The mass number of a nucleus of an element is

Options

A. Always less than its atomic number
B. Always more than its atomic number
C. Sometimes equal to its atomic number
D. Sometimes more than and sometimes equal to its atomic number.

Answer

To determine the relationship between the mass number (A) and the atomic number (Z) of a nucleus of an element, we need to understand the definitions of these terms:

1. **Atomic Number (Z)**: This is the number of protons in the nucleus of an atom. It defines the element and its position in the periodic table.

2. **Mass Number (A)**: This is the total number of protons and neutrons (collectively called nucleons) in the nucleus. It can be expressed as:
\[ A = Z + N \]
where \(N\) is the number of neutrons.

Now, let's analyze the relationship between A and Z:

### Step 1: Understanding the relationship
- For any element, the mass number (A) can be equal to or greater than the atomic number (Z).
- The mass number is equal to the atomic number when there are no neutrons (as in the case of hydrogen-1, where A = 1 and Z = 1).
- The mass number is greater than the atomic number when there are neutrons present (as in the case of nitrogen-14, where A = 14 and Z = 7).

### Step 2: Examples
- **Hydrogen (H)**:
- Atomic Number (Z) = 1 (1 proton)
- Mass Number (A) = 1 (1 proton, 0 neutrons)

Here, A = Z.

- **Nitrogen (N)**:
- Atomic Number (Z) = 7 (7 protons)
- Mass Number (A) = 14 (7 protons + 7 neutrons)

Here, A > Z.

### Conclusion
- The mass number (A) can be equal to or greater than the atomic number (Z) but is never less than it. Therefore, the statement "the mass number is always less than its atomic number" is incorrect.

### Final Statement
The mass number of a nucleus of an element is either equal to or greater than its atomic number, depending on the number of neutrons present.

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