Question

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

Answer 1

Answer:

the distance between the nucleas and the outermost shell is called atomic size,

it is generally measured in picometer.

# variation in a group -→ it increases as we move down.

# variation in a period = it decrease as we move in a period.

Answer 2

Answer:

distance between nucleus and outermost shell is called atomic size

(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).