Atoms are most stable when they have a ‘full’ outer shell of electron (two electrons for hydrogen, eight electrons for other elements).
To have access to full outer shells, atoms ‘share’ electrons between orbitals.
A covalent bond is the sharing of one pair of electrons between two atoms.
Atomic Structure Recap
Atoms have electrons that travel around a nucleus in ‘orbitals’ and exist in sub-shells. Each orbital can only hold one pair of electrons, and each sub-shell is a different distance from the nucleus (1 being the closest, 2 the next closest, etc.).
In covalent bonding, we are concerned with the outermost electrons and sub-shell of an atom.
The most stable configuration for an atom is to have eight electrons in its outermost shell. The reactivity of an element can often be linked to how easily an atom can ‘get' to having eight electrons in its outermost shell. This explains why the noble gases (group 8 in the periodic table) are so unreactive, as they do not want to change.
In covalent bonding, electrons are shared between atoms so each atom has access to a full outer shell of eight electrons (with the exception being hydrogen, which only wants two electrons).
The group number (one to eight) in the periodic table tells us how many electrons an atom has in its outermost shell.