Question

Which has the largest radius, magnesium (Mg), silicon (Si), sulfur (S), or sodium (Na)?

Answer 1

Sodium (Na)

Explanation:

These are all period 3 elements.

We know that every time you add an electron to a period 3 element they all go into the 3rd energy level (`s` and `p` orbitals don't make a lot of difference to radius), so they don't shield against each other - in other words, the only important factor in the radius is the nucleus, since there are the same number of electrons shielding the valence electrons.

It's also obvious that as you go along a period you're simply adding one proton each element you go along, so the nuclear charge increases by `+1`.

That means that going from left to right along the group, the charge increases while the number of energy levels stays the same, so the valence electrons are drawn in closer.

Therefore the largest atoms are on the left and the smallest on the right.

So sodium (Na) has the largest atomic radius, as the valence electrons are the least attracted to the nucleus.