Those atoms of an element having different number of neutrons are called __________

Explanation

**Step-by-Step Solution:**

1. **Understanding Atoms**: Atoms are the basic units of elements, and they consist of protons, neutrons, and electrons. The number of protons in an atom defines the element.

2. **Identifying Atomic Number and Mass Number**:
- The atomic number (Z) is the number of protons in the nucleus of an atom.
- The mass number (A) is the total number of protons and neutrons in the nucleus.

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