# In which of the following sets do all species have the same number of electrons?

A. N, O 2- , F –

B. Sc 3+ , Y 3+, La 3+

C. Cl -, Ar, Ca 2+

D Cr, Cr 2+, Cr 3+

• Well, an unionised atom has the same number of electrons as protons in its nucleus. The number of protons is given by the atomic number in the periodic table. So just look at the periodic table and you have the number of electrons in unionised atoms. If you ionise an atom (add or remove electrons), then you give it some charge. Cl – for example has been given an extra electron to make it negatively charged. This means instead of having 17 electrons, it now has 18.

This question has been made easy really. It could be a lot more difficult. D can be instantly ruled out, because Cr has two more electrons than Cr 2+.

So you just work through them. O 2- has 8 + 2 = 10 electrons, N only has 7, so A is wrong, without even considering F-.

Sc 3+ can’t have the same number of electrons as Y 3+ or La 3+, because they are different types of atoms. They all started off with different numbers of electrons and the same number of electrons has been removed, so they still have different numbers of electrons.

Part C is therefore the only answer left. But just to check, let’s calculate how many atoms each has. Cl has 17 atoms normally, so Cl- has one more, to make 18. Ar has 18 normally, so that’s how many it has in this case. Ca has 20 normally, but two have been removed to make it 2+, so you have 18.