The mass number of a nucleus is.

Options

A. always less then it atomic number
B. always more than its atomic number
C. sometimes equal to its atomic number
D. sometimes more than 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. It is represented as:
\[ A = Z + N \]
where:
- \( A \) is the mass number,
- \( Z \) is the atomic number (number of protons),
- \( N \) is the number of neutrons.

### Step 2: Analyze the Representation of Nuclei
Nuclei are often represented in the form \( \, ^A_ZX \), where:
- \( X \) is the chemical symbol of the element,
- \( Z \) is the atomic number (number of protons),
- \( A \) is the mass number (total number of nucleons).

### Step 3: Identify the Relationship Between Mass Number and Atomic Number
From the definition, we can conclude:
- The mass number \( A \) is always greater than or equal to the atomic number \( Z \) because it includes both protons and neutrons.
- In some cases, especially for isotopes, the mass number can equal the atomic number (e.g., in the case of hydrogen-1, where \( A = 1 \) and \( Z = 1 \)).

### Step 4: Consider Isotopes
For elements with isotopes:
- Hydrogen has three isotopes:
- \( ^1_1H \) (Protium, 1 proton, 0 neutrons, \( A = 1 \)),
- \( ^2_1H \) (Deuterium, 1 proton, 1 neutron, \( A = 2 \)),
- \( ^3_1H \) (Tritium, 1 proton, 2 neutrons, \( A = 3 \)).

In these cases, the mass number can be equal to or greater than the atomic number.

### Step 5: Conclusion
Thus, the mass number of a nucleus can sometimes be equal to its atomic number and sometimes greater than its atomic number.

### Final Answer
The mass number of a nucleus is sometimes equal to its atomic number and sometimes greater than it.