Question

Which of the following statements concerning protium, deuterium, and tritium is not true?
A.They are isotopes of each other
B.They have similar electron configurations
C.They exist in nature in the ratio $1:2:3$
D.Their mass numbers are in the ratio of $1:2:3$

Answer 1

Hint: Isotopes are atoms of the same element having the same atomic number but different mass numbers. This means that they have the same number of protons but differ in the number of neutrons. The atomic number is the number of protons in the nucleus and it is equal to the number of electrons in the neutral atom.

Complete step by step answer:
Hydrogen has 3 isotopes: protium, deuterium, and tritium. The atomic number (Z) of hydrogen is one and the mass number (A) is different for each isotope. Mass number is equal to the total number of nucleons. Hence, statement A is correct
Protium ($${}_1^{1}H$$) has Z $=$ 1 and A $=$ 1, deuterium (${}_1^{2}H$) has Z $=$ 1 and A $=$ 2, and tritium (${}_1^{3}H$) has Z $=$ 1 and A $=$ 3.
Electronic configuration is the distribution of electrons of an atom or a molecule in an atomic or molecular orbital. The three given isotopes have the same atomic number which is equal to one. Therefore the electronic configurations are similar ($1s^1$). Hence, statement B is correct.
The mass number of protium, deuterium, and tritium are 1, 2, and 3 respectively. Therefore, the ratio of their mass numbers is $1:2:3$. Hence, statement D is also correct.
99.985 percent of hydrogen atoms contain only one proton. Therefore, protium is the most abundant isotope of hydrogen followed by deuterium with 0.015 percentage and tritium with ${10}^{-6}$ percentage. Therefore, the ratio of their abundances is not equal to $1:2:3$. Hence, statement C is not true.

The answer is (C) They exist in nature in the ratio $1:2:3$.


Note:
Tritium is found only in trace amounts on the earth because it is formed by the interaction of the atmosphere with cosmic rays.
The chemical properties of atoms are controlled by the number of electrons, which are determined by the number of protons in the nucleus. The number of neutrons present in the nucleus doesn’t have much effect on the chemical properties. Hence, isotopes of the same element show similar chemical behaviour.