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CBSE · Class 9 · Science

Journey Inside the Atom

Standard difficulty · 20 questions · 40 marks

Name: Date: Score: ____ / 40

Show your working for Sections B and C. Answers are on the last page — try every question first!

Section A — Choose the correct answer8 × 1 = 8 marks
1.
How many protons are there in an atom of 126C?
  1. 18
  2. 12
  3. 13
  4. 6
2.
The valency of calcium (Z = 20) is:
  1. 2
  2. 0
  3. 1
  4. 3
3.
The electron was discovered by:
  1. E. Goldstein
  2. J. Chadwick
  3. J. J. Thomson
  4. E. Rutherford
4.
Which pair are isotopes?
  1. 126C and 136C
  2. 147N and 2210Ne
  3. 11H and 146C
  4. 2412Mg and 4018Ar
5.
How many electrons are there in the ion Al3+? (Z of Al = 13)
  1. 10
  2. 16
  3. 13
  4. 11
6.
A sample of neon has 53% atoms of mass 20 u and 47% of mass 22 u. Its average atomic mass is:
  1. 42 u
  2. 21 u
  3. 21.06 u
  4. 20.94 u
7.
An atom has 3 protons, 7 neutrons and 3 electrons. Its mass number is:
  1. 10
  2. 13
  3. 3
  4. 7
8.
Assertion (A): An atom of 136C has 7 neutrons.
Reason (R): Mass number = number of protons + number of electrons.
  1. Both A and R are true, and R is the correct explanation of A.
  2. Both A and R are true, but R is not the correct explanation of A.
  3. A is true, but R is false.
  4. A is false, but R is true.
Section B — Short answer8 × 2 = 16 marks
9.
Hydrogen has atomic number 1 and mass number 1. Find the numbers of protons, neutrons and electrons in its atom, and write its symbol with A and Z.
10.
Write the distribution of electrons in the shells of magnesium (Z = 12). What is its valency?
11.
The atom 4020Ca forms the ion Ca2+. Find the numbers of protons, neutrons and electrons in the ion, and its electronic configuration.
12.
What were the main conclusions of Rutherford's alpha-particle scattering experiment?
13.
Silver has two isotopes of mass 107 u and 109 u. In a sample, 59% of the atoms are silver-107. Find the average atomic mass.
14.
The average atomic mass of a sample of boron is 10.44 u. It contains only boron-10 and boron-11. What percentage of the atoms is boron-10?
15.
Are these pairs isotopes, isobars or neither? (a) 157N and 147N (b) 2010Ne and 2210Ne
16.
An element has the electronic configuration 2, 8, 8. What is its atomic number and name? What is its valency?
Section C — Word problems4 × 4 = 16 marks
17.
Describe Rutherford's alpha-particle scattering experiment and its observations.
18.
Complete the table for beryllium (94Be), silicon (2814Si), chlorine (3517Cl): number of protons, neutrons and electrons, electronic configuration and valency.
19.
An element X has three isotopes: mass 58 u (81%), 59 u (11%) and 60 u (8%). (a) Find its average atomic mass. (b) Why is the atomic mass of an element often not a whole number? (c) Do the isotopes have the same chemical properties? Why?
20.
Compare an atom of 199F with its ion F−: (a) protons, (b) neutrons, (c) electrons, (d) electronic configuration, (e) which noble gas the ion's configuration matches, (f) the charge and why.
AmbifulWorksheets
Practice that never repeats
CBSE · Class 9 · Science

Answer key

Journey Inside the Atom · AW-9S-STA-2026-V1 · For parents — fold back or tear off before handing the sheet to your child.

QAnswerWorking
1(d) 6Z = 6, A = 12
2(a) 2configuration 2, 8, 8, 2
3(c) J. J. Thomson—
4(a) 126C and 136Csame atomic number, different mass numbers
5(a) 10lost 3
6(d) 20.94 u20 × 53/100 + 22 × 47/100
7(a) 10mass number = protons + neutrons
8(c) A is true, but R is false.A is true: neutrons = 13 − 6 = 7. R is false.
9Protons 1, neutrons 0, electrons 1; 11Hneutrons = A − Z
102, 8, 2; valency 2outer shell has 2 electrons
11Protons 20, neutrons 20, electrons 18; 2, 8, 8the atom loses 2 electrons
12Most of the space inside the atom is empty; there is a small, dense, positively charged centre (the nucleus) where almost all the mass is; the nucleus is very small compared with the atom.—
13107.82 u107 × 59/100 + 109 × 41/100
1456% (and 44% boron-11)10x + 11(100 − x) = 1,044
15(a) isotopes; (b) isotopesisotopes: same Z; isobars: same A, different Z
16Z = 18, argon; valency 0total electrons = 2 + 8 + 8
17Fast-moving alpha particles were aimed at a very thin gold foil. Most passed straight through; some were deflected by small angles; about one in 12,000 bounced back. He concluded that the atom is mostly empty space, with a tiny, dense, positive nucleus in which almost all its mass is concentrated, and electrons revolving around it.—
18Beryllium: p 4, n 5, e 4; 2, 2; valency 2. Silicon: p 14, n 14, e 14; 2, 8, 4; valency 4. Chlorine: p 17, n 18, e 17; 2, 8, 7; valency 1—
19(a) 58.27 u. (b) It is the average of the masses of its isotopes, weighted by how common each is. (c) Yes: they have the same number of electrons and the same electronic configuration.58 × 81/100 + 59 × 11/100 + 60 × 8/100
20(a) 9 and 9. (b) 10 and 10. (c) 9 and 10. (d) 2, 7 and 2, 8. (e) neon. (f) −1: the atom gains 1 electron, so protons and electrons no longer balance.—