I). Covalent Bonds:
The bond formed between atoms that share electrons is called a covalent bond. After bonding, each electron attains the electronic configuration of a noble gas, or attains stability. This bond is formed between a non-metal and a non-metal.
A molecule is a group of two or more atoms held together by covalent bonds. Covalent compounds do not have chemical formulae; instead, they have molecular formulae.
Formation and Arrangement:
Many non-metallic elements exist as diatomic molecules. This means that they always exist in the molecular state, with two identical atoms joined together through covalent bonding. For example, hydrogen gas always exists as H2, which is its diatomic form.
Fig 8.1. Covalent bond in Hydrogen molecule.
Note: always show the electrons of different members with different signs, i.e. one with a dot and the other with a cross.
There are other ways of representing a hydrogen molecule as well:
Molecular Formula: H2
Dot & cross diagram: H·×H
Structural Formula: H_H (The single line shows a single covalent bond.)
The sharing of two electrons, or a ‘pair of electrons’, forms a single covalent bond.
Between each molecule of a covalent compound, there are weak van der Waal’s forces holding the molecules together.
Other Examples:
Fig 8.3. Cl2 molecule, Cl_Cl
Fig 8.2. O2 molecule, O=O
Covalent Compounds:
Molecules made from two or more different types of atoms linked together by covalent bonding are called molecular compounds or covalent compounds.
A common example is H2O, water. It is made up of two single covalent bonds.
Fig 8.3. H2O molecule, 
Another example is CH4, methane. It has four single bonds in it.
Fig 8.4. CH4 molecule, 
Carbon dioxide, CO2, has two double bonds.
Fig 8.5. CO2 molecule, O=C=O
Physical Properties:
- They have low melting and boiling points.
- They are mostly gases or volatile liquids at room temperature.
- Most are insoluble in water, and soluble in organic compounds.
- Most do not conduct electricity in either solid, liquid or gaseous state.