Question

Given below is a part of the periodic table

H							He
Li	Be	B	C	N	O	F
Na
K
Rb
Cs

Arrange the given elements in order of decreasing size in the (i) Period (ii) Groups.

Answer 1

• The periodic trend of the size of element-

As we move down the group of a periodic table, an extra energy shell is added for electrons which leads to an increase in the size of the atom of an element.

As we move from left to right in a periodic table, the size of an atom decreases because of an addition of an extra proton which leads to a stronger pull of electrons while an extra electron goes to the same energy shell.

• (i) Periods:

Period 1 has H and He. As we move from left to right, size decreases, so the order of decreasing size is H > He.

Similarly, for period 2, the order of decreasing size is Li > Be > B > C > N > O > F

• (ii) Groups:

For group 1, the size increases on moving down the group.

Thus, the order of decreasing size is: Cs > Rb > K > Na > Li > H

• Hence, considering trends of both periods and groups, the given elements are arranged in decreasing size as-

Cs > Rb > K > Na > Li > Be > B > C > N > O > F > H > He