The `71^(st)` electron of an element X with an atomic number of 71 enters into the orbital

Options

A. (a)4f
B. (b)6p
C. (c)6s
D. (d)5d

Explanation

To determine the orbital in which the 71st electron of element X (with atomic number 71) enters, we will follow the Aufbau principle and the n + l rule.

### Step-by-Step Solution:

1. **Identify the Atomic Number**:
The atomic number of element X is given as 71. This means it has 71 electrons.

2. **Understand the Aufbau Principle**:
According to the Aufbau principle, electrons fill atomic orbitals in order of increasing energy levels. The order is determined by the n + l rule, where n is the principal quantum number and l is the azimuthal quantum number.

3. **Calculate n + l for Different Orbitals**:
- For the 6s orbital: n = 6, l = 0 → n + l = 6 + 0 = 6
- For the 4f orbital: n = 4, l = 3 → n + l = 4 + 3 = 7
- For the 5d orbital: n = 5, l = 2 → n + l = 5 + 2 = 7
- For the 6p orbital: n = 6, l = 1 → n + l = 6 + 1 = 7

4. **Determine the Order of Filling**:
According to the n + l rule:
- The 6s orbital (n + l = 6) will fill first.
- Next, the 4f orbital (n + l = 7) will fill.
- After that, the 5d orbital (n + l = 7) will fill.
- Finally, the 6p orbital (n + l = 7) will fill.

5. **Fill the Orbitals**:
- 6s can hold 2 electrons: 6s²
- 4f can hold 14 electrons: 4f¹⁴
- 5d can hold 10 electrons: 5d¹⁰
- 6p can hold 6 electrons: 6p⁶

6. **Count the Electrons**:
- After filling 6s² (2 electrons) and 4f¹⁴ (14 electrons), we have filled 16 electrons.
- The next electrons will go into the 5d orbital. The 71st electron will be the first electron in the 5d orbital.

7. **Conclusion**:
Therefore, the 71st electron of element X enters the 5d orbital.

### Final Answer:
The 71st electron enters the **5d orbital**.

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