Q38P - Chemistry: Molecular Nature Of Matter And Change

Question

Even though several elements have only one naturally occurring isotope and atomic nuclei have whole numbers of protons and neutrons, no atomic mass is a whole number. Use the data from Table 2.2 to explain this fact.

Step-by-Step Solution

Answer

The atomic mass of several elements is not in the whole number because atomic mass is a sum of the mass of protons and neutrons, which are not the whole number that is why their sum also comes as a fraction.

Step 1: Isotopes

The isotopes of an element are atoms that have different mass numbers due to a different number of neutrons.

Step 2: Mass of proton and neutron

$$\begin{gathered} \mathbf{Mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{proton}\mathbf{=}\mathbf{1}\mathbf{.}\mathbf{00727}\mathbf{ }\mathbf{amu} \\ \mathbf{Mass}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{neuron}\mathbf{ }\mathbf{=}\mathbf{ }\mathbf{1}\mathbf{.}\mathbf{00866}\mathbf{ }\mathbf{amu} \end{gathered}$$

Let us take the example of oxygen to explain why mass is not a whole number of elements.

$$\begin{gathered} \mathbf{Number}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{proton}\mathbf{ }\mathbf{in}\mathbf{ }\mathbf{Oxygen}\mathbf{=}\mathbf{8} \\ \mathbf{Number}\mathbf{ }\mathbf{of}\mathbf{ }\mathbf{neutrons}\mathbf{ }\mathbf{in}\mathbf{ }\mathbf{Oxygen}\mathbf{=}\mathbf{8} \\ \mathbf{8}\mathbf{\times }\mathbf{1}\mathbf{.}\mathbf{00727}\mathbf{ }\mathbf{amu}\mathbf{+}\mathbf{8}\mathbf{\times }\mathbf{1}\mathbf{.}\mathbf{00866}\mathbf{ }\mathbf{amu} \\ \mathbf{=}\mathbf{16}\mathbf{.}\mathbf{12744}\mathbf{ }\mathbf{amu} \end{gathered}$$

Result:

Hence the atomic mass of oxygen comes out be fractions, as the mass of the protons and neutron is fractioned, so their sum is also a fraction.