Q51P - Chemistry: Molecular Nature Of Matter And Change

Question

How can iodine (Z = 53) have a higher atomic number yet a lower atomic mass than tellurium (Z = 52)?

Step-by-Step Solution

Answer

Iodine has a lower atomic mass than tellurium because the most stable tellurium has two more neutrons compared to iodine.

Step1: Define an atom and atomic mass number

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.

Step 2: Find the number of neutrons present in each 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.