Question

If A is the mass number and Z is the atomic number of an element, then the number of neutrons are

• A. A+Z
• B. Z-A
• C. A-Z
• D. A/Z

Answer 1

The correct option is C

A-Z

The correct option is (C).

Explanation of correct option:

1. The sum of the number of neutrons and protons in an atom of the element is called a mass number of an atom.
2. The number of protons present in the nucleus of an atom of an element is known as its atomic number.
3. Hence, the number of protons is equal to the atomic number.
4. A is the mass number and Z is the atomic number of an element.
5. Mass number (A)= Number of Proton + Number of Neutron
6. From this, we can say that;

$$\begin{gathered} \mathrm{Mass}\mathrm{number}\left(\mathrm{A}\right)=\mathrm{Atomic}\mathrm{number}\left(\mathrm{Z}\right)+\mathrm{Number}\mathrm{of}\mathrm{Neutrons} \\ \mathrm{Number}\mathrm{of}\mathrm{Neutrons}=\mathrm{Mass}\mathrm{number}\left(\mathrm{A}\right)-\mathrm{Atomic}\mathrm{number}\left(\mathrm{Z}\right) \end{gathered}$$

Explanation of incorrect option:

Option (A): This is an incorrect option as Mass number (A) is always equal to the sum of a number of protons and number of neutrons.

Option (B): This is also an incorrect option. Because the number of neutrons is always calculated with the subtraction of atomic number (Z) from the mass number (A).

Option (D): A/Z is also an incorrect option.

Thus, If A is the mass number and Z is the atomic number of an element, then the number of neutrons is A-Z Mass number (A) - Atomic number (Z).