Hello, George,
I suggest you look at the fundamental reasons that affect elemental sizes. I'll try to highlight some of the factors in y answers.
a) sodium of lithium: Na, since it has a new outer shells of electrons ( 2p and 3s over Li's 2s).
b) silicon or magnesium: Si, since it has added the 3p shell.
c) carbon or boron: B, the atomic size generally decreases as one moves from the left to the right on the same row. The nuclear charge is more powerful as protons are added to the nucleus.
d) selenium or oxygen: Se, because it lies two complete rows down from O, so has several additional electron shells. The other electrons are pushed outward due to the proximity of other electron in the same shell, and since they are also further from the nucleus, so the electrical attraction to the protons is dimished.
Bob
