class 9 science chapter 8 notes and book solutions

Class 9 Science Chapter 8 – Journey Inside the Atom | StudyCBSE

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Part 1: Short Concept Notes

1. What Is an Atom?

An atom is a tiny building block of matter. Atoms contain three main subatomic particles:

Proton: Positive charge (+1), found in the nucleus.
Neutron: No charge (0), found in the nucleus.
Electron: Negative charge (−1), found around the nucleus.

Almost all the mass of an atom is concentrated in its nucleus. Most of the atom is empty space.

2. Early Ideas About Atoms

Acharya Kanada: Suggested that matter is made of tiny particles called parmanus.
Leucippus and Democritus: Proposed indivisible particles called atomos.
John Dalton (1808): Developed an atomic theory based on scientific evidence and described atoms as indivisible particles.

Later experiments showed that atoms contain smaller particles.

3. Development of Atomic Models

Diagram comparing Dalton, Thomson, Rutherford and Bohr atomic models — Evolution of atomic models. Image source.

Scientist	Model and main idea	Limitation
John Dalton	Atom is an indivisible particle.	Could not explain subatomic particles.
J. J. Thomson	Electrons are embedded in a positively charged sphere.	Could not explain the gold foil experiment.
Ernest Rutherford	A tiny, positive nucleus is surrounded by electrons; most of the atom is empty space.	Could not explain atomic stability.
Niels Bohr	Electrons occupy fixed energy levels or shells.	Does not fully explain all atoms.
James Chadwick	Discovered the neutron in 1932.	His discovery added a missing particle to the atomic picture.

4. Discovery of the Electron

In 1897, J. J. Thomson studied cathode rays and discovered negatively charged particles called electrons.

Since electrons are present in atoms of different elements, atoms are divisible and contain smaller particles.

Thomson’s model: The atom was pictured as a positively charged sphere with negative electrons embedded in it, like seeds in a watermelon or plums in a pudding.

5. Rutherford’s Gold Foil Experiment

Rutherford and his team directed positively charged alpha particles at a thin gold foil.

Most particles passed straight through: most of the atom is empty space.
Some particles were deflected: positive charge is concentrated in a small region.
A very few bounced back: the nucleus is extremely small, dense and contains most of the atom’s mass.

Conclusion: Rutherford proposed the nuclear or planetary model of the atom.

6. Limitation of Rutherford’s Model

Rutherford’s model could not explain why atoms remain stable.

A moving electron should lose energy according to the classical explanation and spiral towards the nucleus. Since real atoms are stable, another model was needed.

7. Bohr’s Model of the Atom

Niels Bohr proposed his model in 1913.

Electrons occupy fixed energy levels called shells.
Shells are named K, L, M, N, and so on.
They can also be numbered as n = 1, 2, 3, 4, and so on.
Electrons do not lose energy while remaining in an allowed shell.
Electrons absorb or release a fixed amount of energy when moving between energy levels.

The K-shell is closest to the nucleus and has the lowest energy.

8. Protons and Neutrons

Proton: Positive charge, relative charge +1.
Neutron: No electric charge, relative charge 0.
Electron: Negative charge, relative charge −1.

Protons and neutrons are present in the nucleus. They are called nucleons.

James Chadwick discovered the neutron in 1932. Neutrons contribute to atomic mass and help the nucleus remain bound.

Particle	Symbol	Relative charge
Electron	e⁻	−1
Proton	p⁺	+1
Neutron	n⁰	0

9. Symbols of Elements

Chemical symbols are internationally recognised abbreviations for elements.

The first letter is always capital: H, C, O.
If there is a second letter, it is lowercase: He, Al, Cl.
Some symbols come from Latin or other historical names.

Element	Symbol	Element	Symbol
Hydrogen	H	Oxygen	O
Carbon	C	Nitrogen	N
Sodium	Na	Potassium	K
Iron	Fe	Copper	Cu
Calcium	Ca	Magnesium	Mg
Chlorine	Cl	Aluminium	Al
Silver	Ag	Gold	Au
Zinc	Zn	Lead	Pb

10. Atomic Number and Mass Number

Atomic number (Z): Number of protons in the nucleus.

Atomic number = Number of protons

For a neutral atom:

Number of electrons = Number of protons

Mass number (A): Total number of protons and neutrons.

Mass number = Protons + Neutrons

Neutrons = Mass number − Atomic number

Example: Sodium-23 has 11 protons, 11 electrons and 12 neutrons.

Atomic notation is written as ^A_ZX, where X is the element symbol, A is the mass number and Z is the atomic number.

11. Electronic Configuration

Electrons occupy shells around the nucleus.

Maximum capacity of a shell = 2n².
K-shell: maximum 2 electrons.
L-shell: maximum 8 electrons.
M-shell: maximum 18 electrons by the 2n² rule.
For the first 18 elements, electrons are arranged as 2, 8, and then up to 8 in the third shell.

Element	Atomic number	Electronic configuration
Hydrogen	1	1
Helium	2	2
Carbon	6	2, 4
Oxygen	8	2, 6
Sodium	11	2, 8, 1
Magnesium	12	2, 8, 2
Chlorine	17	2, 8, 7
Argon	18	2, 8, 8

12. Valence Electrons and Valency

Valence shell: The outermost occupied shell.
Valence electrons: Electrons in the outermost shell.
Valency: Combining capacity of an atom; it is generally related to the electrons lost, gained or shared to reach a stable configuration.

Element	Configuration	Valence electrons	Common valency
Sodium	2, 8, 1	1	1
Magnesium	2, 8, 2	2	2
Carbon	2, 4	4	4
Oxygen	2, 6	6	2
Chlorine	2, 8, 7	7	1
Neon	2, 8	8	0

Atoms with complete outer shells are generally stable and less reactive.

13. Isotopes

Isotopes are atoms of the same element that have the same atomic number but different mass numbers.

They have the same number of protons.
Neutral isotopes have the same number of electrons.
They have different numbers of neutrons.
Their chemical properties are generally similar.

Examples: Hydrogen-1, hydrogen-2 and hydrogen-3; carbon-12, carbon-13 and carbon-14.

Uses: Uranium-235 in nuclear fuel, cobalt-60 in some cancer treatments, iodine-131 in thyroid treatment, and carbon-14 for dating ancient organic materials.

14. Average Atomic Mass

The average atomic mass takes into account the masses of naturally occurring isotopes and their relative abundances.

Average atomic mass = Σ (isotope mass × fractional abundance)

For chlorine-35 (75%) and chlorine-37 (25%):

(35 × 0.75) + (37 × 0.25) = 35.5 u

The result is a weighted average. It does not mean that each chlorine atom has a mass of 35.5 u.

15. Isobars and the Modern Atomic Model

Isobars: Atoms of different elements with the same mass number but different atomic numbers.

Example: calcium-40, potassium-40 and argon-40.

In the modern quantum mechanical model, electrons are described by regions of probability called orbitals or electron clouds, rather than exact circular paths.

Quick Revision Formulas

Atomic number = Number of protons.
Neutral atom: electrons = protons.
Mass number = Protons + Neutrons.
Neutrons = Mass number − Atomic number.
Maximum shell capacity = 2n².
Isotopes: same atomic number, different mass numbers.
Isobars: same mass number, different atomic numbers.
Valency is the combining capacity of an atom.

Part 2: NCERT Book Solutions

Answers are given in the same order as the supplied chapter. The original end-of-chapter questions and options are retained.

Pause and Ponder — Section 8.2.1

1. Suppose you made up your own ‘atom’, as Thomson described, using clay for the positive charge and small beads for the electrons spread through it. What will happen if:
(i) the positive charge on the clay is lesser than the total negative charge of the beads?
(ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?

Answer:

(i) The model would have a net negative charge because the negative charge would be greater than the positive charge.

(ii) No. The clay would add negative charge instead of positive charge, so the model would not represent a neutral atom.

2. Could an orange or a lemon, which also contain seeds inside soft pulp, be a good comparison? In what ways does it match Thomson’s idea and where does it fall short?

Answer: Yes. The pulp can represent the positive charge and the seeds can represent electrons. However, real fruit does not contain a uniformly distributed positive electric charge, so the comparison is only a simple model.

3. Why did Thomson conclude that electrons are present in all atoms?

Answer: Cathode rays showed the same negatively charged particles regardless of the cathode material or the gas used. Thomson concluded that electrons are common components of atoms.

Think as a Scientist — Gold Foil Experiment

4. What do you think would happen if α-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?

Answer: Negatively charged particles would be attracted to the positive nucleus and could be deflected towards it. Their exact paths would also depend on their mass, speed and interactions with electrons.

5. Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ʻplum pudding modelʼ of the atom?

Answer: A sharp bounce-back requires a strong repulsive force concentrated in a tiny region. Thomson’s model spread the positive charge throughout the atom, so it could not explain this result.

6. If you could ask Rutherford one question about his work, what would it be?

Answer: How did you feel when a few alpha particles bounced back, contrary to your expectations?

7. Assertion (A): Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus.
Reason (R): According to Thomson’s model, electrons are embedded in a uniformly distributed positive charge sphere.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.

Answer: (ii)

Both statements are true. However, the reason describes Thomson’s model and does not explain Rutherford’s conclusion about the nucleus.

Pause and Ponder — Section 8.4

8. Imagine you are a scientist who has discovered a new element. Name this element after yourself and justify that the symbol you have chosen follows the IUPAC rules.

Answer: I would name the imaginary element “Renium” and propose the symbol Rn. The first letter is capital and the second is lowercase. However, the name and symbol would need official approval and would have to be checked against existing element names and symbols.

9. What problems could arise if every scientist used different symbols for the same element?

Answer: Scientists could misunderstand formulas, make mistakes in experiments and find it difficult to share results. Standard symbols allow everyone to communicate clearly.

Pause and Ponder — Atomic Number and Mass Number

10. An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.

Answer:

Protons = 26
Electrons = 26, for a neutral atom
Neutrons = 56 − 26 = 30

11. The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.

Answer: Neutrons = 41 − 20 = 21 neutrons.

12. An atom has 18 neutrons and an atomic number of 17. What is its mass number?

Answer: Mass number = 17 + 18 = 35.

13. An atom 23A has 11 electrons. Find the number of neutrons in it.

Answer: Assuming the notation means mass number 23 and that the atom is neutral, it has 11 protons. Neutrons = 23 − 11 = 12.

Pause and Ponder — Electronic Configuration

14. Identify the number of electrons in the outermost shell of the following elements:
(i) ¹²₆ C
(ii) ¹⁹₉ F
(iii) ²⁸₁₄ Si

Answer:

(i) Carbon: 2, 4 — 4 electrons.
(ii) Fluorine: 2, 7 — 7 electrons.
(iii) Silicon: 2, 8, 4 — 4 electrons.

15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.

Answer:

Atomic number 12 (magnesium): 2, 8, 2.
Atomic number 16 (sulfur): 2, 8, 6.
Atomic number 18 (argon): 2, 8, 8.

16. Solve this riddle: I am an atom with a mass number of 23 and 11 protons. I am a soft metal and react vigorously with water. Who am I and how many neutrons do I have? You can also create one such riddle.

Answer: The element is sodium (Na). Neutrons = 23 − 11 = 12.

My riddle: I have 12 protons, 12 electrons and 12 neutrons. My electronic configuration is 2, 8, 2. Who am I?

Answer: Magnesium.

Pause and Ponder — Isotopes

17. Two different atoms have 11 protons each, but one has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element or different elements?

Answer:

Both atomic numbers = 11.
First mass number = 11 + 12 = 23.
Second mass number = 11 + 13 = 24.

They are the same element, sodium, but different isotopes.

18. If a bromine atom is available in the form of, say two isotopes, ⁷⁹₃₅ Br (49.7%) and ⁸¹₃₅ Br (50.3%), calculate the average atomic mass of the bromine atom.

Answer:

Average mass = (79 × 0.497) + (81 × 0.503)

= 39.263 + 40.743 = 80.006 u

The average atomic mass is approximately 80.01 u.

Revise, Reflect, Refine — End-of-Chapter Questions

1. Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford’s gold foil experiment:
(i) The experiment clearly showed the existence of neutrons in the nucleus.
(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
(iii) The large deflection of a few alpha particles indicated that most of the mass of the atom and positive charge are packed into a tiny centre.
(iv) The way alpha particles were deflected showed that electrons move around the nucleus.

Answer: Correct options are (ii) and (iii).

(i) Incorrect: The experiment did not establish the existence of neutrons.
(ii) Correct: The observations showed that Thomson’s model was inadequate and supported a central nucleus.
(iii) Correct: Large deflections showed that positive charge and most of the mass are concentrated in a tiny, dense region.
(iv) Incorrect: The experiment did not establish the paths of electrons around the nucleus.

2. Which of the following statements are correct or incorrect according to the Bohr’s atomic model? Give a reason for each statement.
(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
(ii) Electrons can exist anywhere around the nucleus with no fixed energy.
(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
(iv) Electrons can be found between energy levels as they move around the nucleus.

Answer:

(i) Incorrect: Bohr proposed that electrons do not lose energy while in an allowed orbit.
(ii) Incorrect: Bohr’s model allows only specific energy levels.
(iii) Correct: Electrons in allowed orbits have fixed energies and do not lose energy while remaining there.
(iv) Incorrect: In Bohr’s model, electrons occupy allowed energy levels, not intermediate energies.

3. The composition of the nuclei of three atomic species X, Y, and Z are given as follows.

X Y Z
Number of protons 18 17 17
Number of neutrons 19 18 20

Explain the relation between the following:
(i) Y and Z
(ii) Z and X

Answer:

(i) Y has 17 protons and 18 neutrons, so its mass number is 35. Z has 17 protons and 20 neutrons, so its mass number is 37. They are isotopes of the same element, chlorine.

(ii) Z has mass number 37. X has 18 protons and 19 neutrons, giving mass number 37. They have the same mass number but different atomic numbers, so they are isobars.

4. What conclusion did Rutherford draw about the position and characteristics of the atom’s positively charged part based on the few alpha particles that bounced back or were deflected at large angles in the gold foil experiment?

Answer: Rutherford concluded that the positive charge and most of the atom’s mass are concentrated in a tiny, dense nucleus at the centre.

5. Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.
(i) Bohr’s model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
(ii) Thomson’s model depicted the atom as a ʻplum puddingʼ with electrons embedded in a sphere of positive charge.
(iii) Rutherford’s model proposed that atoms have a dense central nucleus.
(iv) Dalton’s model described atoms as indivisible particles.

Answer: (iv) → (ii) → (iii) → (i)

Dalton: atoms were indivisible particles.
Thomson: electrons were embedded in a positive sphere.
Rutherford: the atom had a dense central nucleus.
Bohr: electrons occupied fixed energy levels.

6. Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.

Answer: Negatively charged electrons are attracted to the positively charged nucleus by electrostatic force. In Bohr’s model, electrons remain in allowed energy levels without losing energy while in those levels.

7. Assertion (A): The discovery of subatomic particles helped in understanding the atomic structure.
Reason (R): The number of electrons is equal to the number of protons in an atom.
Choose the correct option:
(i) Both A and R are true, and R is the correct explanation of A.
(ii) Both A and R are true, but R is not the correct explanation of A.
(iii) A is true, but R is false.
(iv) A is false, but R is true.

Answer: (ii)

Both statements are true for a neutral atom. However, equal numbers of electrons and protons do not fully explain how the discovery of subatomic particles improved our understanding of atomic structure.

8. Magnesium is essential for many biological processes, including muscle contraction. For an atom of magnesium with a mass number of 24 and atomic number 12, determine the number of (i) protons, (ii) neutrons, (iii) electrons, and also illustrate the arrangement of electrons in a magnesium atom.

Answer:

(i) Protons = 12.
(ii) Neutrons = 24 − 12 = 12.
(iii) Electrons = 12 in a neutral atom.

Electronic configuration: 2, 8, 2.

Simple schematic of magnesium electron shells. The dots are illustrative.

9. Find the following information for the elements shown in Fig. 8.17:
(i) Name of the element
(ii) Symbol
(iii) Total number of electrons
(iv) Number of valence electrons
(v) Valency of the element
(vi) Number of protons
(vii) Atomic number

Answer

The electronic configurations of the four elements are: (a) 2, 1; (b) 2, 5; (c) 2, 8, 3; and (d) 2, 7.

Information	(a)	(b)	(c)	(d)
Name of element	Lithium	Nitrogen	Aluminium	Fluorine
Symbol	Li	N	Al	F
Electronic configuration	2, 1	2, 5	2, 8, 3	2, 7
Total number of electrons	3	7	13	9
Number of valence electrons	1	5	3	7
Valency	1	3	3	1
Number of protons	3	7	13	9
Atomic number	3	7	13	9

Easy explanation

(a) Lithium (Li): Its configuration is 2, 1. It has 3 electrons and 1 valence electron. Its valency is 1 because it can lose 1 electron.
(b) Nitrogen (N): Its configuration is 2, 5. It has 7 electrons and 5 valence electrons. Its valency is 3 because it needs 3 more electrons to complete its outer shell.
(c) Aluminium (Al): Its configuration is 2, 8, 3. It has 13 electrons and 3 valence electrons. Its valency is 3 because it can lose 3 electrons.
(d) Fluorine (F): Its configuration is 2, 7. It has 9 electrons and 7 valence electrons. Its valency is 1 because it needs 1 more electron to complete its outer shell.

Remember: In a neutral atom, the number of electrons, protons and the atomic number are equal. Valence electrons are the electrons in the outermost shell.

10. Both Rutherford’s and Bohr’s models have electrons orbiting the nucleus. Why did Rutherford’s model fail to explain atomic stability, while Bohr’s model succeeded?

Answer: Rutherford’s model could not explain why accelerating electrons would not lose energy and fall into the nucleus. Bohr proposed that electrons occupy allowed energy levels without losing energy while in those levels, explaining atomic stability within his model.

11. An atom 70 X has 31 electrons. How many neutrons are there in its nucleus?

Answer: Interpreting the notation as mass number 70 and assuming a neutral atom, the number of protons is 31. Therefore, neutrons = 70 − 31 = 39.

12. An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.

Answer:

(i) Neutrons = 197 − 79 = 118.
(ii) Electrons = 79 in a neutral atom.

13. Complete the Table 8.5:

Table 8.5
Atomic number
Mass number
Number of neutrons
Number of protons
Number of electrons
Name of the elements

5 – 6 – – –
– 14 – – 7 Nitrogen
– 24 – 12 – –
15 – 16 – – –
– 1 0 – – –

Answer: Assuming each atom is neutral and using the given atomic and mass numbers:

Atomic number	Mass number	Neutrons	Protons	Electrons	Element
5	11	6	5	5	Boron
7	14	7	7	7	Nitrogen
12	24	12	12	12	Magnesium
15	31	16	15	15	Phosphorus
1	1	0	1	1	Hydrogen

14. Aman was discussing the structure of atom with his classmates. During the discussion, he learnt that an element X has a mass number of 35 and contains 18 neutrons. Based on this information, answer the following questions:
(i) How many electrons and protons does element X have?
(ii) What is its atomic number?
(iii) Identify the element X.
(iv) Write its electronic configuration.
(v) How many valence electrons does it have?
(vi) What will be the mass number if two neutrons are added to its nucleus?
(vii) What will be the relation of X with the new atom?

Answer:

Protons = 35 − 18 = 17.

(i) Protons = 17; electrons = 17 for a neutral atom.
(ii) Atomic number = 17.
(iii) Element X is chlorine (Cl).
(iv) Electronic configuration = 2, 8, 7.
(v) Valence electrons = 7.
(vi) New mass number = 35 + 2 = 37.
(vii) The new atom is an isotope of chlorine. Both atoms have the same atomic number but different mass numbers.

15. In an atom, there are 12 protons and 12 neutrons in the nucleus. Now, imagine that all the electrons are replaced with some hypothetical particles that have the same charge as electrons but are 500 times heavier. What effect will this replacement have on the atom’s:
(i) Atomic number
(ii) Atomic mass
(iii) Mass number
(iv) Overall charge

Answer:

(i) Atomic number: Remains 12 because the number of protons does not change.
(ii) Atomic mass: Increases because the replacement particles are much heavier than electrons.
(iii) Mass number: Remains 24 because mass number counts only protons and neutrons in the nucleus.
(iv) Overall charge: Remains zero if there are 12 replacement particles, each with the same negative charge as an electron.

Final Revision

Dalton described atoms as indivisible particles.
Thomson discovered electrons and proposed the plum pudding model.
Rutherford identified the nucleus through alpha-particle scattering.
Bohr proposed fixed electron energy levels.
Chadwick discovered the neutron.
Atomic number identifies an element.
Mass number is the sum of protons and neutrons.
Isotopes have the same atomic number but different mass numbers.
Isobars have the same mass number but different atomic numbers.
Valency describes an atom’s combining capacity.

Study material for educational purposes. Textbook questions are reproduced from the chapter text supplied by the user.