Question

Atomic size in a period:
(A) Increases
(B) Decreases
(C) Constant
(D) None of these

Answer 1

We know that the distance between the Centre of the nucleus of an atom to its outermost shell is known as the atomic radius or atomic size of that atom. In a molecule of the same element, the atomic radius is half the distance between the adjacent atoms. Helium will have the smallest atomic size while Francium will have the highest atomic size in the periodic table.

Complete answer: We know that on moving left to right across a period the atomic size of the element decreases as the valence electrons are in the same outermost shell also the atomic number increases on moving across the period which in turn also increase the effective nuclear charge and therefore increases the attractive force between the outermost shell and the nucleus of the atom thereby causing a decrease in the atomic size of the element.
Though there are some exceptions such as oxygen whose atomic size is slightly greater than the nitrogen atom. This is because nitrogen has a half filled $p -$orbital with a configuration of $1{s^2}2{p^3}$ which is why it is very stable and hence there is less repulsion between the electrons.

Therefore, the correct answer is Option B. i.e. decreases.

Note: The atomic size of the element increases on moving down a group in the periodic table because one more shell is added to the element which thus decreases the effective nuclear charge on the element. This will weaken the attractive force present between the nucleus and the outermost shell and will therefore increase the atomic size of the element.