Question

Why does caesium \[(Cs)\] have a larger atomic radius than lithium \[(Li)\]?
(A) Atomic radius increases as you move from right to left and top to bottom
(B) Atomic radius increases as you go from left to right and bottom to top on a periodic table
(C) Both \[(Cs)\] and \[(Li)\] have the same atomic radius.
(D) It cannot be determined which element has the higher atomic radius

Answer 1

Hint: Atomic radius of a chemical element in the measure of the size of its atom. Usually, it is the distance of the nucleus to the outermost shell containing electrons. Both \[(Cs)\] and \[(Li)\] belong to group 1 and the atomic radii comparisons can be done by the regular trends we observe when we come from top to bottom in a group. The same is discussed below,

Complete answer:
In the question it is mentioned that Caesium \[(Cs)\] have a larger atomic radius than Lithium \[(Li)\]. Let us see the reason behind the statement. Firstly, let us understand what periodicity means?
Periodicity refers to trends or recurring variation in element properties (such as atomic radii, electronegativity etc) with increasing atomic number periodicity is caused by regular and predictable variations in element atomic structure. We should always remember that radius of any element in the periodic table is determined by two factors, listed below
The number of shells containing electrons around the nucleus
The attraction between the outer electrons and the nucleus
Now we know that in any group element, the outer electrons feel a net pull of 1+ from the nucleus however this positive charge on the nucleus is cut down due to the screening effect of inner electrons.
 Thus, the only factor which affects the size of the atom is the number of shells containing electrons around the nucleus. Now whenever we move from top to bottom in a group the number of shells of consecutive elements starts increasing. The increase in shell, increases the size of the element.
Since, in group $1$ Lithium \[(Li)\]is at the top, so it has least shell and as a result least size as compared to caesium \[(Cs)\] which is placed last in group and so it has largest number of shell and hence biggest size.

So, Option (A) is the correct Option.

Note:Some students might think atoms will get bigger if we add more shells and might conveniently ignore the other effect (the attraction between outer electrons and nucleus). This is true if we talk about atoms in the same group. But this is not true if we try to compare atoms from different parts of the periodic table.