Question

How does atomic size vary across a period and a group?

Answer 1

Atomic size or atomic radius is the distance between the centre of the nucleus and the outermost shell.There are two factors which determine atomic size.They are:

Number of shells- As number of shells increases, atomic size increases.

Nuclear charge- Nuclear charge is equal to the number of protons in the nucleus of an atom. As nuclear charge increases, the attractive force of the nucleus on the valence electrons increases. Hence atomic size decreases.

As we move from left to right in a period atomic size decreases.

Reason:Along a period number of shells remain the same but nuclear charge increases.Hence attractive force of the nucleus on the valence electrons increases.Thus atomic size decreases along a period.

As we move down a group, atomic size increases.

Reason:Down a group both number of shells and nuclear charge increases. But increases in number of shells dominates increase in nuclear charge. Hence atomic size increases down a group.

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).