How many outer-shell d electrons are there in each of the following transition-metal ions?
re(iii) sc(iii) ru(iv) hg(ii)

1) The electron configuration of Re is [Xe] 4f14 5d5 6s2

The ion Re (3+) has lost 3 electrons, so its configuration is  [Xe] 4f14 5d4

Then, the number outer-shell d electrons is 4. 

2) Sc: [Ar] 3d1 4s2

Sc (3+) has lost 3 electrons -> [Ar] which has not d electrons.

Then, the number of d electrons is 0

3) Ru: [Kr] 4d7 5s1

Ru (4+) has lost 4 electrons -> [Kr] 4d4

Then, the number of outer-shell d electrons is 4

4) Hg: [Xe] 4f14 5d10 6s2

Hg (2+) has lost 2 electrons -> [Xe] 4f14 5d10

Then, the number of outer-shell d electrons is 10

the answer is 0.42 is that is an option. this question provides no info