We know about hundred different atoms, and every atom has an own symbol. Electronic formulas show the symbol of the atom with all its present valency electrons. You can dot that for single atoms as well as for atoms in bonds.
So the electronic formula of a Sodium aton: Na·
The dot represents one valency electron. Two dots (i.e. two valency electrons) make an electron pair that also, and often, can be indicated with a dash.
A bonding between two atoms, like in the molecule of Chlorine (Cl2) may thus be given as:
|Cl - Cl|