Isotope of which noble gas is isotonic (same number of neutrons) with Tritium?

• When two elements have same number of neutrons, then it is said to be Isotonic.
• The number of neutrons equals to the difference between the mass number of the atom and the atomic number.
• Tritium is represented as ${}_1{}^3\mathrm{H}$.

Thus the number of neutrons in tritium would be = 3-1=2

Helium is represented as ${}_2{}^4\mathrm{He}$

Thus the number of neutrons in tritium would be = 3-1=2

Therefore, Isotope of which noble gas is isotonic (same number of neutrons) with Tritium is $\mathrm{He}$