Across a period, how does atomic size change?

Across a period, how does atomic size change?

• A. Increases
• B. Stays constant
• C. Decreases
• D. Fluctuates widely

Answer: (C)

As you move across a period, atomic size decreases. The electrons are filling the same principal energy level, so shielding from inner electrons stays about the same. Meanwhile, the nucleus gains more protons, increasing the effective nuclear charge it exerts on the outer electrons. That stronger attraction pulls the electron cloud closer to the nucleus, shrinking the atomic radius. This trend continues across the period until a new energy level is started in the next row, at which point the radius increases again. The net effect is a progressively smaller atom as you go from left to right across a period.

As you move across a period, atomic size decreases. The electrons are filling the same principal energy level, so shielding from inner electrons stays about the same. Meanwhile, the nucleus gains more protons, increasing the effective nuclear charge it exerts on the outer electrons. That stronger attraction pulls the electron cloud closer to the nucleus, shrinking the atomic radius. This trend continues across the period until a new energy level is started in the next row, at which point the radius increases again. The net effect is a progressively smaller atom as you go from left to right across a period.