Question

The atom having smallest size in the list below is:
$Na,\text{ }Cl,\text{ }Si,\text{ }Ar$
(a)- Na
(b)- Cl
(c)- Si
(d)- Ar

Answer 1

When we move along the period the nuclear force of attraction increases because of the increase in the number of electrons and the electrons come closer to the nucleus. When we move down the group, the nuclear force of attraction decreases because the addition of shell and the electrons go far away from the nucleus.

Complete step by step answer:
We know that there are various properties of the elements along the period and the group. These properties have trends along the period and along with the group.
As we move along the period from left to right, the number of electrons increases within the same shell, so the nuclear force of attraction increases, and the electrons come closer to the nucleus, so the size of the atom decreases. But the noble gas of every period has the largest size because the shell of the noble gas is fully-filled due to which the nuclear force of attraction weakens and the size increases.
When we move down the group, the nuclear force of attraction decreases because the addition of shell and the electrons go far away from the nucleus, due to which the size increases.
The sodium element ($Na$) is placed in group 1, silicon element ($Si$) is placed in group 16, chlorine element ($Cl$), and argon element ($Ar$) is placed in group 18 and is a noble gas.
So, the smallest size will be of chlorine element ($Cl$).

Therefore, the correct answer is an option (b)- Cl.

Note: The variation of covalent radius, metallic radius, van der Waal radius, etc has the same variation as the atomic radius. The halogens in every period have the smallest size and the noble gas has the largest size.