Question

In a period, an atom with smaller radius is:
(A) Chalcogen
(B) Halogen
(C) Aerogen
(D) Pnictogen

Answer 1

When we move from left to right in a period, the atomic radii decreases. Find the groups to which the given elements belong. Use the concept of effective nuclear charge to answer the question but do not forget to consider inter-electronic repulsion in case of fulfilled outermost orbital.

Complete step by step answer:
We know that the periodic table is a tabular display of chemical elements found in nature. There are 118 elements in the periodic table. They are arranged according to their atomic number, electronic configuration and repetitive chemical properties.
Periodic table has seven horizontal rows and 18 vertical columns. The horizontal rows are called period and the vertical columns are called groups.
As we move from left to right, the atomic radius decreases. This is due to the increasing effective nuclear charge.
We can describe an effective nuclear charge as the total charge faced by an electron in a poly-electronic atom. We use the symbol $Z^* \ or \ Z_{eff}$ to denote the effective nuclear charge on an electron. Across a period, shielding effect remains constant but effective nuclear charge increases. Due to this higher effective nuclear charge, electrons face higher attraction and thus cause the atomic radii to shrink.
To answer the given question, we have to find out the group which is towards the right side in the periodic table.
Firstly we have chalcogens. It is the name given to the group 16 elements i.e. oxygen group.
Then we have halogens. It is the name given to the group 17 elements i.e. fluorine group.
Next we have aerogenes. It is the name given to the group 18 elements i.e. the noble gases.
Lastly, we have pnictogens. It is the name given to the group 15 elements i.e. nitrogen group.
We can see that the aerogenes lie towards the extreme right so they should have the smallest atomic radii.
However, it is seen that halogens have a smaller atomic radii than aerogenes. This is due to the fact that in aerogenes, the outermost orbital is already fulfilled. Thus, it has high interelectronic repulsion due to which they overcome the attraction force of the nucleus and manage to get a larger atomic radius than the halogens.
Therefore, the correct answer here is option (B) Halogen.

Note: We can also answer this question on the basis of types of radii. We have Van der Waals radii, metallic radii and ionic radii. We can find the radii of an atom by measuring the distance between the nuclei of the two touching atoms and then taking its half. Noble gases don’t form covalent or ionic bonds thus they are believed to have Van der Waal radius. This is calculated experimentally and is found to be larger than metallic or ionic radii.