Ionic Radii The size of ions as measured by ionic radii varies in a systematic m

Question

Ionic Radii

The size of ions as measured by ionic radii varies in a systematic manner. The size of the ion can be explained in part by effective nuclear charge, Zeff, which is the net nuclear charge felt by an electron. The effective nuclear charge takes into account the actual nuclear charge and the shielding of this charge by inner electrons. When an atom loses electrons, the resulting cation is smaller both because the remaining electrons experience a larger Zeff and because these electrons are usually in orbitals closer to the nucleus than the electrons that were lost. The more electrons that are lost, the smaller the ion becomes. Similarly, when an atom gains electrons, the resulting anion is larger owing to both increased electron-electron repulsions and a reduction in Zeff. The more electrons that are gained, the larger the ion becomes.

Part A

Rank the following ions in order of decreasing radius: Rb+,K+,Li+,Cs+, and Na+.

Rank from largest to smallest radius. To rank items as equivalent, overlap them.

Part B

Rank the following items in order of decreasing radius: Ca, Ca2+, and Ca2.

Rank from largest to smallest radius. To rank items as equivalent, overlap them.

Part C

The following ions contain the same number of electrons. Rank them in order of decreasing ionic radii.

Cl^- ; Ca^2+ ; S^2- ; K+ ; P^3-; Sc^3+

Rank from largest to smallest radius. To rank items as equivalent, overlap them.

Explanation / Answer

Part A

Cs+ > Rb+ > K+ > Na+ > Li+

From top to bottom down a group, electronegativity decreases. This is because atomic number increases down a group, and thus there is an increased distance between the valence electrons and nucleus, or a greater atomic radius.

Part B

Ca2-> Ca > Ca2+

Effective nuclear charge, Zeff, increases from anion to neutral to cation.

Part C

The following ions contain the same number of electrons. Rank them in order of decreasing ionic radii.

Cl- ; Ca2+ ; S2- ; K+ ; P3-; Sc3+

They all have the same number of electrons.

The difference in size is due to the strength of the nuclear attraction (or Effective nuclear charge).

The one with the most protons will be smallest.

So put them in atomic number order.

P3- > S2- > Cl- > K+ > Ca2+ > Sc3+

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