Atomic radius

[ecoli]

In my MCAT review book, it says that as the number of electrons increases, in ground state elements, the atomic radius decreases, because of increased attractiveness to the nucleus. It goes on to say that ionized atoms increase in size when electrons are added.

 

How is this apparent contradiction reconciled?

[Tartaglia]

I think the first statement implies a proton is added with each electron. But I'm not sure exactly what the second implies about protons. Also it would depend which orbital were next in the second statement (and possibly the first too)

[ecoli]

I think the first statement implies a proton is added with each electron.

Ah... true. It was about a periodic table trend across a row.

 

But I'm not sure exactly what the second implies about protons. Also it would depend which orbital were next in the second statement (and possibly the first too)

the formation of an anion, I guess with nucleus remaining the same.

[swansont]

the formation of an anion, I guess with nucleus remaining the same.

 

It would have to. If you change the number of protons, it's not the same element.

 

As you go across a row, adding a proton and electron, the nuclear charge grows, but the screening effect of the electrons doesn't grow as quickly, since they orbitals tend to be spread out. So the net attraction that new electron sees gets larger, and the atom gets smaller, as you move across the row. But an additional electron which forms an ion won't be attracted as much, so it will have a larger radius.

[Mr Skeptic]

Also, a positive ion will be smaller, and a negative ion larger, as compared to a neutral atom.

[hermanntrude]

to understand the sizes of ions and atoms, you must consider the numbers of electrons, the numbers of protons and also the number of electrons which shield any given electron from the positively charged nucleus. I don't remember the exact details but some electrons are better at shielding than others

[swansont]

to understand the sizes of ions and atoms, you must consider the numbers of electrons, the numbers of protons and also the number of electrons which shield any given electron from the positively charged nucleus. I don't remember the exact details but some electrons are better at shielding than others

 

I would imagine s-shell electrons would be best at shielding. The other orbitals tend to be more localized and spread out.

[hermanntrude]

it'd be in any decent chemistry textbook... i just can't remember it offhand

[ecoli]

yeah, I'll look it up to be sure. It's probably a minor point they won't ask about, but I was just curious.