2.1.2 Trends of Period 3 Elements : Atomic Radius

Question 1

How does atomic radius change across a period?

Answer 1

• Number of protons increases
• Number of shells remains the same (shielding remains same)
• So greater force of attraction between positive protons in nucleus and outer shell of electrons
• So atomic radius decreases across period

Question 2

Size of cations (+) compared to the parent atom across a period?

Answer 2

Cations (+) are smaller because they have lost their valence electron

Less shielding too because of lost electrons.

Going across a period from Na+ to Si4+, ions get smaller due to increasing nuclear charge and decreased shielding.

Question 3

Size of anions (-) compared to their parent atom?

Answer 3

Anions (-) are larger because they have gained electrons to form another electron shell

More repulsion between more electrons, so more shielding while nuclear charge remains same per element.

Going across from P3- to Cl-, ionic radii decreases as nuclear charge increases and fewer electrons are gained by each atom