The arrow indicates that HCL possesses a dipole moment in the direction of the Cl atom, the partially negatively charged one. We need a criterion by which to predict whether a bond is nonpolar covalent, polar covalent, or ionic. It is a measure of how strongly an atom attracts the bonding electrons in a chemical bond. The higher the electronegativity, the stronger an atom's attraction for bonding electrons. We will ignore the noble gases since their compounds are of recent origin.
Electronegativity decreases from top to bottom .
This indicates a normal nonpolar covalent bond. Remember this is an attempt at classification and the real world just doesn't obey these manmade rules, so there will be exceptions. The formal charge of an atom in a Lewis formula is the hypothetical charge you obtain by assuming that bonding electrons are equally shared between bonded atoms and that the electrons of each lone pair belong completely to one atom. When several Lewis Structures can be written for a given compound, those which contain the smallest formal charges are preferred. When two proposed Lewis formulas for a molecule have the same magnitudes of formal charges, choose the one with the negative formal charge on the more electronegative atom.