Which property generally decreases across a period in the periodic table?

Which property generally decreases across a period in the periodic table?

• A. Atomic radius increases across a period.
• B. Atomic radius generally decreases across a period.
• C. Electron affinity is the same across the period.
• D. Ionization energy decreases across a period.

Answer: (B)

As you move across a period, the number of protons increases while the electrons are added to the same principal energy level. This raises the effective nuclear charge felt by the outer electrons because shielding by inner electrons doesn’t increase much. The stronger pull from the nucleus draws the outer electrons closer, so the atomic radius shrinks. That’s why the statement about atomic radius decreasing across a period is the best fit. The other ideas don’t align with this trend: the radius does not typically grow across a period, ionization energy generally increases (not decreases) as you go right, and electron affinity is not the same for all elements across a period (noble gases have very low or undefined affinities, while many nonmetals become more favorable to gain an electron as you move to the right).

As you move across a period, the number of protons increases while the electrons are added to the same principal energy level. This raises the effective nuclear charge felt by the outer electrons because shielding by inner electrons doesn’t increase much. The stronger pull from the nucleus draws the outer electrons closer, so the atomic radius shrinks. That’s why the statement about atomic radius decreasing across a period is the best fit.

The other ideas don’t align with this trend: the radius does not typically grow across a period, ionization energy generally increases (not decreases) as you go right, and electron affinity is not the same for all elements across a period (noble gases have very low or undefined affinities, while many nonmetals become more favorable to gain an electron as you move to the right).