GRADE-9|SCIENCE| CHAPTER-8|10 WORKSHEETS (CBSE)

📚 CBSE & NCERT-ALIGNED QUESTION PATTERNS These 10 comprehensive worksheets are designed around the current competency-based assessment approach followed in CBSE and reflected in the evolving NCERT examination pattern. The questions go beyond simple recall and focus on understanding, application, analysis, interpretation, critical thinking and reasoning. 📝 Question Patterns Included: 🔹 Source-Based Questions – Questions based on extracts, passages, quotations or provided source material. 🔹 Case-Based Questions – Real-life situations and contexts that require learners to apply concepts and analyse information. 🔹 Image-Based Questions – Visual prompts, illustrations and pictures that test observation, interpretation and conceptual understanding. 🔹 Assertion–Reasoning Questions – Questions that develop logical thinking by examining the relationship between an assertion and its reason. 🔹 Multiple-Choice Questions (MCQs) – Carefully framed questions with meaningful distractors that assess conceptual understanding rather than random guessing. 🔹 True or False Statements – Statement-based questions that test accuracy, comprehension and conceptual clarity. 🔹 Data Interpretation Questions – Questions based on charts, tables, graphs or other data that develop analytical and interpretative skills. 🔹 Statement-Based Questions – Identification of correct and incorrect statements with appropriate reasoning. 🔹 Competency-Based Questions – Application-oriented tasks that connect learning with practical and meaningful contexts. 🔹 Higher-Order Thinking Questions (HOTS) – Questions that encourage students to analyse, evaluate, infer, justify and think beyond the textbook. These worksheets are designed to provide students with varied question formats, helping them prepare for modern competency-based assessments instead of relying only on traditional question patterns. Which question pattern do you find most useful for your students? 👇 Leave your comment below!

🧪 Journey Inside the Atom — Worksheet Bank

EnglishWithMRK · Science Resource Series

Journey Inside the Atom

NCERT Grade 9 · Exploration · Chapter 8

10 Practice Worksheets · 10 Marks Each · Aligned to NEP 2020 Competency-Based Assessment

Question formats: MCQs · True/False · Assertion–Reason · Source-Based · Image-Based · Case-Based & Data Interpretation

10 Worksheets

100 Total Marks

6 Question Formats

Teacher Answer Keys Included

📑 Worksheet Index

1. Ancient Atomic Theory to Dalton's Atomic Theory
2. J. J. Thomson & the Cathode Ray Experiment
3. The Gold Foil Experiment & Rutherford's Model
4. Limitations of Rutherford's Model & Discovery of the Proton
5. Bohr's Model of the Atom
6. Discovery of the Neutron & Subatomic Particles
7. Symbols of Elements
8. Atomic Number & Mass Number
9. Electronic Configuration & Valency
10. Isotopes, Isobars & Average Atomic Mass

EnglishWithMRK · Science

Worksheet 1: Ancient Atomic Theory to Dalton's Theory

NCERT Grade 9 · Chapter 8 · Section 8.1

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) Who among the following suggested that matter, when divided repeatedly, leads to the smallest indivisible particle called parmanu?

1. (i) Democritus
2. (ii) Acharya Kanada
3. (iii) John Dalton
4. (iv) Leucippus

(b) In which year did John Dalton propose his atomic theory based on scientific experiments?

1. (i) 1897
2. (ii) 1913
3. (iii) 1808
4. (iv) 1932

True/False Q2. State True or False2 Marks (1×2)

(a) The Greek word atomos means "divisible into smaller parts." [True / False]

(b) Dalton's atomic theory became the starting point for the current understanding of atomic structure. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): The concept of the atom originated as a scientific discovery.
Reason (R): Acharya Kanada and the Greek philosophers Leucippus and Democritus based their ideas on direct experimental observation.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is false, but R is false too, since the idea was imaginary, not experimental.
4. (iv) A is true, but R is false.

Source-Based Q4. Read the passage and answer2 Marks

"Acharya Kanada suggested that if matter (dravya) is divided repeatedly, you will reach a stage where you would encounter the smallest particles that can no longer be divided. He called these particles parmanus... Combinations of these forms dyads (groups of two parmanus) and triads (groups of three parmanus)... However, this description does not specify the proportions in which parmanus combine to form different substances."

(i) What did Acharya Kanada call the smallest, indivisible particles of matter? (1)

(ii) What was one limitation of Kanada's description of matter? (1)

Case-Based / Data Interpretation Q5. Study the table and answer2 Marks

Thinker(s)	Civilisation	Term Used	Basis of Idea
Acharya Kanada	Ancient India	Parmanu	Imagination / Philosophy
Leucippus & Democritus	Ancient Greece	Atomos	Imagination / Philosophy
John Dalton	1808, England	Atom	Scientific Experiments

(i) Based on the table, which thinker's idea was the first to be based on scientific experimentation rather than imagination? (1)

(ii) Name one similarity between the Indian and Greek concepts of matter shown in the table. (1)

Answer Key — Worksheet 1

1. (a) (ii) Acharya Kanada   (b) (iii) 1808
2. (a) False — atomos means "indivisible."   (b) True
3. (iv) A is true, but R is false — the concept of the atom originated as an imaginary idea, not from experimental observation.
4. (i) Parmanus.   (ii) It did not specify the proportions in which parmanus combine to form different substances.
5. (i) John Dalton, since his theory (1808) was based on scientific experiments.   (ii) Both civilisations independently proposed that matter, when divided repeatedly, reaches an indivisible smallest particle (parmanu / atomos).

EnglishWithMRK · Science

Worksheet 2: J. J. Thomson & the Cathode Ray Experiment

NCERT Grade 9 · Chapter 8 · Sections 8.2, 8.2.1

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) Cathode rays, studied by J. J. Thomson, are streams of:

1. (i) Protons
2. (ii) Neutrons
3. (iii) Negatively charged particles
4. (iv) Positively charged particles

(b) Thomson's model of the atom is popularly compared to:

1. (i) A cricket ground
2. (ii) A plum pudding
3. (iii) The solar system
4. (iv) A cloud

True/False Q2. State True or False2 Marks (1×2)

(a) The charge on an electron is conventionally taken as +1. [True / False]

(b) The nature of cathode rays was found to be independent of the cathode material and the gas used in the tube. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): Thomson proposed that an atom is a sphere of positive charge with electrons distributed throughout it.
Reason (R): This model successfully explained the large-angle scattering observed in the gold foil experiment.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Study the diagram of the cathode ray tube2 Marks

(i) Label X and Y: In the diagram, rays travel from electrode X to electrode Y. Identify X and Y. (1)

(ii) What conclusion did Thomson draw about the nature of these rays after studying them in electric and magnetic fields? (1)

Case-Based Q5. Read and answer2 Marks

"J. J. Thomson's most significant discovery was that of the electron, the first subatomic particle to be identified... He received the Nobel Prize in Physics in 1906 for his studies of the electrical conductivity of gases... As the head of the famous Cavendish Laboratory in Cambridge, he guided and inspired many scientists, including Ernest Rutherford."

(i) In which year did Thomson receive the Nobel Prize, and for what work? (1)

(ii) Name the famous laboratory Thomson headed, and one scientist he inspired. (1)

Answer Key — Worksheet 2

1. (a) (iii) Negatively charged particles   (b) (ii) A plum pudding
2. (a) False — the charge of an electron is taken as –1.   (b) True
3. (iii) A is true, but R is false — Thomson's model actually failed to explain the gold foil scattering results.
4. (i) X = Cathode (negative electrode), Y = Anode (positive electrode).   (ii) He concluded that cathode rays are streams of negatively charged particles (electrons) with much smaller mass than atoms.
5. (i) 1906, for his studies of the electrical conductivity of gases.   (ii) Cavendish Laboratory, Cambridge; he inspired Ernest Rutherford (among others).

EnglishWithMRK · Science

Worksheet 3: The Gold Foil Experiment & Rutherford's Model

NCERT Grade 9 · Chapter 8 · Sections 8.2.2, 8.2.2(A)

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) The gold foil experiment was performed by Geiger and Marsden working under:

1. (i) J. J. Thomson
2. (ii) Niels Bohr
3. (iii) Ernest Rutherford
4. (iv) James Chadwick

(b) The deflection of alpha particles from their straight-line path is called:

1. (i) Radiation
2. (ii) Ionisation
3. (iii) Scattering
4. (iv) Refraction

True/False Q2. State True or False2 Marks (1×2)

(a) According to Rutherford's model, most of an atom is empty space. [True / False]

(b) The nucleus of an atom is about 10⁵ times larger than the atom itself. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): Most alpha particles passed straight through the gold foil without any deflection.
Reason (R): Most of the volume of an atom is empty space.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Study the schematic of the gold foil experiment2 Marks

(i) What does the straight, undeflected beam (passing through undisturbed) tell us about the structure of the atom? (1)

(ii) What does the sharply bent / bounced-back path tell us about the nucleus? (1)

Data Interpretation Q5. Study the data and answer2 Marks

Quantity	Approximate Value
Diameter of an atom	≈ 10⁻¹⁰ m
Diameter of the nucleus	≈ 10⁻¹⁵ m

(i) Using the data, how many times smaller is the nucleus compared to the whole atom? (1)

(ii) The textbook compares the atom to a cricket ground (100 m across). What object is the nucleus compared to, at that scale? (1)

Answer Key — Worksheet 3

1. (a) (iii) Ernest Rutherford   (b) (iii) Scattering
2. (a) True   (b) False — the nucleus is about 10⁵ times smaller than the atom, not larger.
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) It shows that most of the atom is empty space, so alpha particles pass through undisturbed.   (ii) It shows that the positive charge and most of the mass of the atom are concentrated in a tiny, dense region called the nucleus.
5. (i) 10⁵ (one lakh) times smaller.   (ii) A tiny black pepper grain (a few mm) at the centre of the ground.

EnglishWithMRK · Science

Worksheet 4: Limitations of Rutherford's Model & Discovery of the Proton

NCERT Grade 9 · Chapter 8 · Sections 8.2.2(B), 8.2.2(C)

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) The positively charged subatomic particle discovered and named by Rutherford is the:

1. (i) Neutron
2. (ii) Electron
3. (iii) Proton
4. (iv) Positron

(b) Rutherford's atomic model could not satisfactorily explain the:

1. (i) Existence of electrons
2. (ii) Stability of the atom
3. (iii) Existence of the nucleus
4. (iv) Scattering of alpha particles

True/False Q2. State True or False2 Marks (1×2)

(a) A charged particle moving in a circular path is constantly changing direction and is therefore accelerating. [True / False]

(b) For an atom to be electrically neutral, its number of protons must equal its number of electrons. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): According to classical physics, an electron revolving around the nucleus should eventually spiral into it.
Reason (R): An accelerating charged particle continuously loses energy.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Study the diagram2 Marks

(i) What does the spiral path of the electron in this diagram represent? (1)

(ii) If this really happened in nature, what would be the consequence for all matter? Why does this NOT match reality? (1)

Case-Based Q5. Study the data and answer2 Marks

Atom	Protons	Electrons
Helium	2	2
Sodium	11	11

(i) Why are both helium and sodium atoms electrically neutral despite having a different number of protons? (1)

(ii) An atom has 15 protons. How many electrons must it have to remain neutral? (1)

Answer Key — Worksheet 4

1. (a) (iii) Proton   (b) (ii) Stability of the atom
2. (a) True   (b) True
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) It represents an electron losing energy as it accelerates around the nucleus, causing it to spiral inward.   (ii) All atoms would collapse and matter would not exist; this does not match reality because atoms are observed to be stable.
5. (i) In both, the total positive charge (protons) equals the total negative charge (electrons), so the net charge is zero.   (ii) 15 electrons.

EnglishWithMRK · Science

Worksheet 5: Bohr's Model of the Atom

NCERT Grade 9 · Chapter 8 · Section 8.2.3

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) In Bohr's model, the fixed circular paths of electrons are also called:

1. (i) Nucleons
2. (ii) Energy levels
3. (iii) Isotopes
4. (iv) Isobars

(b) Which shell is closest to the nucleus and has the least energy?

1. (i) L-shell
2. (ii) M-shell
3. (iii) K-shell
4. (iv) N-shell

True/False Q2. State True or False2 Marks (1×2)

(a) According to Bohr, an electron loses energy continuously while moving within a fixed shell. [True / False]

(b) An electron can move to another shell by absorbing or releasing a fixed amount of energy. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): Bohr's model successfully explained why atoms are stable.
Reason (R): Bohr proposed that electrons revolve in fixed "stationary states" without losing energy while in a given shell.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Study Bohr's atomic model diagram2 Marks

(i) Name the four shells labelled in the diagram, in order from nucleus outward. (1)

(ii) Which shell shown has the highest energy, and why? (1)

Source-Based Q5. Read and answer2 Marks

"Why are Bohr's shells called K, L, M, N... and not, A, B, C, D? The naming came from early X-ray experiments by the physicist, Charles Barkla, who called the first observed X-ray line K. He didn't start naming from A to leave room for possible discovery of a series earlier than the K series, although none were ever found. Bohr adopted the same notation for atomic shells."

(i) Who first used the letter 'K' to name an observed line, and in what type of experiment? (1)

(ii) Why did Barkla not begin naming from the letter 'A'? (1)

Answer Key — Worksheet 5

1. (a) (ii) Energy levels   (b) (iii) K-shell
2. (a) False — electrons do not lose energy while moving in a fixed shell (stationary state).   (b) True
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) K, L, M, N (from nucleus outward).   (ii) The N-shell (outermost shown), because energy increases as shells get farther from the nucleus.
5. (i) Charles Barkla, in early X-ray experiments.   (ii) To leave room in case a series earlier than K was discovered later (though none ever was).

EnglishWithMRK · Science

Worksheet 6: Discovery of the Neutron & Subatomic Particles

NCERT Grade 9 · Chapter 8 · Section 8.3, 8.3.1

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) The neutron was discovered in 1932 by:

1. (i) J. J. Thomson
2. (ii) Ernest Rutherford
3. (iii) James Chadwick
4. (iv) Niels Bohr

(b) The relative charge of a neutron is:

1. (i) +1
2. (ii) –1
3. (iii) 0
4. (iv) +2

True/False Q2. State True or False2 Marks (1×2)

(a) Neutrons are found in the nuclei of all atoms, including the most common hydrogen isotope. [True / False]

(b) As atoms get heavier, their nuclei generally contain many more neutrons than protons. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): The mass of a helium atom is about four times that of a hydrogen atom, not double.
Reason (R): A helium nucleus contains two protons and two neutrons, while a hydrogen nucleus (protium) contains only one proton and no neutron.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Data Interpretation Q4. Complete Table 8.1 (Subatomic Particles)2 Marks

S.No.	Particle	Symbol	Relative Charge
1	Electron	e⁻	____ (i)
2	Proton	____ (ii)	+1
3	Neutron	n⁰	0

Fill in the blanks (i) and (ii) above. (2)

Case-Based Q5. Read and answer2 Marks

"Lighter atoms often have an equal number of protons and neutrons (like carbon with six each). However, as atoms get heavier, their nuclei have many more neutrons than protons. Iron has 26 protons and 30 neutrons, and by the time we reach uranium, the nucleus has 92 protons and 146 neutrons... Neutrons being neutral help reduce this repulsion... by strengthening the force, called the nuclear force, that binds all particles together."

(i) Why do heavier atoms need many more neutrons to stay bound together? (1)

(ii) Calculate the difference between the number of neutrons and protons in a uranium nucleus. (1)

Answer Key — Worksheet 6

1. (a) (iii) James Chadwick   (b) (iii) 0
2. (a) False — neutrons are found in the nucleus of all atoms except hydrogen (protium).   (b) True
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) –1   (ii) p⁺
5. (i) Neutrons help reduce the repulsion between positively charged protons and strengthen the nuclear force that binds particles together, so heavier atoms with more protons need extra neutrons for stability.   (ii) 146 − 92 = 54 more neutrons than protons.

EnglishWithMRK · Science

Worksheet 7: Symbols of Elements

NCERT Grade 9 · Chapter 8 · Section 8.4

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) Who introduced the first pictorial symbols to represent elements, in 1803?

1. (i) Berzelius
2. (ii) John Dalton
3. (iii) Niels Bohr
4. (iv) IUPAC

(b) The chemical symbol for iron, "Fe", is derived from its Latin name:

1. (i) Aurum
2. (ii) Ferrum
3. (iii) Argentum
4. (iv) Plumbum

True/False Q2. State True or False2 Marks (1×2)

(a) Berzelius suggested in 1813 that element symbols should be derived from their Latin names. [True / False]

(b) The second letter of a chemical symbol, if present, is always written as a capital letter. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): The symbol for tungsten is "W".
Reason (R): "W" is derived from the German name for tungsten, "wolfram".

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Identify Dalton's pictorial symbols2 Marks

These are Dalton's 1803 pictorial symbols for Hydrogen, Carbon, Oxygen, and Phosphorus (in some order). (i) Match each symbol shape (i)-(iv) to its element using the textbook figure 8.9. (1)

(ii) Why did scientists eventually replace pictorial symbols with alphabetic (letter-based) symbols? (1)

Data Interpretation Q5. Study the table and answer2 Marks

Element	Symbol	Latin/Greek/German Name
Gold	Au	Aurum (Latin)
Sodium	Na	Natrium (Latin)
Mercury	Hg	Hydrargyros (Greek)
Potassium	K	Kalium (Latin)

(i) Which element's symbol comes from a Greek name, according to the table? (1)

(ii) What organisation today approves the official names and symbols of elements? (1)

Answer Key — Worksheet 7

1. (a) (ii) John Dalton   (b) (ii) Ferrum
2. (a) True   (b) False — the second letter is written in lowercase (small letter).
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) (i) Hydrogen — dot; (ii) Carbon — shaded circle; (iii) Oxygen — open circle; (iv) Phosphorus — triangle (as per Fig. 8.9).   (ii) Pictorial symbols were not standard or easy to reproduce/communicate internationally; alphabetic symbols (from Latin/Greek/German names) allowed clear, universal scientific communication.
5. (i) Mercury (Hg, from Hydrargyros).   (ii) The International Union of Pure and Applied Chemistry (IUPAC).

EnglishWithMRK · Science

Worksheet 8: Atomic Number & Mass Number

NCERT Grade 9 · Chapter 8 · Sections 8.5, 8.6

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) The number of protons in the nucleus of an atom is known as its:

1. (i) Mass number
2. (ii) Atomic number
3. (iii) Valency
4. (iv) Valence number

(b) Mass number (A) of an atom is equal to:

1. (i) Protons − Neutrons
2. (ii) Protons + Neutrons
3. (iii) Protons + Electrons
4. (iv) Neutrons − Electrons

True/False Q2. State True or False2 Marks (1×2)

(a) Protons and neutrons present in the nucleus are together called nucleons. [True / False]

(b) Electrons contribute significantly to the mass number of an atom. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): The mass number of a lithium atom is 7.
Reason (R): A lithium atom's nucleus contains 3 protons and 4 neutrons.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Study the lithium atom diagram2 Marks

(i) From the diagram, how many protons, neutrons, and electrons does this atom of lithium have? (1)

(ii) Determine the atomic number and mass number of this atom. (1)

Data Interpretation Q5. Standard atomic notation2 Marks

The standard notation for carbon is written as: ¹²₆C (mass number 12, atomic number 6).

(i) Using this notation style, if an atom is written as ³¹₁₅P, state its atomic number, mass number, and number of neutrons. (1)

(ii) An atom has atomic number 11 and mass number 23. How many neutrons does it have? (1)

Answer Key — Worksheet 8

1. (a) (ii) Atomic number   (b) (ii) Protons + Neutrons
2. (a) True   (b) False — the electron's mass is negligible and ignored in mass number calculations.
3. (i) Both A and R are true, and R is the correct explanation of A. (3 protons + 4 neutrons = mass number 7)
4. (i) 3 protons, 4 neutrons, 3 electrons.   (ii) Atomic number = 3, Mass number = 7.
5. (i) Atomic number = 15, Mass number = 31, Neutrons = 31 − 15 = 16.   (ii) Neutrons = 23 − 11 = 12.

EnglishWithMRK · Science

Worksheet 9: Electronic Configuration & Valency

NCERT Grade 9 · Chapter 8 · Sections 8.7, 8.7.1, 8.8

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) The maximum number of electrons that a shell can hold is given by the formula:

1. (i) n²
2. (ii) 2n
3. (iii) 2n²
4. (iv) n/2

(b) Carbon has the electronic configuration 2, 4. Its valency is:

1. (i) 2
2. (ii) 4
3. (iii) 6
4. (iv) 8

True/False Q2. State True or False2 Marks (1×2)

(a) The outermost (valence) shell of an atom can hold a maximum of 8 electrons. [True / False]

(b) Elements with a complete octet of valence electrons are usually highly reactive. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): Sodium (electronic configuration 2, 8, 1) has a valency of 1.
Reason (R): Sodium can lose its single valence electron to attain a stable octet configuration.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Identify the elements from their shell diagrams2 Marks

(i) Using the electron distributions shown (a: 2,8 and b: 2,8,1), name the elements represented. (1)

(ii) State the valency of each atom shown. (1)

Data Interpretation Q5. Study the table and answer2 Marks

Element	K	L	M	Valence electrons
Magnesium (Mg)	2	8	2	?
Sulfur (S)	2	8	6	?
Argon (Ar)	2	8	8	?

(i) State the valency of magnesium and sulfur based on their configurations. (1)

(ii) Which of the three elements is likely to be the most chemically unreactive, and why? (1)

Answer Key — Worksheet 9

1. (a) (iii) 2n²   (b) (ii) 4
2. (a) True   (b) False — elements with a complete octet are largely unreactive and stable.
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) (a) Neon (2, 8); (b) Sodium (2, 8, 1).   (ii) Neon: valency 0 (already stable/octet complete); Sodium: valency 1 (loses 1 electron).
5. (i) Magnesium: valency 2 (loses 2 electrons); Sulfur: valency 2 (gains 2 electrons to complete octet).   (ii) Argon, because its outermost (M) shell already has a complete octet of 8 electrons, making it stable and largely unreactive.

EnglishWithMRK · Science

Worksheet 10: Isotopes, Isobars & Average Atomic Mass

NCERT Grade 9 · Chapter 8 · Section 8.9

10

Marks

Name: ____________________

Class/Sec: _______

Date: _______

Time: 20 min

Read each question carefully. Attempt all questions. Marks are indicated against each question.

MCQ Q1. Multiple Choice Questions2 Marks (1×2)

(a) Atoms of the same element having the same atomic number but different mass numbers are called:

1. (i) Isobars
2. (ii) Isotopes
3. (iii) Isomers
4. (iv) Ions

(b) Which carbon isotope is used in archaeology to date fossils and artefacts?

1. (i) Carbon-12
2. (ii) Carbon-13
3. (iii) Carbon-14
4. (iv) Carbon-16

True/False Q2. State True or False2 Marks (1×2)

(a) Isobars are atoms of different elements with the same atomic number but different mass numbers. [True / False]

(b) Isotopes of the same element have the same chemical properties but different physical properties. [True / False]

Assertion–Reason Q3.2 Marks

Assertion (A): Calcium (Z=20), potassium (Z=19), and argon (Z=18) are examples of isobars.
Reason (R): All three have the same mass number of 40, but different atomic numbers.

1. (i) Both A and R are true, and R is the correct explanation of A.
2. (ii) Both A and R are true, but R is not the correct explanation of A.
3. (iii) A is true, but R is false.
4. (iv) A is false, but R is true.

Image-Based Q4. Identify the isotopes of hydrogen2 Marks

(i) Name the three hydrogen isotopes shown as (i), (ii), and (iii): (protium, deuterium, tritium). (1)

(ii) How many neutrons does the isotope in diagram (iii) have? (1)

Data Interpretation Q5. Calculate the weighted average atomic mass2 Marks

Isotope	Mass (u)	Abundance
Bromine-79 (⁷⁹Br)	79	49.7%
Bromine-81 (⁸¹Br)	81	50.3%

(i) Using the formula for weighted average atomic mass, calculate the average atomic mass of bromine. Show your working. (1.5)

(ii) Is the calculated value closer to the mass of ⁷⁹Br or ⁸¹Br? Why? (0.5)

Answer Key — Worksheet 10

1. (a) (ii) Isotopes   (b) (iii) Carbon-14
2. (a) False — isobars have the same mass number but different atomic numbers.   (b) True
3. (i) Both A and R are true, and R is the correct explanation of A.
4. (i) (i) Protium; (ii) Deuterium; (iii) Tritium.   (ii) 2 neutrons.
5. (i) Average atomic mass = (79 × 49.7/100) + (81 × 50.3/100) = 39.263 + 40.743 = ≈ 80.0 u.   (ii) It lies almost midway between the two, very slightly closer to ⁸¹Br since it has a marginally higher abundance (50.3% vs 49.7%).

© Journey Inside the Atom — Worksheet Bank · Prepared for EnglishWithMRK Science Resources · Based on NCERT Grade 9 Exploration, Chapter 8 · Aligned to NEP 2020 competency-based assessment guidelines.