Largest in Atomic Radii?

[Sophia Diaz - Dis 1B]

In a review session, there was a question asking which of the following was the largest in terms of atomic radii? Ca+2, Ar, or Cl-?
Apparently, the answer is Cl- since it has the least number of protons. But I don't understand why? Why does the number of protons have anything to do with the atomic radii? Is it because the elements are isoelectric?

Thanks!

[Tina Wen 1G]

Having more protons mean that the atom/ion has stronger ability to attract electrons towards the center nucleus. Hence the smaller the atomic radii. Hope this helps.

[Amanda 1A]

More protons would lead to a greater nuclear pull and a smaller atomic radius since the greater nuclear charge pulls the electrons closer to the nucleus.
So fewer protons would lead to a lower nuclear pull and a larger atomic radius due to the lower nuclear charge.

[Alexander Hari 1L]

If you think of it in terms of gravity this may help: If you know that an element has a large number of protons, it tends to have a larger pull on its electrons because the "gravitational pull" of the protons is higher. If you have a single Cl atom its atomic number is 17 which means it has 17 protons, and 17 electrons. If you take away an electron making it a Cl- this means the electron cloud has less mass for the center of the atom to pull from. Making the atomic radii larger. So, following the trend of atomic radii decreasing across a period: Cl- is bigger than Cl and Cl is bigger than Ar, so Cl- is bigger than Ar. So now we only have Cl- and Ca+2. Since cations are always smaller than their ground state, Ca+2 is almost like Ar. So Cl- is also larger than Ca+2. Hope that didn't confuse you further.

[Chem_Mod]

When you count the number of electrons for the 3 chemical species, you will find that they have same number of electrons (isoelectronic). So, the only thing that is different about those 3 is the number of protons. Protons are positively charged and electrons are negatively charged, so having more protons will imply that the nucleus will attract the electrons much more. This leads to a decrease in the radius, meaning for a set of isoelectronic species, those having more protons will have a smaller radius.

[yuetao4k]

More protons means that the pull on electrons surrounding the atom is greater. This pull makes the atomic radius smaller.