The element with electronic configuration `[Rn] 5f^2 6d^1 7s^2` has atomic number

Options

A. 90
B. 91
C. 95
D. 100

Explanation

To determine the atomic number of the element with the electronic configuration `[Rn] 5f^2 6d^1 7s^2`, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Base Configuration**:
The given electronic configuration starts with `[Rn]`, which stands for Radon. Radon is a noble gas with an atomic number of 86.

2. **Count the Electrons in the Given Configuration**:
The configuration `[Rn] 5f^2 6d^1 7s^2` indicates the following:
- `5f^2`: This means there are 2 electrons in the 5f subshell.
- `6d^1`: This means there is 1 electron in the 6d subshell.
- `7s^2`: This means there are 2 electrons in the 7s subshell.

3. **Calculate the Total Number of Electrons**:
To find the total number of electrons, we add the electrons from Radon and the additional electrons from the given configuration:
\[ \text{Total Electrons} = \text{Electrons from Radon} + \text{Electrons in 5f} + \text{Electrons in 6d} + \text{Electrons in 7s} \]
\[ \text{Total Electrons} = 86 + 2 + 1 + 2 = 91 \]

4. **Determine the Atomic Number**:
The atomic number of an element is equal to the number of protons, which is equal to the number of electrons in a neutral atom. Therefore, the atomic number of the element is 91.

5. **Identify the Element**:
The element with atomic number 91 is Protactinium (Pa).

### Final Answer:
The atomic number of the element with the electronic configuration `[Rn] 5f^2 6d^1 7s^2` is **91**.

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