The mass number of a nucleus is equal to

Options

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

Answer

To solve the question regarding the mass number of a nucleus, we can follow these steps:

### Step 1: Understand the Definition of Mass Number
The mass number (A) of a nucleus is defined as the total number of protons and neutrons present in that nucleus.

### Step 2: Identify the Components of Mass Number
- **Protons**: The number of protons in a nucleus is also known as the atomic number (Z).
- **Neutrons**: The number of neutrons can vary even for atoms of the same element (isotopes).

### Step 3: Write the Formula for Mass Number
The formula for mass number can be expressed as:
\[ A = Z + N \]
where:
- \( A \) = mass number
- \( Z \) = number of protons (atomic number)
- \( N \) = number of neutrons

### Step 4: Analyze the Relationship Between Mass Number and Atomic Number
From the formula, we can see that:
- The mass number (A) is always equal to or greater than the atomic number (Z) because neutrons (N) are always non-negative.
- For hydrogen (which has an atomic number of 1 and typically 0 neutrons), the mass number is equal to the atomic number.

### Step 5: Conclusion Based on Options
Given the relationship derived, we conclude that:
- The mass number can be equal to the atomic number (as in the case of hydrogen) or greater than it (for other elements).
- Therefore, the correct option states that the mass number is sometimes equal to and sometimes greater than the atomic number.

### Final Answer
The mass number of a nucleus is equal to the number of protons plus the number of neutrons, and it can be sometimes equal to and sometimes greater than the atomic number.