Vidaara.orgClass 9 · Chemistry
CodeVID-C9-03-T2-01
Assignment — Atoms, Molecules, Ions & Chemical Formulae
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The symbol of sodium is:
- A.So
- B.Sd
- C.Na
- D.S
2.
A positively charged ion is a:
- A.anion
- B.cation
- C.molecule
- D.atom
3.
The valency of the sulphate ion SO42− is:
- A.1
- B.2
- C.3
- D.4
4.
The formula of aluminium oxide is:
- A.AlO
- B.Al₂O₃
- C.Al₃O₂
- D.AlO₂
5.
The atomicity of ozone (O3) is:
- A.1
- B.2
- C.3
- D.4
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define valency and atomicity with one example each.
7.
Write the formulae of (a) potassium oxide and (b) magnesium chloride.
8.
Distinguish between a cation and an anion with one example each.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Using the criss-cross method, write the formulae of calcium phosphate and ammonium sulphate.
10.
Name the elements and give the number of atoms of each in (a) CaCO₃ and (b) Al₂(SO₄)₃.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Explain the criss-cross method of writing chemical formulae, then use it to write the formulae of sodium sulphate, aluminium nitrate and calcium carbonate.
Answer Key
Section A — Multiple Choice Questions
- (C) Na
- (B) cation
- (B) 2
- (B) Al₂O₃
- (C) 3
Section B — Short Answer (2 marks)
- Valency is the combining capacity of an element or ion (e.g. oxygen has valency 2). Atomicity is the number of atoms in one molecule of an element (e.g. oxygen O₂ has atomicity 2).
- Potassium oxide: K₂O (K = 1, O = 2). Magnesium chloride: MgCl₂ (Mg = 2, Cl = 1).
- A cation is a positively charged ion formed by loss of electrons (e.g. Na⁺); an anion is a negatively charged ion formed by gain of electrons (e.g. Cl⁻).
Section C — Short Answer (3 marks)
- Calcium phosphate: Ca²⁺ and PO₄³⁻ give Ca₃(PO₄)₂. Ammonium sulphate: NH₄⁺ and SO₄²⁻ give (NH₄)₂SO₄.
- CaCO₃: 1 calcium, 1 carbon, 3 oxygen. Al₂(SO₄)₃: 2 aluminium, 3 sulphur, 12 oxygen.
Section D — Long Answer (5 marks)
- In the criss-cross method, write the symbols of the cation and anion side by side, place each one's valency above it, then cross over each valency so it becomes the subscript of the other, finally reducing to the simplest ratio and using brackets for more than one polyatomic ion. Sodium sulphate: Na⁺ (1) and SO₄²⁻ (2) give Na₂SO₄. Aluminium nitrate: Al³⁺ (3) and NO₃⁻ (1) give Al(NO₃)₃. Calcium carbonate: Ca²⁺ (2) and CO₃²⁻ (2) cross to Ca₂(CO₃)₂, which reduces to CaCO₃.
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