Question

Be sure to answer all parts.

For this isoelectronic series, S²⁻, Cl⁻, and K⁺, identify the ion with the largest radius and the ion with the smallest radius.

Answer 1

Anions are bigger in size while cations are smaller in size than their parent atoms.

Largest Radius Ion - S^2-

Smallest Radius Ion - K⁺

More specifically, potassium, which has an atomic number equal to 19, has 19 protons in its nucleus. Chlorine, on the other hand, only has 17.This is important because in the case of the potassium atom, the outermost electrons will be attracted by the nucleus more.In other words, K+ has bigger effective nuclear charge than Cl−, which translates to a bigger net positive charge felt by the outermost electrons.This will compress the energy levels a bit and make the ionic radius smaller for the potassium cation.Therefore, the chloride anion will have the larger atomic radius.

Both sulphide ion and chloride ion belongs to the third period and the general trend along the third period is that size decreases along the period. So the size of sulphur is larger than chlorine. Apart from this fact we also know that the size of an anion is larger than its parent atom due to inter electronic repulsion. As the repulsion is more in case of S^2- than Cl^- therefore the size of S^2- is larger than Cl^-.