Electronegativity refers to an atom's ability to attract electrons in a chemical bond.

Across a period in the periodic table, electronegativity generally increases. This is because, as you move from left to right, the atomic radius decreases, and the effective nuclear charge increases. The decreased atomic radius brings valence electrons closer to the nucleus, resulting in a stronger attraction and higher electronegativity. Atoms across a period have a greater ability to attract electrons and form polar covalent or ionic bonds.

Down a group in the periodic table, electronegativity generally decreases. This is due to the increasing atomic radius and the shielding effect from inner electron shells. The larger atomic radius and shielding reduce the effective nuclear charge, weakening the attractive force on electrons. Consequently, atoms down a group have a lower ability to attract electrons and form polar covalent or ionic bonds.