• the number of spaces in the outer energy level left to be filled, and

• the distance of the outer energy level from the nucleus. (The smaller the atom, the stronger the tendency of the atom to attract electrons.) 

Atoms whose outermost energy levels are almost full have a stronger attraction for electrons (i.e., a higher electronegativity) than atoms whose outermost energy levels have four or more vacancies.  Thus an atom like sodium, with seven vacancies, attracts electrons less strongly (that is, is less electronegative) than nitrogen, which has three vacancies, or oxygen, which has two vacancies.  Fluorine, a small atom with only one electron vacancy, is highly electronegative. 

The figures below relate the negativity of various atoms to their size and position on the periodic table of elements.  Note that oxygen (O) and nitrogen (N), two important elements in living systems, are both highly electronegative.  

(click images for larger versions)

            The relative tendency of an atom to attract electrons depends on the number of spaces in the outer shell left to be filled and on the distance of the outer shell from the nucleus.  Hence, lithium (Li) with seven vacancies is less attractive to electrons (that is, less electronegative) than carbon (C), which has four. Oxygen (O), with only two missing electrons, is yet more electronegative.  This graph also helps explain why in methane (CH₄) the shared electrons will be nearer the carbon atom, while in carbon dioxide (CO₂) they will be nearer the oxygens: carbon is more electronegative  than hydrogen, but less electronegative than oxygen.  The electronegativity of the noble gases (which have filled outer shells) cannot be measured, and so is simply estimated.  This knowledge of relative electronegativity permits us to make important predictions about many biochemical reactions.  (The covalent bonding capacity of each atom is shown in parentheses.)