Where a compound only contains nonmetal atoms, a covalent bond
is formed by atoms sharing two or more electrons. Nonmetals have
4 or more electrons in their outer shells (except boron). With
this many electrons in the outer shell, it would require more energy
to remove the electrons than would be gained by making new bonds.
Therefore, both the atoms involved share a pair of electrons.
Each atom gives one of its outer electrons to the electron pair,
which then spends some time with each atom. Consequently, both
atoms are held near each other since both atoms have a share in
More than one electron pair can be formed with half of the electrons
coming from one atom and the rest from the other atom. An important
feature of this bond is that the electrons are tightly held and
equally shared by the participating atoms. The atoms can be of
the same element or different elements. In each molecule, the
bonds between the atoms are strong but the bonds between molecules
are usually weak. This makes many solid materials with covalent
bonds brittle. Many ceramic materials have covalent bonds.
Compounds with covalent bonds may be solid, liquid or gas at
room temperature depending on the number of atoms in the compound.
The more atoms in each molecule, the higher a compound’s
melting and boiling temperature will be. Since most covalent compounds
contain only a few atoms and the forces between molecules are
weak, most covalent compounds have low melting and boiling points.
However, some, like carbon compounds, can be very large. An example
is the diamond in which carbon atoms each share four electrons
to form giant lattices.
Some Common Features of Materials with Covalent Bonds:
- Good insulators
- Brittle or cleave rather than deform