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 the electrons.
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
Some Common Features of Materials with Covalent
enthalpies of fusion and vaporization
- Good insulators
- Solids can be soft or brittle
- If brittle often transparent and cleave rather than