Which one of the following properties decreases across the periodic table from left to right and increases from top to bottom?

Atomic radius is the distance from the nucleus to the outermost electron shell. Across a period from left to right, the atomic radius decreases because the effective nuclear charge increases while electron shielding remains relatively constant, pulling the electron cloud closer to the nucleus. Conversely, the atomic radius increases from top to bottom down a group because new energy levels (electron shells) are added, increasing the distance between the nucleus and valence electrons. Other properties like ionization energy, electron affinity, and electronegativity generally follow the opposite pattern: they increase from left to right across a period due to stronger nuclear attraction and decrease from top to bottom down a group as electrons are further from the nucleus and more shielded.