in the form of a hydronium ion, H3O+ and follows the reaction:
acid + H2O → H3O+ + conjugate base

Arrhenius base – An Arrhenius base is a substance that when added to water increases the number of OH ions in the water. Arrhenius bases follow the reaction:
base + H2O → conjugate acid + OH

Arrhenius rate equation – The Arrhenius rate equation is a mathematical expression which relates the rate constant of a chemical reaction to the exponential value of the temperature.

arsenic – Arsenic is the name for the metalloid element with atomic number 33 and is represented by the symbol As.

aryl – An aryl group is a functional group derived from a simple aromatic ring compound where one hydrogen atom is removed from the ring.

aryl halide – An aryl halide is a molecule where a halogen atom is bonded to a carbon atom that is part of an aryl ring.
Also known as: haloarene, halogenoarene
Examples: Chlorobenzene, fluorobenzene and bromobenzene are all aryl halide molecules.

asbestos – Asbestos is the general name for a class of materials comprised of silicate fibers. The material is known for its resistance to heat, electrical resistance, and chemical inertness. The use of asbestos was reduced when it was discovered it was linked to lung disorders and lung cancer.

asn – Asn is an abbreviation for the amino acid asparagine. Asparagine is also abbreviated as N.

asp – Asp is an abbreviation for the amino acid aspartic acid. Aspartic acid is also abbreviated as D.

asphalt or asphaltic – Asphalt is a brownish-black semi-solid or solid mixture of bitumens, either from a native source or as a petroleum by-product. Sometimes the term asphalt refers to a mixture of asphalt with sand, gravel or crushed stone.
Also Known as: asphaltic (adjective)
Common Misspellings: aphsalt

asphyxiant – An asphyxiant is a gas or vapor that can displace or dilute air. Asphyxiants can cause unconsciousness and/or death if inhaled.
Examples: hydrogen gas, helium gas, propane, carbon dioxide are all asphyxiants.

astatine – Astatine is the name for the halogen element with atomic number 85 and is represented by the symbol At.

astrochemistry – Astrochemistry is the chemistry of outer space. It is usually applied to regions beyond the solar system (which is sometimes termed cosmochemistry). Astrochemistry is an integration of astronomy and chemistry.

atactic – Atactic refers to a polymer chain configuration in which the side groups are positioned randomly on one or the other side of the polymer backbone.

atmosphere – Atmosphere refers to the gases surrounding a star or planetary body held in place by gravity.
Atmosphere is also a unit of pressure. One atmosphere (1 atm) is defined to be equal to 101,325 Pascals.

atom – An atom is the defining structure of an element, which cannot be broken by any chemical means. A typical atom consists of a nucleus of protons and neutrons with electrons orbiting this nucleus.
Examples: Hydrogen, carbon-14, zinc, cesium, Cl are all atoms. A substance can be an atom and an isotope or ion at the same time.

atomic ion – An atomic ion is an atom which has gained or lost at least one electron resulting in a net positive or negative charge on the atom.
Example: The hydride ion, H is an atomic ion

atomic mass – Atomic mass or atomic weight is the average mass of atoms of an element, calculated using the relative abundance of isotopes in a naturally-occurring element.
Also known as: atomic weight

atomic mass unit (amu) – An atomic mass unit or amu is one twelfth of the mass of an unbound atom of carbon-12. It is a unit of mass used to express atomic masses and molecular masses.
Also known as: unified atomic mass unit (u), Dalton (Da) or universal mass unit

atomic number – The number of protons in an element. The atomic number identifies the element.
Examples: The atomic number of hydrogen is 1; the atomic number of carbon is 6.
Also known as: The atomic number is also known as the proton number. It may be represented by the capital letter Z.

atomic radius – The atomic radius is a term used to describe the size of the atom, but there is no standard definition for this value. Atomic radius may refer to the ionic radius, covalent radius, metallic radius, or van der Waals radius. In all cases, the size of the atom is dependent on how far out the electrons extend. The atomic radius for an element tends to increase as one goes down an element group. The electrons become more tightly packed as you move across the periodic table, so while there are more electrons for elements of increasing atomic number, the atomic radius actually may decrease.

atomic solid – An atomic solid is one in which atoms of an element are bonded to other atoms of the same atom type. Atomic solids in which the atoms are covalently bonded to each other are network solids.
Examples: Atomic solids include pure metals, silicon crystals, and diamond.

atomic volume – The atomic volume is the volume one mole of an element occupies at room temperature. Atomic volume is typically given in cubic centimeters per mole – cc/mol.
The atomic volume is a calculated value using the atomic weight and the density using the formula:
atomic volume = atomic weight/density

atomic weight – Atomic weight is a term used interchangeably with atomic mass. Technically, atomic weight is the average weight of an element based on its natural abundance. Atomic mass is the average mass of an element based on its natural abundance.

ATP – ATP is the acronym for the molecule adenosine triphosphate. The empirical formula of ATP is C10H16N5O13P3.

atto – Atto is a decimal prefix for SI units equal to 10-18. The symbol for atto is a.
Example: 100,000 carbon atoms weighs approximately 20 attograms.

Aufbau principle – The Aufbau principle, simply put, means electrons are added to orbitals as protons are added to an atom.

The Aufbau principle outlines the rules used to determine how electrons organize into shells and subshells around the atomic nucleus.

  • Electrons go into the subshell having the lowest possible energy.
  • An orbital can hold at most 2 electrons obeying the Pauli exclusion principle.
  • Electrons obey Hund’s rule, which states that electrons spread out before they pair up if there are two or more energetically equivalent orbitals (e.g., p, d).

aurum – Aurum is the Latin name for the element gold. Gold’s symbol Au comes from the name aurum.

austenite – Austenite is face-centered cubic iron. The term austenite is also applied to iron and steel alloys that have the FCC structure (austenitic steels).

autoionization – Autoionization is an ionization reaction which occurs between identical molecules.

Avogadro’s Law – Avogadro’s Law is the relation which states that at the same temperature and pressure, equal volumes of all gases contain the same number of molecules.

Avogadro’s number – Avogadro’s number is the number of particles found in one mole of a substance. It is the number of atoms in exactly 12 grams of carbon-12. This experimentally determined value is approximately 6.022 x 1023 particles per mole.
Also known as: Avogadro’s constant

azeotrope – An azeotrope is a solution that retains its composition when distilled.
Also known as: azeotropic mixture, azeotropy
Example: Boiling a 95% (w/w) ethanol solution in water would produce a vapor that is 95% ethanol. Distillation cannot be used to obtain higher percentages of ethanol.

azide – An azide is an anion with the molecular formula N3. The azide functional group has the general molecular formula of RN3.
Also known as: atisine

azimuthal quantum number – Azimuthal quantum number is another name for angular momentum quantum number. See definition above.

azo compound – An azo compound is a compound containing a diazene molecule (HN=NH) where the hydrogen atoms are replaced with alkyl or aryl groups. The general formula for an azo compound is R-N=N-R’.

azo group – The azo group is a functional group consisting of a diazene molecule (HN=NH) where the hydrogen atoms are replaced with alkyl or aryl groups. The general formula for an azo group is R-N=N-R’.
Also known as: diimide functional group.