Q is a semi-conductor. The chloride of Q reacts with water to form white fumes and an acidic solution. Which Period 3 element is Q?
A. magnesium
B. aluminium
C. silicon
D. phosphorus
▶️ Answer/Explanation
Ans: C
Silicon (Si) is the only semiconductor in Period 3. Its chloride, SiCl4, reacts vigorously with water, producing white fumes of HCl and an acidic solution (silicic acid). This matches the given properties, confirming Q as silicon.
What are the acid–base nature and structure of SO₂?
▶️ Answer/Explanation
Ans: B
Acid-Base Nature: SO₂ is acidic because it dissolves in water to form sulfurous acid (H₂SO₃). It acts as a Lewis acid by accepting electron pairs.
Structure: SO₂ has a bent (V-shaped) molecular geometry due to the lone pair of electrons on sulfur, which causes repulsion and reduces the bond angle from the ideal 120° (trigonal planar) to around 119°.
Thus, the correct combination is acidic and bent, making option B the right answer.
Q is a semi-conductor. The chloride of Q reacts with water to form white fumes and an acidic solution. Which Period 3 element is Q?
▶️ Answer/Explanation
Ans: C
1. **Semiconductor Property**: Silicon (Si) is the only Period 3 element that is a semiconductor (used in electronics). 2. **Chloride Reaction**: Silicon tetrachloride (SiCl4) reacts vigorously with water, producing white fumes of HCl and an acidic solution (H2SiO3). 3. **Elimination of Other Options**: – Magnesium (A) forms an ionic chloride (MgCl2), which dissolves without fumes. – Aluminium (B) forms AlCl3, which hydrolyzes but is not a semiconductor. – Phosphorus (D) forms PCl3/PCl5, but it is not a semiconductor. Thus, **Q must be silicon (C)**.
Elements X and Y are in Period 3 of the Periodic Table. Element X is either phosphorus or sulfur. Element Y is either sodium or magnesium. Element X forms an oxide that reacts with water to give a solution containing the aqueous anion \(XO_4^{2–}\). One mole of element Y reacts with one mole of chlorine molecules. At the end of the reaction, all of the element Y and all of the chlorine molecules have been used up. What are elements X and Y?
▶️ Answer/Explanation
Ans: D
Element X: The oxide of sulfur (\(SO_3\)) reacts with water to form \(SO_4^{2–}\) (sulfate ion), while phosphorus oxide (\(P_4O_{10}\)) forms \(PO_4^{3–}\). Since the given anion is \(XO_4^{2–}\), X must be sulfur (S).
Element Y: Sodium (Na) reacts with chlorine in a 2:1 mole ratio (\(2Na + Cl_2 \rightarrow 2NaCl\)), while magnesium (Mg) reacts in a 1:1 mole ratio (\(Mg + Cl_2 \rightarrow MgCl_2\)). The condition matches magnesium, so Y is magnesium (Mg).
Thus, the correct combination is S and Mg (Option D).