Question

Fill in the blanks
The biggest atom is ________.
A. Radium
B. Radon
C. Caesium
D. Promethium

Answer 1

Hint: Size of an atom depends on the radius of the atom, which is the mean distance from the center of the nucleus to the outer shell. And the size order we can find from the periodic table order size.

Step by step solution:
We know that Radium(Ra) belongs to 2nd group and 7th period in s-block, Radon(Rn) belongs to 6th period and 18th group from p-block, caesium(Cs) belongs to 1st group and 6th period from s-group and promethium(Pm) belongs to Lanthanides and 6th period from f-block.
And as the general trend of periodic table atomic sizes increases as one moves downwards in the Periodic Table of the Elements, as electrons fill outer electron shells.
Atomic radius decreases, however, as one moves from left to right, across the Periodic Table.
As the atomic number increases along each row of the periodic table, the additional electrons go into the same outermost shell, causing the atomic radius to decrease due to the increasing nuclear charge.
When moving down a group of the periodic table, the atomic radius increases because of the presence of additional principal energy levels.
So, the order size of Ra, Rn, Cs and Pm, by their position in: Ra > Cs > Pm > Rn
Then the correct option is “A”.

Note: Order of size also depends on the charge on that element. Atomic radius of an atom is different when that atom is in neutral form or in charged state. Order of radius: anion > neutral > cation.