Molecules and Covalent Bonds

Molecules

A molecule is the smallest particle of a compound that retains the chemical properties of that compound.
Molecules are formed when two or more atoms join together. This can be the same atom, such as an oxygen molecule (O2), or different atoms, like water molecule (H2O).
The atoms in a molecule are held together by chemical bonds.
The type and number of atoms in a molecule are shown by its chemical formula. For example, the formula for water, H2O, shows that each water molecule contains two hydrogen atoms and one oxygen atom.

Covalent Bonds

Covalent bonds are formed when two atoms share a pair of electrons. This type of bond is common between non-metals.
Each shared pair of electrons forms a single covalent bond. Some molecules contain double or triple covalent bonds, where two or three pairs of electrons are shared, respectively.
Covalent bonding leads to the formation of molecules or giant covalent structures. For example, diamond and graphite (forms of carbon) are giant covalent structures.
Covalently bonded substances generally have low melting and boiling points, and do not conduct electricity (except for graphite) because there are no free electrons or ions to carry the charge.
The number of covalent bonds an atom can form is typically equal to the number of electrons needed to fill its outer energy level. For example, oxygen can form two covalent bonds, as it has six electrons in its outer shell and needs two more to fill the shell.