Final answer:
Electronegativity decreases as atomic size increases and increases from left to right across a period due to the increase in nuclear charge; it decreases down a group due to the increase in atomic size.
Step-by-step explanation:
In general, electronegativity decreases as atomic size increases because a shared electron pair is closer to the nucleus of a smaller atom, which therefore attracts these electrons more strongly. Electronegativity tends to increase from left to right across a period of the periodic table and tends to decrease down a group.
The periodic trend in electronegativity is such that it generally increases due to an increase in nuclear charge as you go from left to right across a period. This is because the atomic size decreases which makes the nucleus more effective in pulling the bonding electron pair to itself. Conversely, as you move down a group, the atomic size increases, making the effective pull of the nucleus on the bonding electrons weaker, thus decreasing the electronegativity.