Question

In the third period of the periodic table, the element having the smallest size is:
A. Na
B. Ar
C. Cl
D. Si

Answer 1

Hint: Atomic size or atomic radius is the distance from the nucleus to the outermost electrons. It is found that Helium is the smallest element and Francium is the largest element.

Complete step by step solution:
- Atomic radius varies in a way across the periodic table. Generally, it is found that the atomic radius increases from top to bottom of the periodic table.
- And it decreases from left to right across a period. Hence, we can see that in the third period, Argon (Ar) is present in the last period of the periodic table, which has the smallest atomic radius.
- From left to right across a period, basically more number of protons are added to the nucleus, but the electrons are added to the valence shell, and not to the lower energy levels.
- What happens, here is that as the protons are added to the nucleus, the electrons present in the valence shell feel a higher effective nuclear charge, which is the sum of the charges on the inner, core electrons and charges on the protons in the nucleus.
- Therefore, the valence shell electrons are held more tightly and hence, the size of the atom decreases across a period.

Hence, we can say that the correct option is (B), that is in the third period of the periodic table, the element having smallest size is Ar.

Note: There are several factors that affect the atomic radius of an atom like electron shells tend to increase the atomic radius, nuclear charge tends to decrease atomic radius and shielding effect tend to increase the atomic radius.