4. Which pair of elements has the smaller element listed first? (a) Al, B (b) C, N (c) Li, Na (d) CI, F (e) Si, P Solution The rule is: As we move down a group, atomic radius increases. this is because an additional shell is added each time As we move from left to right, atomic radius decreases. this is because an additional proton is added to some shell which increases attraction a) they are in same group Al will be bigger than B b) they are in same period C will be bigger than N c) they are in same group Na will be bigger than Li d) they are in same group Cl will be bigger than F e) they are in same period Si will be bigger than P Answer: c.