|Most of the substances we encounter are not ionic. Rather than existing
as collections of electrically charged particles (ions), they occur as electrically
neutral combinations of atoms called molecules. Water, H2O,
consists of molecules, table sugar C12H22O11
and gasoline C8H18 are also examples.
|Ionic bonding results because of the energy-lowering lattice energy
and the energy raising IE and EA. Many times this is not possible, particulary
when the IE of all the atoms involved is large, as happens when non-metals
combine with other non-metals. In such instances, nature uses a different
way to lower the energy - electron sharing.
|What happens when two hydrogen atoms join together to form an H2
molecule? As the two atoms approach each other, the electron of each atom
begins to feel the attraction of both nuclei. This causes the electron density
around each nucleus to shift toward the region between the two atoms. Therefore,
as the distance between the nuclei decreases, there is an increase in the
probability of finding either electron near either nucleus. In effect, then,
each of the hydrogen atoms in this H2 molecule now has a share
of two electrons.
|When the electron density shifts to the region between the two hydrogen
atoms, it attracts each of the nuclei and pulls them together. Being of the
same charge, however, the two nuclei also repel each other, as do the two
electrons. In the molecule that forms, therefore, the atoms are held at a
distance at which these attractions and repulsions are balanced. Overall,
the nuclei are kept from separating, and the net force of attraction produced
by the sharing of the pair of electrons is called a covalent bond.
|Every covalent bond is characterized by two quantities, the average
distance between the nuclei held together by the bond, and the energy needed
to separate the two atoms to produce neutral atoms again. In the hydrogen
molecule, the attractive forces pull the nuclei to a distance of 75 pm, and
this distance is called the bond length or bond distance. Because a covalent
bond holds atoms together, work must be done to separate them. When a bond
is formed, an equivalent amount of energy is released. The amount of energy
released when the bond is formed is called the bond energy.
|Formation of a covalent bond releases the bond energy, which means that as the bond forms, the energy of the atom decreases. In the diagram below you can see how the energy changes when two hydrogen atoms form H2. The minimum energy occurs at a bond distance of 75 pm, and that 1 mol of hydrogen molecules is more stable than 2 mol of hydrogen atoms by 435 kJ. In other words the bond energy of H2 is 435 kJ/mole.|
|Before joining, each of the separate hydrogen atoms has one electron
in its 1s orbital. When these electrons are shared, the 1s orbital of each
atom has, in a sense, become filled. The electrons have also become paired,
as required by Pauli's Exclusion Principle, (each pair of atoms has a different
spin and hence opposite magnetic poles). For this reason a covalent bond is
sometimes referred to as an electron pair bond.
|Covalent bonds which are sharing an electron pair bond are indicated
with a dash, just like what was used in the organic unit.
H· + ·H ---> H:H which is shown as H-H. (the line indicates a covalent bond pair of electrons).