Vidaara.orgClass 9 · Chemistry
CodeVID-C9-04-T3-01
Assignment — Valency, Atomic Number, Mass Number & Isotopes
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. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
The atomic number of an element is the number of:
- A.neutrons
- B.protons
- C.nucleons
- D.shells
2.
Mass number A is equal to:
- A.protons − neutrons
- B.protons + electrons
- C.protons + neutrons
- D.neutrons only
3.
Atoms with the same atomic number but different mass numbers are called:
- A.isobars
- B.isotopes
- C.ions
- D.isomers
4.
The valency of a noble gas such as neon is:
- A.0
- B.1
- C.2
- D.8
5.
The isotope used as fuel in nuclear reactors is:
- A.Iodine-131
- B.Cobalt-60
- C.Uranium-235
- D.Carbon-12
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
Define valence electrons and valency.
7.
An atom is written as 4020Ca. Find its protons, electrons and neutrons.
8.
State one use each of Cobalt-60 and Iodine-131.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
Distinguish between isotopes and isobars with one example each.
10.
Find the valencies of nitrogen (Z = 7) and aluminium (Z = 13) from their configurations.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Define atomic number and mass number. For the atom 3717Cl, find protons, electrons, neutrons and valency. Then explain why the atomic mass of chlorine is 35.5 u and not a whole number.
Answer Key
Section A — Multiple Choice Questions
- (B) protons
- (C) protons + neutrons
- (B) isotopes
- (A) 0
- (C) Uranium-235
Section B — Short Answer (2 marks)
- Valence electrons are the electrons in the outermost shell of an atom; valency is the combining capacity, i.e. the number of electrons an atom loses, gains or shares to complete its outermost shell.
- Protons = Z = 20; electrons = 20 (neutral atom); neutrons = A − Z = 40 − 20 = 20.
- Cobalt-60 is used in radiotherapy to treat cancer; Iodine-131 is used to treat goitre and diagnose thyroid disorders.
Section C — Short Answer (3 marks)
- Isotopes are atoms of the same element with the same atomic number but different mass numbers, e.g. ¹H, ²H, ³H. Isobars are atoms of different elements with the same mass number but different atomic numbers, e.g. ⁴⁰Ca (Z=20) and ⁴⁰Ar (Z=18).
- Nitrogen (2, 5): outermost has 5 electrons, valency = 8 − 5 = 3. Aluminium (2, 8, 3): outermost has 3 electrons, valency = 3.
Section D — Long Answer (5 marks)
- Atomic number (Z) is the number of protons in the nucleus; mass number (A) is the total number of protons and neutrons. For ³⁷Cl: protons = 17, electrons = 17, neutrons = A − Z = 37 − 17 = 20. Configuration 2, 8, 7 gives valency = 8 − 7 = 1. The atomic mass of chlorine is 35.5 u because natural chlorine is a mixture of about 75% ³⁵Cl and 25% ³⁷Cl; the listed atomic mass is the weighted average of these isotopes, (0.75 × 35) + (0.25 × 37) = 35.5 u, which is a fraction.
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