Covalent bonds
form between non-metal atoms.
Covalent bonds involve sharing one or more pairs
of electrons between the atoms
bonded together.
Noble gases, the elements in
the last column of the periodic table, as have been mentioned before,
have completely filled valence shells and therefore they are energetically stable.
The rest of the non-metals however have less than 8 electrons
(hydrogen less than 2 electrons), incomplete valence shells and
therefore are energetically unstable.
To become stable like the noble gases, these non-metal atoms, when they
react with each other, share electrons to attain a completely filled
valence shell.