Question

Define atomic size. how does it vary in a period and in a group ?

Answer 1

atomic size is the size of a atom (from center of nucleus till its last orbital). Going from left to right in a period, atomic size decreases . Going down a group atomic size increases as orbitals are increased.

Answer 2

Answer:

(1) In a period : Atomic radius decreases in a period from left to right (upto group 17). This is because as we move across a period, nuclearncharge increases but electrons enter the same shell. Thus, the screening effect of the inner core remains the same. As a result the valence electrons are tightly held by the nucleus and atomic size (radius) decreases. For example, in 2nd Period, it decreases from Li(152 pm) to F(64 pm).

(2) In a group : Down the group, as the atomic number increase, nuclear charge increses but number of shells increase. As a result, shielding effect increases and valence electrons are held by weaker attrctive forces. Thus, atomic radius increases. For example, atomic radius increases. For group 17 elements, the value increase from 1st member F (64 pm) to the last member At (140 pm).