Consider the Mg2+, Cl-, K+, and Se2- ions. The four spheres below represent these four ions, scaled according to ionic size. A) Match each ion to its appropriate sphere. B) In terms of size, between which of the spheres would you find the (i) Ca2+ and (ii) S2- ions?
The correct answer and explanation is :
Answer:
A) Matching the ions to spheres (from smallest to largest ionic radius):
- Smallest sphere – Mg²⁺
- Second smallest – K⁺
- Second largest – Cl⁻
- Largest sphere – Se²⁻
B) Estimating positions of other ions:
(i) Ca²⁺ would be between Mg²⁺ and K⁺.
(ii) S²⁻ would be between Cl⁻ and Se²⁻.
Explanation (approx. 300 words):
Ionic size is influenced primarily by the number of electrons, electron shells, and the effective nuclear charge (Z_eff). Here’s how that applies to each ion:
- Mg²⁺ loses two electrons from its 3s orbital, leaving it with a noble gas configuration (Ne, 1s²2s²2p⁶). It has 12 protons pulling on just 10 electrons, resulting in a very small ionic radius due to a strong effective nuclear charge.
- K⁺, though it also has the noble gas configuration of Ar, has 19 protons and loses one 4s electron, leaving it with 18 electrons. Since it still retains the third energy level, it is larger than Mg²⁺, but smaller than any anion due to the reduced electron–electron repulsion.
- Cl⁻ gains one electron to complete its outer shell (3p⁶), becoming isoelectronic with Ar. With only 17 protons to hold 18 electrons, the increased repulsion and reduced effective nuclear charge cause the radius to expand compared to its neutral atom.
- Se²⁻ gains two electrons, becoming isoelectronic with Kr. The 34 protons have to hold 36 electrons, leading to even greater repulsion and larger size than Cl⁻.
For Ca²⁺, with 20 protons and 18 electrons, it is similar to Mg²⁺ but larger due to the extra shell before ionization. Thus, it falls between Mg²⁺ and K⁺.
For S²⁻, which gains two electrons like Se²⁻ but has fewer shells and protons (16), it is smaller than Se²⁻ but larger than Cl⁻, placing it between Cl⁻ and Se²⁻.
These trends align with periodic trends—cations are smaller than their neutral atoms, and anions are larger. Among isoelectronic ions, those with more protons are smaller due to stronger attraction pulling electrons inward.