How do covalent bonds form between atoms? why is it that diatomic molecules always form nonpolar covalent bonds?

Covalent bonds are formed when two or more non metal atoms share one or more electrons to achieve stable configuration.
Diatomic molecules form non polar covalent bonds because they are linear and the charge distribution across the bond is the same; therefore, resulting in no net charge.

D)can be increased in a closed system