How do you place these ions in order of decreasing size?

Question

There are three sets I need help with:

1) F -, N3-, Mg2 , Na

2) Cl -, Sc3 , Ca2 , P3-

3) I -, Te2-, La3 , Cs

If you could explain how you determine the order that would be great, at least for the first one so I can get the hang out of it. Thanks!

Answer 1

Rank these ions from largest to smallest

1) F-, N3-, Mg2+, Na+

The ions are all isoelectronic (same number of electrons).

They get smaller as the effective nuclear charge gets larger.  The effective nuclear charge increases as the number of protons increases.

N^3-, F^-, Na+, Mg^2+

2) Cl-, Sc3+, Ca2+, P3-

All are isoelectronic.  Same explanation.

P3-, Cl-, Ca2+, Sc3+

3) I-, Te2-, La3+, Cs+

Ditto

Te2+, I-, Cs+, La3+

Answer 2

F- is a fluorine with an extra electron in its outer orbital, meaning it’s basically Neon now (in terms of electrons).  Mg2+ is a magnesium with two electrons missing from its outer orbital, meaning it’s also basically Neon now.  Same deal with N3-.. The number of electrons might be the same, but the protons still differ which affects the size.  Even though F has an octet, it’s not happy because it doesn’t have 8 protons to counter it so the extra electrons in the orbital repel, making it larger. N also has an octet but it has even less protons than F to balance the negative charge so it repels even more, making it bigger than F-. Mg2+ is basically losing a full orbital, but it has more protons in the nucleus to attract the electrons so it’s smaller than F-. Sodium is the biggest because it has more orbitals and is neutral. You already know that neutral Na is definitely bigger than N3- or F- because it has an extra orbital, but what about Mg2+? It’s lost it’s outer two electrons and has a surplus of protons now and is pulling it’s electrons tighter, so Na is bigger now.

Mg2+  < F- < N3- <  Na

I think that’s the right answer.. hope that helps!

Answer 3

If I remember correctly, atoms on the periodic table get smaller as you move to the right in a row (because they have the same number of electron orbitals, but more electrons so the orbitals (negative) are more attracted to the protons (positive) in the nucleus), but larger as you move down a column (because they have more orbitals).  Hope that helps!