Question

The element with the largest atomic size from the elements of period 1, 2 and 3 is:
A. Li
B. Na
C. F
D. Cl

Answer 1

Hint: To answer this question, you must recall the trend of change in the atomic radius of an atom in a periodic table. As we move from left to right in a period, the atomic size decreases and when we move down the group, an increase in the size is observed.

Complete step by step answer:
- In order to explain the behaviour of an atom or ion or molecule, we must have an idea about its size. Atomic radius is the most common way of expressing the size of an atom. It can be termed as half of the distance between the nuclei of two identical atoms bonded together.
- When we move from left to right in a period, protons and electrons are added to the same energy level. As the nuclear charge increases, the pull on the electrons increases and thus the size decreases. But ultimately in the last elements of the period, the size increases due to inter electronic repulsions. As we move down the group, the number of shells keep on increasing and thus the size of the atom increases.
- Thus, we can conclude that in each period the first element has the largest size. The largest elements in group 1, 2 and 3 are helium, lithium and sodium respectively. Out of these elements, sodium is the largest in size due to the presence of three shells whereas lithium has two and hydrogen has one shell only.

Thus, the correct answer is B.

Note:
The trend in the atomic radius is also affected by several other factors. Shielding effect of the inner shells tend to increase the size of the atoms and if the shielding is poor, like in the cases of shells containing d and f orbitals, the atomic size doesn’t increase significantly and may also decrease in some cases.