Question

Atomic & lonic Size 4. In each of the following pairs, pick the species with the larger radius. Explain your answers in terms atomic structure. (a) Cu and Cu? (b) F and F (c) Na and K

Answer 1

4a) the free metal Cu has larger radius than the ion Cu2+.

This is because Cu loses two electrons to form Cu2+ and this leads to less repulsion between electrons in the same shell along with increase in the effective nuclear charge of the other electrons. This leads to shrinking of the cation and therefore the size of Cu2+ is less than the parent Cu.

b) F- is larger in size than F

On adding an electron to the neutral F atom to form the F- anion, there is an increase in the repulsion between electrons which causes the effective nuclear charge to decrease. This causes an increase in the size of the anion and therefore F- is larger than F.

c) Between Na and K, K is larger in size.

Atomic radii increase from top to bottom down the column of the periodic table. The principal quantum number increased down the group and this addition of a new shell leads to an increase in the atomic radii. Na lies above K in the group and is therefore smaller in size compared to K.

When a neutral parent atom gives up electron to form cation, remember that the number of electron decreases and the nuclear pull will be more now which will lead to the cation shrinking respective to the parent atom.

Similarly, when electrons are added, the number of electron increases and this leads to a decrease in the effective nuclear pull and this will cause an expansion in radius.

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