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 \({\text{110}} \times {\text{1}}{{\text{0}}^{ - 12}}{\text{ m}}\) and \({\text{212}} \times {\text{1}}{{\text{0}}^{ - 12}}{\text{ m}}\).” Evaluate this hypothesis.

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

same number of shells/energy levels / shielding effect 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/\({\text{110}} \times {\text{1}}{{\text{0}}^{ - 12}}{\text{ m}}\);

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

The trend of atomic radii was described well, but many candidates explained the increased attraction between nucleus and electrons with increasing amount of electrons instead of protons. Very few mentioned that electrons are being added to the same level.

Many thought that the ionic radius in the data booklet was for \({{\text{P}}^{3 + }}\). Others suggested that the \({{\text{P}}^{3 + }}\) radius would be smaller, but did not specify what it would be smaller than. Many thought that fewer outer electrons in the outer shell mean weaker attraction from the nucleus.