Question

The biggest atom is:
A. radium
B. Francium
C. Caesium
D. Rhodium

Answer 1

Hint: As per the general trend of Periodic table, atomic sizes of elements increases as one moves downwards in the Periodic table of the elements while when one moves from left to right across the Periodic table, the atomic sizes decreases. Knowing which atoms belong to which group and period will help you to answer this question. It is the most electropositive element.

Complete step by step solution:
To answer this question, we have to know in what group and period these elements tend to belong in the Periodic table.
So, first of all we should write the configuration of each element so that we can draw a conclusion to which period and group they belong.
So, electronic configuration of Radium is $[Rn]7{{s}^{2}}$, which mean it belongs to second group and seventh period, while that of Francium is $[Rn]7{{s}^{1}}$meaning it belongs to first group and seventh period whereas, the electronic configuration of Caesium is $[Xe]6{{s}^{1}}$means it belongs to first group and sixth period and that of Rhodium is $[Kr]4{{d}^{8}}5{{s}^{1}}$means it belongs to ninth group and fifth period.
As the general trend of Periodic table says,
When one moves downward along a group in the Periodic table, the atomic size of elements increases, however, as one moves from left to right across the Periodic table, the atomic size of the elements decreases.
So, here the order of atomic size of the given elements based on their position will be,
$Francium>Caesium>Radium>Rhodium$
But, due to the highly unstable and radioactivity property of Francium, the award goes to Caesium.

Hence, the correct option is C.

Note: Nuclear charge increases as you go from left to right in the Periodic table while it decreases when one moves downwards in the Periodic table. This tends to increase or decrease the atomic size of the elements. By this logic you can get confused by Francium being the biggest atom but Francium is radioactive and is highly unstable. Hence, Caesium is known as the biggest atom.