i. Variation in atomic size down the group : 

a. As we move down the group from top to bottom in the periodic table, the atomic size increases with the increase in atomic number. 

b. This is because, as the atomic number increases, nuclear charge increases but simultaneously the number of shells in the atoms also increases.

c. Asa result, the effective nuclear charge decreases due to increase in the size of the atom and shielding effect increases down the group. 

Thus,

The valence electrons experience less attractive force from nucleus and are held less tightly.

d. Hence, the atomic size increases in a group from top to bottom.

e. g.

• In group 1, as we move from top to bottom i.e., from Li to Cs, a new shell gets added in the atom of the elements and the electrons are added in this new shell.
• As a result of this, the effective nuclear charge goes on decreasing and screening effect goes on increasing down a group.
• Therefore, The atomic size is the largest for Cs and is the smallest for Li in group 1.

[Note : Atomic radii of Li and Cs are 152 pm and 262 pm respectively.]

ii. Variation in atomic size across a period : 

a. As we move across a period from left to right in the periodic table, the atomic size of an element decreases with the increase in atomic number.

b. This is because, as the atomic number increases, nuclear charge increases gradually but addition of electrons takes place in the same shell.

c. Therefore, as we move across a period, the effective nuclear charge increases but screening effect caused by the core electrons remains the same.

d. As a result of this, attraction between the nucleus and the valence electrons increases. 

Therefore,

Valence electrons are more tightly bound and hence, the atomic radius goes on decreasing along a period resulting in decrease in atomic size.

e. g.

• In the second period, as we move from left towards right i.e., from Li to F, the electrons are added in the second shell of all the elements in second period (except noble gas Ne).
• As a result of this, the effective nuclear charge goes on increasing from Li to F, however, screening effect remains the same.
• Therefore, the atomic size is the largest for Li (alkali metal) and is the smallest for F (halogen).

[Note : Atomic radii of Li and F are 152 pm and 64 pm respectively.]