Explanation:
The number of possible diatomic molecules for three isotopes of hydrogen will be
\(\mathrm{{ }^{1} \mathrm{H}-{ }^{1} \mathrm{H},{ }^{1} \mathrm{H}-\mathrm{D},{ }^{1} \mathrm{H}-\mathrm{T}, \mathrm{D}-\mathrm{D}, \mathrm{D}-\mathrm{T}, \mathrm{T}-\mathrm{T}}\).