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# How many of the following properties show a clear periodic variation along with a group ?a.) First ionisation energyb.) Molar mass of the elementc.) Number of isotopes of an elementd.) Atomic radius

Last updated date: 11th Sep 2024
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Hint: The answer to this question is the property which is constant in every situation and is determined by knowing the molecular formula. There is experimental evidence available for this property but theoretical results can also be obtained. We can also calculate that property theoretically.

Let us see all these properties one by one. Then, we will be able to get a clear picture of their periodic trend.
The first option given to us is the first ionisation energy. It is the amount of energy that is required to remove the first electron from an atom.
The first ionization energy shows some irregularities while moving down a group in the periodic table. In the starting elements of a family, the valence electron is very close to the nucleus. But later on, the addition of shells decreases nuclear force to some extent.
For ex - When we move down the group in group 13, there are many exceptions. So, this can not be the perfect answer.

The second option given is the molar mass of the element. The molar mass constantly increases in the group. So, it could be an answer.

The third option given is the number of isotopes of an element. This number could be any. It does not have any trend in the group.

The fourth property given is the atomic radius. The atomic radius increases when we move down a group in the periodic table. But still, there are some variations. So, this can also not be the correct answer.
So, the correct answer is “Option B”.

Note: It must be noted that although the atomic radius increases with increase in atomic number because every time there is an addition of a new shell. But in some elements that involve filling of f shell also, in such cases due to lesser shielding effect of f block the atomic radius may decrease also. So, this can not be the perfect answer.