Describe and explain the trend in atomic radius across period 3.

A student formulates the following hypothesis: “If phosphorus were to form a positive ion, \({{\text{P}}^{3 + }}\), its ionic radius would probably be between \(110 \times {10^{ - 12}}{\text{ m}}\) and \(212 \times {10^{ - 12}}{\text{ m}}\).” Evaluate this hypothesis.

decreases (from left to right/across period 3);

same number of shells/energy levels / shielding remains the same;

number of protons/nuclear charge increases so attraction of nucleus on outer electrons increases / OWTTE;

hypothesis is wrong since ionic radius should be smaller than atomic radius/\(110 \times {10^{ - 12}}{\text{ m}}\);

greater attraction of the nucleus on outer electrons / effective charge of nucleus greater / repulsive forces between electrons smaller;

The trend in atomic radius was generally correctly identified (although some seemed to use ionic radius data) but many tried to explain in terms of electronegativity.

Part (b) was felt to be challenging and it required calm and logical thought with a clear explanation; many realized that the hypothesis was wrong but very few could explain why.