The atomic numbers of elements `X,Y`,and `Z` are `19,21`, and `23`, respectively. The number of electron present in the `M` shells of these elements follows the order

Options

A. `Z gt X gt Y`
B. `X gt Y gt Z`
C. `X gt Y gt Z`
D. `Z gt Y gt X`

Explanation

To determine the number of electrons present in the M shell (n=3) for elements X, Y, and Z with atomic numbers 19, 21, and 23 respectively, we will follow these steps:

### Step 1: Determine the electronic configuration of each element.

1. **Element X (Atomic Number 19)**:
- The electronic configuration is:
\[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^1 \]

2. **Element Y (Atomic Number 21)**:
- The electronic configuration is:
\[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^1 \]

3. **Element Z (Atomic Number 23)**:
- The electronic configuration is:
\[ 1s^2 \, 2s^2 \, 2p^6 \, 3s^2 \, 3p^6 \, 4s^2 \, 3d^5 \]

### Step 2: Count the number of electrons in the M shell (n=3).

- The M shell corresponds to the principal quantum number \( n = 3 \), which includes the 3s and 3p subshells.

1. **For Element X**:
- Electrons in M shell = \( 3s^2 + 3p^6 = 2 + 6 = 8 \)

2. **For Element Y**:
- Electrons in M shell = \( 3s^2 + 3p^6 + 3d^1 = 2 + 6 + 1 = 9 \)

3. **For Element Z**:
- Electrons in M shell = \( 3s^2 + 3p^6 + 3d^5 = 2 + 6 + 5 = 13 \)

### Step 3: Arrange the number of electrons in M shell in order.

- From the calculations:
- Element X has 8 electrons in the M shell.
- Element Y has 9 electrons in the M shell.
- Element Z has 13 electrons in the M shell.

Thus, the order of the number of electrons in the M shell is:
\[ X < Y < Z \quad \text{(8 < 9 < 13)} \]

### Final Answer:
The number of electrons present in the M shells of elements X, Y, and Z follows the order: **X < Y < Z**.

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