Chemical bonds are the glue that hold atoms and ions together to form molecules and crystals. When we talk about chemical bonding, the main types of bonds are the strong ones that attract atoms to one another and form molecules. These are intramolecular bonds or bonds within molecules. However, there are also intermolecular forces that attract (and repel) atoms belonging to different molecules. These forces include weaker chemical bonds, such as hydrogen bonds. Here is a look at the types of chemical bonds, with examples.
3 Main Types of Chemical Bonds
The three main types of chemical bonds between atoms and ions are ionic, covalent, and metallic bonds:
- Ionic bonds form between a metal and a nonmetal. The metal donates a valence electron to the nonmetal to form the bond.
- Covalent bonds form when two nonmetals share electrons in a chemical bond.
- Metallic bonds form between metal atoms, where the valence electrons float between multiple atoms.
Ionic Bonds
Ionic bonds form when there is a large electronegativity difference between atoms or ions. Generally, this type of bond forms between a metal and a nonmetal. However, ammonium ion (NH4+) consists of nonmetals and forms ionic bonds with other nonmetals. The valence electron of the metal (the electropositive species) transfers to the valence shell of the nonmetal (the electronegative species), forming a chemical bond. Ionic bonds tend to be strong, forming ionic crystals that are hard and brittle. A good example of an ionic bond is the bond between the sodium atom and chlorine atom in sodium chloride or table salt (NaCl).
Covalent Bonds
Covalent bonds form when atoms or ions have comparable electronegativity values. The atoms form a chemical bond by sharing valence electrons. Covalent bonds form between two nonmetals. Examples of molecules made via covalent bonds include molecular oxygen (O2), water (H2O), and carbon dioxide (CO2). Compounds containing only covalent bonds tend to form relatively soft solids, although some are brittle. They tend to have lower melting and boiling points than ionic compounds and don’t conduct heat or electricity well. However, the covalent bond is not necessarily weaker than the ionic bond. For example, diamond consists of carbon atoms connected by covalent bonds.
The two main types of covalent bonds are nonpolar and polar, plus there are also coordinate covalent bonds.
- Nonpolar Covalent Bonds: A nonpolar covalent bond forms between atoms with identical or nearly identical electronegativities, resulting in equal sharing of electrons. Examples: diatomic and triatomic molecules like H2, O2, O3, and N2.
- Polar Covalent Bonds: A polar covalent bonds involves unequal sharing of electrons due to a significant difference in electronegativity. This results in partial charges, making one end of the molecule slightly positive and the other slightly negative. Examples: H2O, CO2.
- Coordinate covalent bonds (dative bonds): This type of bond forms when one atom provides both electrons for a shared pair in a covalent bond. Example: the formation of the ammonium ion NH4+, where a nitrogen atom donates a lone pair to a proton (H+).
Metallic Bonds
Metal atoms form metallic bonds with each other. Here, the valence electrons are delocalized. What this means is that these valence electrons move between atoms rather than associate with just one (like in an ionic or covalent bond). The “electron sea model” describes metallic bonding as a sea of electrons that are delocalized across a lattice of metal cations. This type of bond promotes high electrical conductivity and helps metals be ductile and malleable. Pure metallic elements like gold or silver form this type of bond. It also occurs in alloys, such as brass, bronze, and steel.
Comparing Chemical Bonds
- Ionic Bonds: Strong electrostatic attractions; high melting and boiling points; typically soluble in water; conduct electricity when molten or dissolved.
- Covalent Bonds: Variable strength; lower melting and boiling points for molecular compounds; generally non-conductive in solid and liquid states.
- Metallic Bonds: Malleable and ductile; excellent conductors of electricity and heat; variable melting points depending on the metal.
| Property | Ionic Bond | Covalent Bond | Metallic Bond |
|---|---|---|---|
| Formation | Electron transfer | Electron sharing | Delocalized electron sea |
| Example | NaCl | H₂, H₂O | Cu, Fe |
| Strength | High | Variable | Variable |
| Melting/Boiling Point | High | Low to moderate | Variable |
| Electrical Conductivity | Only in molten/dissolved state | None (except for certain exceptions) | High |
| Physical Properties | Hard, brittle | Soft (most) | Malleable, ductile |
Chemical Bond Type Calculator
Input the electronegativity values of two elements. See the electronegativity difference and the type of bond (ionic, polar covalent, or nonpolar covalent) that forms between the elements.
Bond Type Calculator
This calculator uses the following electronegativity difference cut-off points to determine bond type:
- Ionic Bond:
- Electronegativity difference greater than 1.7.
- These large differences indicate a transfer of electrons, leading to ion formation.
- Polar Covalent Bond:
- Electronegativity difference between 0.5 and 1.7.
- The unequal sharing of electrons results in partial charges.
- Nonpolar Covalent Bond:
- Electronegativity difference less than or equal to 0.4.
- Electrons are shared equally or nearly equally.
These values are the commonly accepted thresholds, but it’s worth noting that the precise boundaries are debated by chemists and differ according to the reference.
Hydrogen Bonding
Hydrogen bonding is another type of chemical bonding. It occurs between a hydrogen atom of one molecule and an electronegative atom (a nonmetal) from another molecule or another portion of the same molecule. A hydrogen bond is a bit different from ionic, covalent, or metallic bonding because it involves partial electrical charge. It’s closest to the electron sharing of covalent bonding. While a hydrogen bond is weaker than the bonds that hold atoms within molecules, it’s still a significant factor in how molecules arrange themselves. Hydrogen bonding occurs between the hydrogen and oxygen atoms of two water molecules. But, it also occurs between hydrogen and other atoms. For example, hydrogen bonding occurs between the chlorine atoms of Cl2 and the hydrogen atoms of water (H2O).
Single, Double, and Triple Bonds
Another way of looking at chemical bonds is whether they are single, double, or triple bonds. These are varieties of covalent bonds. A single bond forms when the two atoms share one pair of valence electrons. A double bond forms when the atoms share two pairs of valence electrons. When the atoms share three pairs of valence electrons, the results is a triple bond. Triple bonds are stronger than double or single bonds and are also shorter. Similarly, a single bond is longer and weaker than a double or triple bond.
References
- Atkins, Peter; Loretta Jones (1997). Chemistry: Molecules, Matter and Change. New York: W.H. Freeman & Co. ISBN 978-0-7167-3107-8.
- Housecroft, Catherine E.; Sharpe, Alan G. (2005). Inorganic Chemistry (2nd ed.). Pearson Prentice-Hal. ISBN 0130-39913-2.
- Lewis, Gilbert N. (1916). “The Atom and the Molecule”. Journal of the American Chemical Society. 38 (4): 772. doi:10.1021/ja02261a002
- Pauling, Linus (1960). “The Concept of Resonance”. The Nature of the Chemical Bond – An Introduction to Modern Structural Chemistry (3rd ed.). Cornell University Press. ISBN 978-0801403330.