Compound X contains two elements, Y and Z. Element Y is in Period 2 of the Periodic Table. In one atom of element Y, the p sub-shell has all three orbitals occupied; only one of these three orbitals is fully occupied. Element Z is in Period 3 of the Periodic Table. In one atom of element Z, the p sub-shell has only two orbitals occupied. What is the formula of compound X?
▶️ Answer/Explanation
Ans: C
Element Y is in Period 2 with three p-orbitals occupied, but only one fully occupied (2 electrons). This gives Y an electron configuration of \(1s^2 2s^2 2p^2\), which is Carbon (C). Element Z is in Period 3 with two p-orbitals occupied, giving an electron configuration of \(1s^2 2s^2 2p^6 3s^2 3p^2\), which is Silicon (Si). The stable compound formed between Carbon and Silicon is SiO₂ (Silicon dioxide), as Carbon does not form a stable binary compound with Silicon, and the other options do not fit the given conditions.
Which species contains a different number of electrons from the other three?
▶️ Answer/Explanation
Ans: D
To determine the electron count:
- A: \(ClO_4^–\) has \(17 (Cl) + 4 \times 8 (O) + 1 (charge) = 50\) electrons.
- B: \(H_2SO_4\) has \(2 (H) + 16 (S) + 4 \times 8 (O) = 50\) electrons.
- C: \(SO_4^{2–}\) has \(16 (S) + 4 \times 8 (O) + 2 (charge) = 50\) electrons.
- D: \(Te^{2–}\) has \(52 (Te) + 2 (charge) = 54\) electrons.
Thus, D (\(Te^{2–}\)) has a different electron count (54) compared to the other three (50).
Compound X contains two elements, Y and Z. Element Y is in Period 2 of the Periodic Table. In one atom of element Y, the p sub-shell has all three orbitals occupied; only one of these three orbitals is fully occupied. Element Z is in Period 3 of the Periodic Table. In one atom of element Z, the p sub-shell has only two orbitals occupied. What is the formula of compound X?
▶️ Answer/Explanation
Ans: C
Element Y (Period 2) has three p-orbitals occupied, with only one fully occupied. This means its electron configuration is \(1s^2 2s^2 2p^4\), identifying Y as Oxygen (O). Element Z (Period 3) has two p-orbitals occupied, giving the configuration \(1s^2 2s^2 2p^6 3s^2 3p^2\), which is Silicon (Si). The stable compound formed between Si and O is \(SiO_2\), as Si shares its 4 valence electrons with two O atoms, each needing 2 electrons to complete their octet.
In this question Q is used to represent a halogen atom. Magnesium and calcium each form a compound with chlorine and a compound with bromine. One of these compounds contains:
- the element in Group 2 with the higher first ionisation energy and
- the element in Group 17 with the higher Q–Q bond energy.
What is the formula of this compound?
▶️ Answer/Explanation
Ans: A
In Group 2, magnesium (\(Mg\)) has a higher first ionisation energy than calcium (\(Ca\)) because ionisation energy decreases down the group. In Group 17, chlorine (\(Cl\)) has a higher \(Q-Q\) bond energy than bromine (\(Br\)) due to smaller atomic size and stronger bonding. Thus, the correct compound is \(MgCl_2\), as it satisfies both conditions.