Which property is described as increasing across periods and groups in the material?

• A. Ionization energy
• B. Atomic size
• C. Electronegativity
• D. Nuclear charge

Answer: (A)

The key idea is how the nucleus pulls on its electrons. Nuclear charge is the total positive charge of the nucleus (the number of protons). As you move across a period, the number of protons increases, so the nucleus becomes more positively charged and pulls electrons more strongly. As you move down a group, you add entire electron shells, but you also have a nucleus with more protons overall in heavier elements, so the net positive charge that effectively affects the outer electrons still increases with atomic number. In both directions, the nucleus’ positive charge grows.

Ionization energy doesn’t follow that pattern. It generally rises across a period (stronger attraction makes it harder to remove an electron) but falls down a group (additional shells and shielding make it easier to remove a valence electron). Electronegativity behaves similarly to ionization energy: increasing across a period but decreasing down a group. Atomic size, on the other hand, decreases across a period and increases down a group, so it doesn’t rise in both directions.