[T]

Here is the question:

Which one of the following elements would be expected to form the largest ion with a noble gas electron configuration?

a) Al
b) Cl
c) P
d) K
e) S

So firstly I looked at the periodic table. Al, Cl, P and S are in the same period and K is in the next.

Al can be eliminated because it will lose its electrons so it will only have 2 electron shells left compared to 3 shells for the other elements.

The higher the atomic number, the more protons in the nucleus, therefore, the atom will be smaller because the electrons will be pulled in closer.

The elements in the order of increasing atomic number is P, S, Cl, K. Therefore P will be the largest ion.

But apparently, Cl is the largest ion. Could someone explain this to me? Thanks.

[Borek]

Why do you think Cl^- is the largest?

I see nothing wrong with your approach, and the trend is confirmed by known radii of Cl^- and S^2-.

Compare http://www.chemguide.co.uk/atoms/properties/atradius.html

[T]

Why do you think Cl^- is the largest?

I see nothing wrong with your approach, and the trend is confirmed by known radii of Cl^- and S^2-.

Compare http://www.chemguide.co.uk/atoms/properties/atradius.html

Hi Borek, thanks for replying. I have looked at the website and it confirms that P^3- has a greater ionic radius than Cl^-. I don't know why Cl^- is the largest. Could the answer sheet just be wrong? Thanks

[Borek]

I have looked at the website and it confirms that P^3- has a greater ionic radius than Cl^-.

Well, the radius of P^3- is given in parentheses, to mark the fact it is not entirely clear whether it can be trusted. But it clearly fits the trend.

I don't know why Cl^- is the largest. Could the answer sheet just be wrong?

That would be my bet.

[T]

I see, thanks Borek.