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Neutrons, protons, and electrons are the three subatomic particles that make up an atom. The nucleus of an atom contains neutrons and protons, with electrons rotating around it. Electrons are placed in the shell (orbit) of atoms in order of increasing energy levels.

Atomic mass number: It is defined as the sum of the number of protons and neutrons in the nucleus of an atom.

The atomic number, Z of iodine is given as 53 and its mass number, A is 127. So, the number of neutrons, N is,

$$\begin{gathered} \mathbf{N}\mathbf{=}\mathbf{A}\mathbf{-}\mathbf{Z} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{127}\mathbf{-}\mathbf{53} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{74} \end{gathered}$$

The given atomic number of tellurium is 52 and its mass number, A is 128 (most stable isotope). So, the number of neutrons, N is,

$$\begin{gathered} \mathbf{N}\mathbf{=}\mathbf{A}\mathbf{-}\mathbf{Z} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{128}\mathbf{-}\mathbf{52} \\ \mathbf{}\mathbf{}\mathbf{}\mathbf{}\mathbf{=}\mathbf{76} \end{gathered}$$

The number of neutrons in iodine is less than that of tellurium. We knew that atomic mass equals the sum of the number of protons and neutrons. Hence, iodine has a lower atomic mass than tellurium.