Question

Which amongst the following does not have neutrons in their atom?

• A. Protium
• B. Deuterium
• C. Tritium
• D. Helium

Answer 1

The correct option is A

Protium

Protium, deuterium and tritium are the isotopes of hydrogen.
Generally an element is represented as $AZX$, where A is the mass number and Z is the atomic number of the element X.
$\bullet$ The above isotopes are represented as:
Protium- $11H$, Deuterium- $21H$, Tritium - $31H$
$\bullet$ Also, mass number (A) = Atomic number (Z) + Number of neutrons

The number of electrons, protons and neutrons in each of the given atoms are given below.
$$\begin{array}{cccc}\text{Atom}& \text{Electrons}& \text{Protons}& \text{Neutrons (A-Z)}\\ \text{Protium}& \text{one}& \text{one}& \text{1-1=0}\\ \text{Deuterium}& \text{one}& \text{one}& \text{2-1=1}\\ \text{Tritium}& \text{one}& \text{one}& \text{3-1=2}\\ \text{Helium}& \text{two}& \text{two}& \text{4-2=2}\end{array}$$
For protium, number of neutrons is zero.
So, it is the only atom which does not contain a neutron.