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Question

The maximum number of electrons that can be accommodated in M shell is:

This question was previously asked in
RRB ALP 2018 CBT 2 Fitter Question Paper (21-Jan-2019) (Shift 3)
The correct answer is

18

Understanding Electron Shells and Maximum Capacity

In an atom, electrons occupy specific energy levels or shells around the nucleus. These shells are designated by letters starting from the one closest to the nucleus: K, L, M, N, and so on. Each shell corresponds to a principal quantum number, denoted by 'n'.

  • K shell: n = 1
  • L shell: n = 2
  • M shell: n = 3
  • N shell: n = 4

Calculating Maximum Electrons in a Shell

The maximum number of electrons that can be accommodated in a principal shell with quantum number 'n' is given by the formula:

\begin{equation*} \text{Maximum electrons} = 2n^2 \end{equation*}

Applying the Formula to the M Shell

The question asks for the maximum number of electrons in the M shell. As established, the M shell corresponds to the principal quantum number n = 3.

Using the formula, we substitute n = 3:

\begin{equation*} \text{Maximum electrons in M shell} = 2 \times (3)^2 \end{equation*}

\begin{equation*} = 2 \times 9 \end{equation*}

\begin{equation*} = 18 \end{equation*}

Therefore, the maximum number of electrons that can be accommodated in the M shell is 18.

Analyzing the Options

Let's look at the given options based on our calculation:

  • 8: This is the maximum capacity of the L shell (n=2, $2 \times 2^2 = 8$).
  • 2: This is the maximum capacity of the K shell (n=1, $2 \times 1^2 = 2$).
  • 18: This matches our calculation for the M shell (n=3, $2 \times 3^2 = 18$).
  • 32: This is the maximum capacity of the N shell (n=4, $2 \times 4^2 = 32$).

Based on the formula and calculation, the correct number for the M shell is 18.

Maximum Electron Capacity of First Few Shells
Shell Principal Quantum Number (n) Maximum Electrons ($2n^2$)
K 1 $2 \times 1^2 = 2$
L 2 $2 \times 2^2 = 8$
M 3 $2 \times 3^2 = 18$
N 4 $2 \times 4^2 = 32$

Revision Table: Electron Shells and Capacity

Shell Name n Value Maximum e- Capacity ($2n^2$)
K 1 2
L 2 8
M 3 18
N 4 32

Additional Information: Subshells within the M Shell

While the maximum capacity of the M shell is 18 electrons, these electrons are distributed among subshells within the M shell. The M shell (n=3) contains three types of subshells:

  • s subshell (l=0): Can hold a maximum of 2 electrons.
  • p subshell (l=1): Can hold a maximum of 6 electrons.
  • d subshell (l=2): Can hold a maximum of 10 electrons.

The total maximum capacity of the M shell is the sum of the capacities of its subshells: 2 (s) + 6 (p) + 10 (d) = 18 electrons. This distribution aligns with the $2n^2$ formula for the principal shell.

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