Valency

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 (2 electrons).
L-shell (n=2) can hold a maximum of .
M-shell (n=3) can hold a maximum of (though it often fills up to 8 before the N-shell starts filling for elements up to Calcium, according to the (2n ) 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 = 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.

Concept: To write the chemical formula of an ionic compound, we use the "criss-cross" method, where the numerical value of the valency (or charge) of one ion becomes the subscript of the other ion. The formula should represent the simplest whole number ratio of ions that results in a neutral compound. Step 1: Identify the valencies (or charges) of the element Z and the sulphate ion
Valency of element Z is given as 3. This means the ion formed by Z will likely be (assuming it's a metal or forms a cation).
The sulphate ion is a polyatomic ion with the formula . Its charge (and thus its valency in ionic compounds) is 2-. Step 2: Apply the criss-cross method Write the symbols of the ions with their valencies (ignoring the sign for now for subscript determination): Z with valency 3 with valency 2 Criss-cross the valencies:
The valency of Z (3) becomes the subscript for the group.
The valency of (2) becomes the subscript for Z. This gives the formula: . The parentheses around are important because the subscript 3 applies to the entire sulphate group. Step 3: Check if the formula is in its simplest ratio The subscripts are 2 and 3. These are already in their simplest whole number ratio (their GCD is 1). Step 4: Verify neutrality (optional but good practice) Charge from ions = . Charge from ions = . Total charge = . The compound is neutral. The formula for the compound formed by element Z (valency 3) and sulphate ions ( ) is . This matches option (3). The handwritten criss-cross on the image also indicates this.