atoms get larger the more right and down you go on the Periodic Table.

 

so....in general...if two elements occupy the same quantum number, if the number of protons is larger, the atomic size is smaller? or am I missing something key to this?

 

Hmm you're right....let me look in my old chem books.

 

Well yeah so basically the answer is that the radii get smaller because you have more protons attracting each electron as you go up in atomic number. The reason n = 4 atoms are bigger than n = 3 atoms is because the inner shell shields the outer electrons from the nucleus.

 

awesome, thanks