Electronegativity is the ability of an atom to attract shared electrons towards itself.

1. Horizontal Trend: Increases from left to right across a period. This is due to the increase in effective nuclear charge and a decrease in atomic size, which increases the attraction for electrons.
2. Vertical Trend: Decreases down a group. This is because the atomic size increases and the valence electrons are farther from the nucleus, weakening the attraction.