2.1 Chlorine (Cl2)
Before considering how two chlorine atoms form a chemical bond you need to consider the appropriate valence electrons.
How many valence electrons does chlorine have?
The electronic configuration of chlorine is: 1s2 2s2 2p6 3s2 3p5. Alternatively this can be written [Ne] 3s2 3p5 in other words the core electrons are 1s2 2s2 2p6 corresponding to the electronic configuration of the noble gas neon. Thus the valence electrons are 3s2 3p5 i.e. there are seven.
Because chlorine has an electronic configuration 1s2 2s2 2p6 3s2 3p5, all it needs is a single electron to give it 8 electrons in its outer shell, and thus the electronic configuration of argon.
Suggest how chlorine could acquire this extra electron?
It could share an electron with another atom, or one electron could be transferred to it from another atom.
In chlorine an electron pair is shared between the two atoms in Cl2.
This is called covalent bonding.
So by sharing electrons through covalent bond formation, atoms are able to fill their valence shell and so attain a noble gas configuration.