order Se, S, As, by increasing atomic radius? explain why?

Answer 1

I would go with S, As, Se.
Look at the periodic table. S is one row up so everything S has extra shells which means larger.
As and Se are consecutive in the table. Se contains one more proton and 1 more electron but no more shells. Therefore, I would expect the Se to be slightly smaller because of the extra proton attracting outside electrons.