[Rutherford]

I will ask two questions here, because they are short:

1.Which of the following molecules has the smallest dipole moment?
a) CO₂
b) SiO₂
c) SO₃
d) SF₆
e) UF₆
f) XeF₆
Till now, I was used to think that these and similar molecules have zero net dipole moment, which is obviously not true. How to answer this question?

2.Which of the following ions has the smallest ionic radius?
a) Ag⁺
b) F^-
c) H^-
d) Al³⁺
e) Na⁺
H^- is from the first period but it somehow isn't the smallest. Why?
Aside this, it is easy to guess that the answer is Al³⁺.

[Rutherford]

I found out that the first question has a mistake, it should be biggest dipole moment, and it is f) then.

What about 2.?

[9-92-6-19]

I believe for questions like this calculating the effective nuclear charge can get you the answer; the greater the Z* then the smaller the atom/ion/whatever is.

[Rutherford]

Good suggestion. It will be: 1, -1, -1, 3, 1, -biggest for aluminium. It will be another factor I have to take into consideration.
I was surprised to see that the hydride anion has a radius almost equal as Rb⁺!

[Big-Daddy]

How does one calculate an accurate estimate for effective nuclear charge?

As for question 1, don't know about UF6 but were we right to think that all others actually do have 0 dipole moment? Except for XeF6 due to the lone pair which ruins the symmetry.

[CafeYen]

For Ionic Radius, always consider
1) positive will be smaller, why? think of electron cloud being sucked in because the atom is now positive!!
2) Then for the rest of the positive ones follow regular atomic radius

BUt for this one it's Al3+ the more positive the smaller
think of the positive like a magnet sucking electron clouds in !!!!

[Big-Daddy]

For Ionic Radius, always consider
1) positive will be smaller, why? think of electron cloud being sucked in because the atom is now positive!!
2) Then for the rest of the positive ones follow regular atomic radius

BUt for this one it's Al3+ the more positive the smaller
think of the positive like a magnet sucking electron clouds in !!!!

I'm sure this can all be subsumed into a quantitative argument i.e. effective nuclear charge. Which I'm waiting for Raderford (or someone else) to outline.

[Rutherford]

You have it explained here: http://en.wikipedia.org/wiki/Effective_nuclear_charge. I did some mistake in the calculation.

[Big-Daddy]

You have it explained here: http://en.wikipedia.org/wiki/Effective_nuclear_charge. I did some mistake in the calculation.

Are you sure application of Slater's rules will lead to Al³⁺ having a smaller ionic radius than H^-?

[Rutherford]

It has to be so .

[Big-Daddy]

In any case the effective nuclear charge for H itself is much lower than the effective nuclear charge for Al yet Al is a larger atom. Clearly this approach does not fully work.

[Corribus]

Ionic radius is a predominantly artificial quantity that is determined empirically.  Neither atoms or ions are hard spheres, so it doesn't really have much of an absolute meaning.  I think it's pretty hard to predict in any absolute sense what relative values will be, especially for elements with very different nuclei and charge states.

[Rutherford]

In any case the effective nuclear charge for H itself is much lower than the effective nuclear charge for Al yet Al is a larger atom. Clearly this approach does not fully work.

The lower the nuclear charge, the bigger the ion is. Also you have to take in mind the periods.

[Big-Daddy]

In any case the effective nuclear charge for H itself is much lower than the effective nuclear charge for Al yet Al is a larger atom. Clearly this approach does not fully work.

The lower the nuclear charge, the bigger the ion is. Also you have to take in mind the periods.

Yes, mistake on my part, but an easy example still demonstrates the problem. Al³⁺ has a higher effective nuclear charge from Slater's Rules than H⁺ (9.2:1) but a much larger ionic radius. Of course you have to take the periods into account, but I was hoping the effective nuclear charge system would be able to handle that as well.

[Rutherford]

Nothing is perfect  . And after all, H⁺ doesn't have any electrons.