Question

Name the element whose M-Shell has 2 electrons :

• A. Magnesium
• B. Sodium
• C. Aluminium
• D. Carbon

Hint:

Electron Shells and Filling:
K-shell (1st shell): max 2 electrons.
L-shell (2nd shell): max 8 electrons.
M-shell (3rd shell): starts filling after K and L are full. If M-shell has 2 electrons, it means: K-shell = 2 electrons L-shell = 8 electrons M-shell = 2 electrons Total electrons = \(2 + 8 + 2 = 12\). The element with atomic number 12 (and thus 12 electrons in a neutral atom) is Magnesium (Mg). Electronic configuration of Mg: 2, 8, 2.

Answer 1

The Correct Option is A

Concept: Electron shells (or energy levels) in an atom are designated by letters K, L, M, N, ... starting from the shell closest to the nucleus.
K-shell (n=1) can hold a maximum of \(2e^-\) (2 electrons).
L-shell (n=2) can hold a maximum of \(8e^-\).
M-shell (n=3) can hold a maximum of \(18e^-\) (though it often fills up to 8 before the N-shell starts filling for elements up to Calcium, according to the (2n\(^2\)) rule for capacity and the Bohr-Bury scheme for filling). For stability, an octet (8 electrons) or a duplet in the outermost shell (for K-shell) is preferred. Electrons fill these shells in order, starting from the K-shell. Step 1: Understand the filling order For an element to have electrons in the M-shell, its K and L shells must be completely filled first according to the general filling rules for elements up to atomic number 20. K-shell filled = 2 electrons L-shell filled = 8 electrons Step 2: Determine the total number of electrons If the M-shell has 2 electrons, and the K and L shells are full: Total electrons = (Electrons in K) + (Electrons in L) + (Electrons in M) Total electrons = \(2 + 8 + 2 = 12\) electrons. Step 3: Identify the element with this number of electrons For a neutral atom, the number of electrons is equal to the atomic number (number of protons). An element with 12 electrons has an atomic number of 12. The element with atomic number 12 is Magnesium (Mg). Its electronic configuration is 2, 8, 2. (K=2, L=8, M=2). Step 4: Analyze the options
(1) Magnesium (Mg): Atomic number 12. Electronic configuration: 2, 8, 2. (M-shell has 2 electrons). Correct.
(2) Sodium (Na): Atomic number 11. Electronic configuration: 2, 8, 1. (M-shell has 1 electron). Incorrect.
(3) Aluminium (Al): Atomic number 13. Electronic configuration: 2, 8, 3. (M-shell has 3 electrons). Incorrect.
(4) Carbon (C): Atomic number 6. Electronic configuration: 2, 4. (L-shell has 4 electrons, M-shell is empty). Incorrect. The handwritten "K L M, 2 8 2 = 12" and "Mg" on the image also indicates this. Therefore, Magnesium is the element whose M-shell has 2 electrons.