Electron Sharing: The Covalent Bond
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).
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