Molecules and Covalent Bonds

Molecules are made up of two or more atoms joined together by one or more chemical bonds.
They can be formed from atoms of the same element, like oxygen gas (O2), or different elements, like water (H2O).
The atoms within a molecule are held together by strong covalent bonds.

A covalent bond is formed when two atoms share a pair of electrons.
This type of bond usually occurs between non-metals that have similar electronegativities.
The shared electron pair is attracted to the nucleus of both atoms, which holds the atoms together.
The number of covalent bonds an atom can form is generally equal to the number of electrons needed to fill its outer shell.
For instance, hydrogen needs one electron, so it can form one covalent bond, and oxygen needs two, so it can form two.

If one pair of electrons is shared, it is a single covalent bond.
When two pairs of electrons (four electrons in total) are shared, a double bond is formed. An example is the double bond between the two oxygen atoms in oxygen gas.
Similarly, a triple covalent bond involves the sharing of three pairs of electrons (six in total), as seen in nitrogen gas.
A ‘dative’ or ‘coordinate’ bond is a special type of covalent bond where both electrons originate from the same atom.

Atoms in a covalent network molecule are linked by strong covalent bonds in a giant lattice structure.
This makes them very hard and high melting point substances. Diamond and Graphite, both forms of carbon, are examples of covalent network structures.

Covalent compounds exist as small molecules, giant covalent structures, or polymers.
Properties of simple molecular substances include low melting and boiling temperatures, being poor conductors of electricity, and being generally insoluble in water.
Covalent substances with giant structures, however, can have very high melting and boiling points, but they also do not conduct electricity.
Polymers have very large molecules and the intermolecular forces increase with the size of the molecules, therefore, many polymers are solids at room temperature.

A dot-and-cross diagram can be used to represent covalent bonds in a molecule.
In these diagrams, the dots and crosses represent the outer shell electrons from different atoms.