Answer:
It is present in third period that's why its valance electrons are present in 3rd energy level.
Its atomic number is greater than lithium when compared in group wise.
There are more electrons in sodium to shield the outer valance electron thus nuclear attraction becomes weak and size increase.
Explanation:
The size of sodium is greater than lithium because atomic number of sodium is 11 and lithium is 3. Both are present in first group but sodium is present down to the lithium. As we move from top to bottom in a group atomic size increases with addition of electrons. The nuclear effect become weaker on valance electrons and atomic size increase. Same time shielding effect is also produces which shield the outer electrons from the influence of nucleus. While in case of lithium less electrons are present to shield the valance electrons.
As we note the position of both elements along period. The sodium is present in third period while lithium is present in second period. So, in case of sodium third energy level is involved. That's why its size is greater than lithium.
