Which of the following has an equal number of neutrons and protons?

Options

A. hydrogen
B. deuterium
C. fluorine
D. chlorine

Answer

To determine which of the given options has an equal number of neutrons and protons, we will analyze each element one by one.

### Step-by-Step Solution:

1. **Understanding the Concepts**:
- The atomic number (Z) of an element represents the number of protons in the nucleus.
- The mass number (A) of an element is the sum of the number of protons and neutrons in the nucleus.
- The number of neutrons can be calculated using the formula:
\[ \text{Number of neutrons} = A - Z \]

2. **Analyzing Hydrogen (H)**:
- Atomic number (Z) = 1
- Mass number (A) = 1
- Number of neutrons = A - Z = 1 - 1 = 0
- Protons = 1, Neutrons = 0 → Not equal.

3. **Analyzing Deuterium (D)**:
- Atomic number (Z) = 1
- Mass number (A) = 2
- Number of neutrons = A - Z = 2 - 1 = 1
- Protons = 1, Neutrons = 1 → Equal.

4. **Analyzing Fluorine (F)**:
- Atomic number (Z) = 9
- Mass number (A) = 19
- Number of neutrons = A - Z = 19 - 9 = 10
- Protons = 9, Neutrons = 10 → Not equal.

5. **Analyzing Chlorine (Cl)**:
- Atomic number (Z) = 17
- Mass number (A) = 35
- Number of neutrons = A - Z = 35 - 17 = 18
- Protons = 17, Neutrons = 18 → Not equal.

6. **Conclusion**:
- The only element with an equal number of neutrons and protons is Deuterium (D).

### Final Answer:
**Deuterium (D)** has an equal number of neutrons and protons.

---