What Is a Compound in Chemistry? Definition and Examples

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Atoms, Molecules, Compounds
A compound consists of two or more elements chemically bonded in a fixed ratio. All compounds are examples of molecules, but molecules also include substances consisting of only one element, chemically bonded to itself.

In chemistry, a compound is a substance made of two or more elements chemically bonded to each other in a fixed ratio. The chemical bonds between atoms of elements involve a transfer or sharing of valence of electrons. This give a compound different properties from its elements. A compound may be represented by a chemical formula that shows the element symbols of the atoms and their proportions.

Examples of Compounds

Examples of compounds include any substance with more than one element and a fixed ratio between them. For example, the ratio between hydrogen and oxygen atoms in a water molecule (H2O) is always 2:1. Whether you have a milliliter or 50 liters of water, every particle has the same 2:1 ratio between hydrogen and oxygen atoms. This ratio is important because the same elements can combine in different proportions to form compounds with completely different properties. For example, water (H2O) is quite different from hydrogen peroxide (H2O2), even though they both consist of hydrogen and oxygen atoms.

Examples of compounds and their names include:

  • Water (H2O)
  • Hydrogen peroxide (H2O2)
  • Carbon monoxide (CO)
  • Carbon dioxide (CO2)
  • Methane (CH4)
  • Sodium chloride (NaCl)
  • Glucose (C6H12O6)
  • Sodium bicarbonate (NaHCO3)
  • Acetic acid (C2H4O2)
  • Sulfuric acid (H2SO4)
  • Ammonia (NH3)
  • Nitrous oxide (N2O)
  • Gallium arsenide (GaAs)

Examples of substances which are not compounds include the hydrogen ion (H+) and the noble gas elements (e.g., Ar, Kr, Ne). Because there is only one element, the pure metals and diatomic nonmetals often are not considered to be compounds (e.g., gold, copper, H2, F2).

Difference Between a Compound and a Molecule

All compounds are examples of molecules, but not all molecules are compounds.

According to the International Union of Pure and Applied Chemistry (IUPAC), a molecule is defined as an electrically neutral substance consisting of more than one atom. This includes compounds, plus it includes diatomic and triatomic elements, such as oxygen (O2), chlorine (Cl2), and ozone (O3). By this definition, many superconductors are molecules, but not compounds, because their chemical formula does not have a fixed ratio. An example is a YBCO superconductor, which has a formula YBa2Cu3O7-x. (x could be 0.15).

Types of Compounds

Compounds are classified according to the type of chemical bonds formed between the atoms. These bonds may be ionic, covalent, metallic, or a mixture of ionic and covalent bonds.

  • Covalent or molecular compounds are held together by covalent bonds.
  • Ionic compounds are held together by ionic bonds.
  • Complexes are held together by coordinate covalent bonds.
  • Intermetallic compounds are held together by metallic bonds.

How to Write Compound Formulas

Compound names and formulas are written listing the atom or group of atoms acting as a cation first, followed the atom or group of atoms acting as an anion second. Because atoms of elements have different oxidation states, an element may be in either the beginning or end of the formula, depending on other elements. For example, carbon (C) is a cation in carbon dioxide (CO2) and anion in silicon carbon (SiC).

The number of atoms of an element are indicated using subscripts. If there is only one atom of an element, the subscript is omitted. Water is a compound made of two hydrogen atoms (H) and one oxygen atom (O). H2O is correct, but H2O1 is not. Table salt (sodium chloride) consists of one sodium atom bonded to one chlorine atom. Its chemical formula is NaCl and not Na1Cl1.

References

  • Brown, Theodore L.; LeMay, H. Eugene; Bursten, Bruce E.; Murphy, Catherine J.; Woodward, Patrick (2013). Chemistry: The Central Science (3rd ed.), Frenchs Forest, NSW: Pearson/Prentice Hall. ISBN 9781442559462.
  • IUPAC (1997). “Molecule”. Compendium of Chemical Terminology (2nd ed.) (the “Gold Book). Oxford: Blackwell Scientific Publications. ISBN 0-9678550-9-8. doi:10.1351/goldbook
  • Hill, John W.; Petrucci, Ralph H.; McCreary, Terry W.; Perry, Scott S. (2005). General Chemistry (4th ed.). Upper Saddle River, NJ: Pearson/Prentice Hall. ISBN 978-0-13-140283-6.
  • Whitten, Kenneth W.; Davis, Raymond E.; Peck, M. Larry (2000). General Chemistry (6th ed.). Fort Worth, TX: Saunders College Publishing/Harcourt College Publishers. ISBN 978-0-03-072373-5.