Ch 8 Journey Inside The Atom Question Answers |Class 9 NCERT

The answers to each of the questions presented here are strictly based on Chapter 8, Journey Inside The Atom, of the class 9 science textbook, Exploration.

Here is the text revised for clarity and easy reading:

We have prepared the “Ch 8: Journey Inside The Atom” question answers in a simple, easy-to-read, and easy-to-understand format.

You can prepare the chapter in the best way by:

1. Reading the chapter from the textbook first
2. Preparing the chapter with our short notes
3. Solving the questions of Revise, Reflect, Refine

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Q1.

Choose the correct options and explain the reason for the correct answer 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 the positive charge are packed into a tiny
centre.
(iv) The way alpha particles were deflected showed that electrons
move around the nucleus.

Answers:

(i) INCORRECT. The gold foil experiment only showed that a positive charge is concentrated in a dense nucleus. Neutrons were discovered separately by James Chadwick in 1932.

(ii) CORRECT. Thomson’s model said positive charge is spread evenly. But some alpha particles bounced back sharply — this proved positive charge is packed in a tiny, dense nucleus, not spread out.

(iii) CORRECT. Rutherford concluded: “The nucleus is dense, contains all the positive charge and most of the mass of an atom.”

(iv) INCORRECT. Alpha particle deflection told us about the nucleus, where the positive charge is. It gave no information about how electrons move. Electron behaviour was explained later by Bohr’s model.

Q2.

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.

Answers:

(i) INCORRECT. Bohr specifically stated: “While moving in a fixed shell, an electron does not lose energy.” This was the key improvement over Rutherford’s model.

(ii) INCORRECT. Bohr said electrons follow “fixed circular paths called stationary states, orbits, or shells. In each shell, an electron has a definite amount of energy.”

(iii) CORRECT. “In a stationary state, the energy of an electron remains constant, even though it is in motion around the nucleus.”

(iv) INCORRECT. “Electrons can revolve only in these allowed shells, and not in between them.”

Q3.

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

Explain the relation between the following:

(i) Y and Z
(ii) Z and X

Answers:

The composition of the nuclei of three atomic species X, Y, and Z:

(i) Relation between Y and Z:

Y and Z are Isotopes. Both have the same atomic number (17 = same element, Chlorine) but different mass numbers (Y = 35, Z = 37). “Atoms with the same atomic number but different mass numbers are called isotopes.”

(ii) Relation between Z and X:

Z and X are Isobars. Both have the same mass number (37) but different atomic numbers (Z = 17, X = 18). “When atoms of different elements have the same mass number, but different atomic numbers, they are called isobars.”

Q4.

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?

Answers:

Rutherford drew these conclusions:

Key conclusions about the nucleus:

• It is extremely small — about 10⁵ times smaller than an atom
• Diameter of atom ≈ 10⁻¹⁰ m; Diameter of nucleus ≈ 10⁻¹⁵ m
• It contains all the positive charge and most of the mass
• Electrons revolve around it (planetary model)

Q5.

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.

Answers:

Correct Chronological Order:

So the answer order is: (iv) → (ii) → (iii) → (i)

Q6.

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

Answers:

Two forces are at work:

A. Why electrons don’t fly away:

1. Electrons carry a negative charge (–1)
2. The nucleus carries a positive charge from protons (+1 each)
3. Opposite charges attract each other
4. This electrostatic attraction pulls electrons toward the nucleus and keeps them in their orbits

B. Why electrons don’t fall INTO the nucleus (Bohr’s answer):

1. This keeps them stable at a fixed distance from the nucleus
2. Bohr proposed that electrons occupy fixed “stationary states”
3. In these orbits, energy remains constant — electrons neither gain nor lose energy

Q7.

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.

Answers:

Option (ii) — Both A and R are true, but R is not the correct explanation of A.

Why A is true:
Each subatomic particle discovery improved our understanding — Thomson’s electron led to the plum pudding model, Rutherford’s proton led to the nuclear model, and Chadwick’s neutron explained atomic mass.

Why R is true but NOT the explanation of A:
Yes, electrons = protons (making atoms neutral) — but this fact about charge balance does not explain why subatomic particles helped us understand atomic structure. R is a separate fact about electrical neutrality, not a cause-and-effect explanation. (Page 145, Section C, Para 2)

Q8.

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.

Answers:

Electron Arrangement (Electronic Configuration):

Using the rule 2n²:

• K-shell (n=1): max 2 → filled with 2
• L-shell (n=2): max 8 → filled with 8
• M-shell (n=3): remaining = 12 − 10 = 2

Configuration: 2, 8, 2

Q9.

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

As per Fig. 8.17

Step 1 — Count Electrons in Each Shell

Step 2 — Fill the Answer Table

Note on valency:

Step 3 — How Valency Was Decided

(Page 153, Section 8.8 — Valency rules)

Helium (a) → Valency = 0. Already has 2 electrons in its only shell. First shell is complete with 2. It is stable and does not need to gain or lose any electrons.

Lithium (b) → Valency = 1 Has 1 electron in its outermost shell. It loses that 1 electron to become stable. So valency = 1.

Sodium (c) → Valency = 1 Has 1 electron in its outermost shell. It loses that 1 electron to become stable. So valency = 1.

Sulfur (d) → Valency = 2 Has 6 electrons in its outermost shell. It needs 2 more to complete the octet of 8. So it gains 2 electrons. Valency = 2.

Q10.

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?

Answers:

Q11.

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

Answers:

Number of electrons = 31
Since an atom is neutral → number of protons = 31
Mass number = 70
Neutrons = Mass number − Protons = 70 − 31 = 39

39 neutrons Ans

Q12.

An atom has 79 protons and a mass number of 197.

Calculate

(i) the number of neutrons, and
(ii) the number of electrons.

Answers:

(i) Neutrons = Mass number − Protons = 197 − 79 = 118

(ii) Electrons = Protons (neutral atom) = 79

118 neutrons, 79 electrons Ans (This element is Gold — Au)

Q13.

Complete the Table 8.5:

Answers:

Using: Atomic number (Z) = protons = electrons; Mass number (A) = protons + neutrons

How each was calculated:

1. Row 1: Z=5, n=6 → A=5+6=11; protons=electrons=5 → Boron
2. Row 2: A=14, electrons=7 → Z=7; neutrons=14−7=7 → Nitrogen
3. Row 3: A=24, protons=12 → Z=12; neutrons=24−12=12 → Magnesium
4. Row 4: Z=15, neutrons=16 → A=15+16=31; electrons=15 → Phosphorus
5. Row 5: A=1, neutrons=0 → protons=1; Z=1; electrons=1 → Hydrogen

Q14.

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?

Answers:

Element X: Mass number = 35, Neutrons = 18.

Working:

1. Protons = Mass number − Neutrons = 35 − 18 = 17
2. Atomic number = 17
3. Electrons = 17

(i) Electrons = 17, Protons = 17
(ii) Atomic number = 17
(iii) Element X = Chlorine (Cl)

(iv) Electronic configuration:

• K-shell: 2, L-shell: 8, M-shell: 7
• Configuration: 2, 8, 7

(v) Valence electrons = 7 (electrons in outermost M-shell)
(vi) If 2 neutrons are added: New mass number = 35 + 2 = 37
(vii) Relation of X with new atom: X (mass number 35) and new atom (mass number 37) both have atomic number 17. They are Isotopes — same atomic number, different mass numbers.

Q15.

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

Answers:

—End of Questions Answers of Revise, Reflect, Refine | Chapter 8 Journey Inside The Atom Class 9—

FAQs: Ch 8 Journey Inside The Atom Question Answers

Source: (Page 147, Table 8.1; Page 150, Section 8.5)

Neutrons = Mass Number (A) − Atomic Number (Z)

Example using Sodium:

• Atomic number = 11
• Mass number = 23
• Neutrons = 23 − 11 = 12

Source:(Page 149–150, Sections 8.5 and 8.6)

Source: (Page 141–146, Sections 8.1 and 8.2; Page 157, Fig 8.16)

source: (Page 153, Section 8.8)

The decision rule:

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