Different atoms pull on bonding electrons to differing degrees, and this difference is the pivotal property in determining whether a covalent or ionic bond is formed. When one atom has a much higher pull on bonding electrons than its partner, electron transfer occurs, and an ionic compound is formed. When both atoms have similar attractions for the bonding electrons, they'll share them more equitably and a covalent compound is formed.
Elements can be ranked by their relative attraction for bonding electrons. The more different their ranks, the more likely the bond is to be ionic.
Elements by their ability to attract bonding electrons - Linus Pauling and others considered several elemental properties to develop a consistent ranking scheme. Pauling used the element's ionization energy and electron affinity to predict how it will behave in a bond. The more energy it takes to pull off the outer electron of an atom, he reasoned, the less likely it is to allow another atom to take those electrons. The more energy the atom releases when it gains an electron, the more likely it is to take electrons from another atom in bonding. These two energies were used to compute a numerical score called anelectronegativity. Electronegativity ranks the element's tendency to attract electrons and acquire a more negative charge in a bonding situation.
Electronegativity will increase going left to right across the periodic table. Fluorine's high nuclear charge coupled with its small size make it hold onto bonding electrons more tightly than any other element. Lithium has a lower nuclear charge and is actually larger than fluorine. Its valence electron is not tightly held and it tends to surrender it in chemical bonds.
How are electronegativity values used? Consider coupling two fluorine atoms together. Even though each atom has a high attraction for bonding electrons, both attract them equally. The electronegativity difference between the atoms is zero, and the bond ispure covalent.
