Chemical Bonding

The most stable electron configuration for atoms is to have the outer most, or valence, shell full of electrons. A simple chemical rule (the octet rule) says that atoms will gain, lose or share electrons to achieve this configuration. This is the basis for all chemical bonds.

Ionic Bonds

Some atoms gain or lose electrons to get a stable electron configuration. Once the atoms gain or lose electrons, they become ions and the attractive forces between the ions are called ionic bonds. Metal atoms lose electrons and become positively charged (the amount of positive charge equals the number of electrons lost). Non-metal atoms gain electrons and become negatively charged (the amount of negative charge equals the number of electrons gained). Oppositely charged ions attract each other and form a highly ordered three dimensional array. Compounds formed this way are called ionic compounds.

Covalent Bonds

Covalent bonds are formed when two non-metal atoms react with each other. The atoms come close together and share electrons; the atoms gain access to more valence electrons and get a more stable valence electron configuration. A shared pair of electrons is called a covalent bond. A group of atoms held together by covalent bonds is called a molecule. Substances formed this way are covalent compounds.

When two atoms of the same element share electrons, each atom attracts the shared electrons equally. The equal sharing results in a pure covalent bond or non-polar covalent bond. When atoms of two different elements share electrons, one atom usually attracts the shared electrons more strongly, that is, it is more electronegative. Instead of being shared equally, the electrons on the average are nearer to the more electronegative atom. This results in a polar covalent bond. The amount of charge at each pole is less than 1, or a partial charge. The more electronegative atom has a negative partial charge and the other atom has a partial positive charge. Molecules containing only non-polar covalent bonds are called non-polar molecules; molecules containing one or more polar covalent bonds may or may not be polar molecules.

Hydrogen Bonds

The presence of partial positive and negative charges in polar covalent bonds allows electrostatic attractions between polar molecules known as dipole-dipole interactions. Unusually strong dipole-dipole interactions are found between a hydrogen atom covalently bonded to a small electronegative atom (fluorine, oxygen or nitrogen) and a pair of electrons on another small electronegative atom (fluorine, oxygen or nitrogen). Two factors account for the strength of these interactions:
  1. the strongly electronegative atom the hydrogen is covalently bonded to attracts the electrons so strongly that the hydrogen atom is left as an essentially "bare" nucleus (it has no other electrons other than those in the covalent bond - unique to hydrogen), and
  2. the small size of the hydrogen atom allows the "bare " nucleus to closely approach the pair of electrons on the electronegative atom of an adjacent molecule.

Hydrogen bonding can occur between two molecules if one molecule has a hydrogen atom covalently bonded to nitrogen, oxygen or fluorine and the other molecule has an oxygen, nitrogen or fluorine with an available pair of electrons.