---
title: "NCERT Solutions Class 9 Science Chapter 8 Journey Inside the Atom"
url: https://www.swavid.com/science/class/9/chapter/journey-inside-the-atom/ncert-solutions
dateModified: 2026-10-07T16:23:56+00:00
---

# NCERT Solutions Class 9 Science Chapter 8 Journey Inside the Atom

This chapter's questions cover foundational concepts of atomic structure, historical models of the atom, subatomic particles, electronic configuration, valency, isotopes, and isobars.

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## Pause and Ponder

### Question 1

*3 marks · Short answer*

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?

**Part (i)**

1. The model will carry a net negative charge because the total negative charge of the beads exceeds the positive charge of the clay.
2. Thus, it will not represent a neutral atom.

Answer (i): The model will have a net negative charge and will not be neutral.

**Part (ii)**

1. If the clay itself carries a negative charge along with the embedded negative beads, the entire model will be negatively charged.
2. Since an atom must have equal positive and negative charges to be neutral, this model will not represent a neutral atom.

Answer (ii): No, the model would not represent a neutral atom because it would lack sufficient positive charge to balance the negative charge.

**Answer:** The model would be negatively charged in both cases and would not represent a neutral atom.

> Common mistake: Forgetting that an atom is electrically neutral only when total positive charge equals total negative charge.

### Question 2

*3 marks · Short answer*

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?

**Solution**

1. An orange or a lemon can be compared to Thomson's model because the soft pulp represents the positively charged sphere and the seeds represent the embedded electrons.
2. It matches the model in terms of having smaller structures (seeds) embedded inside a larger mass (pulp).
3. It falls short because the pulp and seeds are discrete biological units, whereas in Thomson's model the positive charge is a continuous sphere and electrons are point subatomic particles distributed uniformly.

**Answer:** It matches by having seeds inside pulp like electrons in a positive sphere, but falls short because real fruit parts are not continuous charges or subatomic particles.

> Common mistake: Stating that fruit is an exact scientific representation without noting the physical differences.

### Question 3

*3 marks · Short answer*

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

**Solution**

1. J. J. Thomson studied cathode rays produced in a discharge tube containing different gases and using different cathode materials.
2. He observed that the nature of cathode rays was completely independent of the gas used and the material of the cathode.
3. This proved that electrons are a fundamental universal component present in all atoms of every element.

**Answer:** Thomson concluded this because cathode ray properties were independent of the gas or electrode material, showing electrons exist in all elements.

> Common mistake: Missing that the independence from material and gas is the key proof.

### Question 4

*3 marks · Short answer*

What do you think would happen if $\alpha$-particles were replaced with negatively charged particles in Rutherford’s gold foil experiment?

**Solution**

1. Alpha particles are massive and positively charged, whereas negatively charged particles would be electrons or beta particles.
2. If positively charged alpha particles are replaced by negatively charged particles, they would be strongly attracted and deflected by the positive nucleus instead of being repelled.
3. Also, being much lighter, these particles would undergo massive deflections or get absorbed easily by the electrons in the atom.

**Answer:** The negatively charged particles would be attracted toward the positive nucleus and scattered differently due to their very small mass compared to alpha particles.

> Common mistake: Ignoring the difference in mass and charge between alpha particles and negative particles.

### Question 5

*3 marks · Short answer*

Rutherford found that a few $\alpha$-particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?

**Solution**

1. According to Thomson's model, positive charge is spread uniformly throughout the atom, so its mass and charge density are very low.
2. Such a diffuse positive sphere cannot exert a strong enough electrostatic force to bounce back a fast-moving, heavy alpha particle.
3. The sharp bouncing back of alpha particles proves that the positive charge and mass are concentrated in a tiny, dense center called the nucleus, completely ruling out Thomson's model.

**Answer:** A uniform positive sphere cannot exert enough force to bounce back alpha particles, proving mass and charge are concentrated in a dense nucleus.

> Common mistake: Stating only that particles deflected without explaining why Thomson's model cannot cause sharp rebounds.

### Question 6

*2 marks · Very short answer*

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

**Solution**

1. Based on the gold foil experiment, one could ask Rutherford how electrons can remain in motion around the dense nucleus without continuously losing energy and collapsing into it.

**Answer:** Why do the revolving electrons not lose energy and spiral into the nucleus?

> Common mistake: Asking a question unrelated to the known limitations of his model.

### Question 7

*1 mark · Assertion and reason*

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:

- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true, but R is false.
- A is false, but R is true.

**Solution**

1. Assertion (A) is true because Rutherford's alpha-particle scattering experiment showed that most mass and positive charge are concentrated in the nucleus.
2. Reason (R) is true as it correctly states Thomson's model, but it is not the explanation for Rutherford's conclusion.

**Answer:** (ii) Both A and R are true, but R is not the correct explanation of A.

> Common mistake: Choosing option (i) by assuming any correct statement about atomic models is an explanation.

### Question 8

*3 marks · Short answer*

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.

**Solution**

1. Let the discovered element be named 'Sharma', so its proposed symbol is Sh.
2. According to IUPAC rules, the first letter of a symbol must be capitalized (S) and the second letter must be lowercase (h).
3. This ensures international standardisation and avoids confusion with symbols of other elements.

**Answer:** Name: Sharma, Symbol: Sh, justified by IUPAC capital-lowercase rule.

> Common mistake: Writing both letters in uppercase like 'SH'.

### Question 9

*2 marks · Very short answer*

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

**Solution**

1. If every scientist used different symbols, it would create chaos and severe communication barriers globally.
2. Chemical formulas and equations would become incomprehensible across different regions and languages.

**Answer:** It would lead to global confusion and communication barriers in sharing chemical knowledge.

> Common mistake: Writing a vague answer without mentioning global communication or confusion.

### Question 10

*3 marks · Numerical*

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

**Solution**

1. Given: Atomic number ($Z$) = 26, Mass number ($A$) = 56.
2. Number of protons = Atomic number = 26, and number of electrons = number of protons = 26 in a neutral atom.
3. Number of neutrons = Mass number - Number of protons = $56 - 26 = 30$.

**Answer:** Number of electrons = 26, Protons = 26, Neutrons = 30.

> Common mistake: Confusing nucleons with electrons.

### Question 11

*3 marks · Numerical*

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

**Solution**

1. Given: Number of protons = 20, Mass number ($A$) = 41.
2. Formula: Mass number = Number of protons + Number of neutrons.
3. Substitution: $41 = 20 + \text{Number of neutrons}$.
4. Result: Number of neutrons = $41 - 20 = 21$.

**Answer:** 21 neutrons

> Common mistake: Adding protons and mass number instead of subtracting.

### Question 12

*3 marks · Numerical*

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

**Solution**

1. Given: Atomic number ($Z$) = 17, Number of neutrons = 18.
2. Number of protons = Atomic number = 17.
3. Formula: Mass number ($A$) = Number of protons + Number of neutrons.
4. Substitution: $A = 17 + 18 = 35$.

**Answer:** 35 u

> Common mistake: Using atomic number directly as mass number without adding neutrons.

### Question 13

*3 marks · Numerical*

An atom $^{23}\text{A}$ has 11 electrons. Find the number of neutrons in it.

**Solution**

1. Given: Mass number (A) = 23, Number of electrons = 11.
2. Since the atom is neutral, the number of protons = number of electrons = 11.
3. Formula: Mass number (A) = Number of protons + Number of neutrons.
4. Substitution: 23 = 11 + Number of neutrons.
5. Result: Number of neutrons = 23 - 11 = 12.

**Answer:** 12 neutrons

> Common mistake: Confusing mass number with atomic number.

### Question 14

*3 marks · Short answer*

Identify the number of electrons in the outermost shell of the following elements:
(i) $^{12}_6\text{C}$
(ii) $^{19}_9\text{F}$
(iii) $^{28}_{14}\text{Si}$

**Part (i)**

1. Carbon (C) has atomic number 6.
2. Electronic configuration is K=2, L=4.
3. Outermost shell has 4 electrons.

Answer (i): 4 electrons

**Part (ii)**

1. Fluorine (F) has atomic number 9.
2. Electronic configuration is K=2, L=7.
3. Outermost shell has 7 electrons.

Answer (ii): 7 electrons

**Part (iii)**

1. Silicon (Si) has atomic number 14.
2. Electronic configuration is K=2, L=8, M=4.
3. Outermost shell has 4 electrons.

Answer (iii): 4 electrons

**Answer:** The number of electrons in the outermost shell are (i) 4, (ii) 7, and (iii) 4.

> Common mistake: Incorrectly filling shells beyond their capacity.

### Question 15

*3 marks · Short answer*

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

**Solution**

1. For atomic number 12 (Magnesium): K=2, L=8, M=2.
2. For atomic number 16 (Sulfur): K=2, L=8, M=6.
3. For atomic number 18 (Argon): K=2, L=8, M=8.

**Answer:** The electronic configurations are 12: 2, 8, 2; 16: 2, 8, 6; 18: 2, 8, 8.

> Common mistake: Not following the K, L, M shell order.

### Question 16

*3 marks · Short answer*

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.

**Solution**

1. The atom has 11 protons, so its atomic number is 11.
2. The element with atomic number 11 is Sodium (Na), which is a soft metal that reacts vigorously with water.
3. Given mass number (A) = 23 and protons = 11.
4. Number of neutrons = A - protons = 23 - 11 = 12.

**Answer:** The element is Sodium (Na) and it has 12 neutrons.

> Common mistake: Identifying the element incorrectly based on mass number instead of atomic number.

### Question 17

*3 marks · Short answer*

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?

**Solution**

1. Both atoms have 11 protons, so their atomic number (Z) is 11.
2. Atom 1 has 11 protons and 12 neutrons, so mass number (A) = 11 + 12 = 23.
3. Atom 2 has 11 protons and 13 neutrons, so mass number (A) = 11 + 13 = 24.
4. Since they have the same atomic number but different mass numbers, they are the same element (isotopes).

**Answer:** They have the same atomic number (11) but different mass numbers (23 and 24); they are the same element.

> Common mistake: Assuming different mass numbers implies different elements.

### Question 18

*3 marks · Numerical*

If a bromine atom is available in the form of, say two isotopes, $^{79}_{35}\text{Br}$ (49.7%) and $^{81}_{35}\text{Br}$ (50.3%), calculate the average atomic mass of the bromine atom.

**Solution**

1. Formula: Average atomic mass = (Mass of isotope 1 × abundance) + (Mass of isotope 2 × abundance).
2. Substitution: (79 × 0.497) + (81 × 0.503).
3. Calculation: 39.263 + 40.743 = 80.006 u.
4. Result: The average atomic mass is 80.006 u.

**Answer:** 80.006 u

> Common mistake: Using simple arithmetic mean instead of weighted average.

## Revise, Reflect, Refine

### Question 1

*3 marks · Short answer*

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.

**Part (i)**

1. Neutrons were discovered by James Chadwick in 1932, whereas the gold foil experiment was conducted in 1911.
2. This statement is incorrect.

Answer (i): Incorrect, because neutrons were discovered later by James Chadwick.

**Part (ii)**

1. The large-angle deflection of alpha particles disproved Thomson's plum pudding model.
2. It proved that the positive charge is concentrated in a tiny central nucleus.

Answer (ii): Correct, it disproved the plum pudding model and established the nuclear model.

**Part (iii)**

1. Only a very few alpha particles experienced large deflections or bounced back.
2. This indicated that the entire positive charge and most of the mass are packed into an extremely small central region.

Answer (iii): Correct, mass and positive charge are concentrated in a tiny centre.

**Part (iv)**

1. The gold foil experiment investigated the nucleus and positive charge distribution, not the motion of electrons.
2. Bohr later described electron orbits.

Answer (iv): Incorrect, the experiment did not show how electrons move around the nucleus.

**Answer:** Statements (ii) and (iii) are correct, while (i) and (iv) are incorrect.

> Common mistake: Confusing the discoveries of the nucleus (Rutherford) and neutrons (Chadwick).

### Question 2

*3 marks · Short answer*

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.

**Part (i)**

1. According to Bohr's model, electrons do not lose energy while revolving in fixed stationary orbits.
2. This statement describes the limitation of Rutherford's model, not Bohr's model.

Answer (i): Incorrect, electrons do not lose energy or fall into the nucleus in fixed orbits.

**Part (ii)**

1. Electrons can only revolve in specific allowed circular paths called stationary states or shells with definite energy levels.

Answer (ii): Incorrect, electrons cannot exist anywhere; they occupy fixed energy levels.

**Part (iii)**

1. Bohr postulated that while moving in a fixed shell or stationary state, an electron does not lose energy.

Answer (iii): Correct, electrons revolve in orbits of fixed energy without losing energy.

**Part (iv)**

1. Electrons are restricted to specific allowed shells (n = 1, 2, 3, ...) and cannot exist in the forbidden spaces between them.

Answer (iv): Incorrect, electrons cannot be found between energy levels.

**Answer:** Statements (iii) is correct; statements (i), (ii), and (iv) are incorrect.

> Common mistake: Confusing Rutherford's unstable spiral path with Bohr's stable stationary states.

### Question 3

*3 marks · Short answer*

The composition of the nuclei of three atomic species X, Y, and Z are given as follows.
Table showing X, Y, Z with protons (18, 17, 17) and neutrons (19, 18, 20).
Explain the relation between the following:
(i) Y and Z
(ii) Z and X

**Part (i)**

1. Species Y has 17 protons and 18 neutrons (mass number = 17 + 18 = 35).
2. Species Z has 17 protons and 20 neutrons (mass number = 17 + 20 = 37).
3. Since they have the same atomic number (17) but different mass numbers, Y and Z are isotopes.

Answer (i): Y and Z are isotopes because they have the same number of protons but different mass numbers.

**Part (ii)**

1. Species Z has 17 protons and a mass number of 37.
2. Species X has 18 protons and 19 neutrons (mass number = 18 + 19 = 37).
3. Since they have different atomic numbers (17 and 18) but the same mass number (37), Z and X are isobars.

Answer (ii): Z and X are isobars because they have different atomic numbers but the same mass number.

**Answer:** Y and Z are isotopes, while Z and X are isobars.

> Common mistake: Mixing up the definitions of isotopes (same atomic number) and isobars (same mass number).

### Question 4

*3 marks · Short answer*

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?

**Solution**

1. Rutherford concluded that the positive charge of an atom is not spread throughout the atom as suggested by Thomson.
2. Instead, it is concentrated in an extremely small, dense region at the centre called the nucleus.
3. He also concluded that the nucleus contains most of the mass of the atom and is surrounded by empty space where electrons revolve.

**Answer:** Rutherford concluded that the positive charge and most of the mass of an atom are concentrated in an extremely small, dense central region called the nucleus.

> Common mistake: Stating that the entire atom's mass is in the nucleus instead of most of the mass.

### Question 5

*3 marks · Short answer*

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 ‘plum pudding’ 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.

**Solution**

1. Step 1: Dalton's model described atoms as indivisible particles (1808).
2. Step 2: Thomson's model depicted the atom as a plum pudding with electrons embedded in a positive sphere (late 19th century).
3. Step 3: Rutherford's model proposed a dense central nucleus based on the gold foil experiment (1911).
4. Step 4: Bohr's model proposed electrons moving in fixed energy orbits around the nucleus (1913).

**Answer:** Chronological order: (iv) Dalton's model -> (ii) Thomson's model -> (iii) Rutherford's model -> (i) Bohr's model.

> Common mistake: Placing Thomson's model before Dalton's model.

### Question 6

*3 marks · Short answer*

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

**Solution**

1. The nucleus contains positively charged protons, while electrons carry negative charges.
2. Unlike charges attract each other due to electrostatic force.
3. This attractive force between the positive nucleus and negative electrons keeps the electrons bound in their orbits and prevents them from flying away.

**Answer:** Electrons do not fly away because of the electrostatic force of attraction between the positively charged nucleus and the negatively charged electrons.

> Common mistake: Forgetting to mention the electrostatic or attractive force between opposite charges.

### Question 7

*1 mark · Assertion and reason*

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:

- Both A and R are true, and R is the correct explanation of A.
- Both A and R are true, but R is not the correct explanation of A.
- A is true, but R is false.
- A is false, but R is true.

**Solution**

1. The discovery of subatomic particles like electrons, protons, and neutrons provided the foundation for understanding atomic structure.
2. An atom is electrically neutral because the number of electrons equals the number of protons, but this property does not explain how subatomic particles helped understand atomic structure.

**Answer:** (ii) Both A and R are true, but R is not the correct explanation of A.

> Common mistake: Confusing a true statement about an atom with the correct scientific explanation for why subatomic particle discoveries advanced atomic structure models.

### Question 8

*3 marks · Short answer*

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.

**Part (i)**

1. The number of protons in an atom is equal to its atomic number Z.
2. Number of protons = 12.

Answer (i): 12

**Part (ii)**

1. Number of neutrons = Mass number - Number of protons.
2. Neutrons = 24 - 12 = 12.

Answer (ii): 12

**Part (iii)**

1. In a neutral atom, number of electrons equals the number of protons.
2. Number of electrons = 12, distributed as K = 2, L = 8, M = 2.

Answer (iii): Electrons = 12, Electronic configuration: 2, 8, 2

**Answer:** Protons = 12, Neutrons = 12, Electrons = 12, Configuration = 2, 8, 2

> Common mistake: Subtracting atomic number from mass number incorrectly or writing incorrect electron distribution.

### Question 9

*3 marks · Short answer*

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

**Part (i)**

1. Observe Fig. 8.17 where the atomic structures show elements from the first eighteen elements.
2. For structure (a) with 2, 1 electrons, the element is Lithium.

Answer (i): Lithium

**Part (ii)**

1. The chemical symbol for Lithium is Li.

Answer (ii): Li

**Part (iii)**

1. Count the total number of electrons in shells (2 + 1 = 3).

Answer (iii): 3

**Part (iv)**

1. Valence electrons are the electrons present in the outermost shell (1).

Answer (iv): 1

**Part (v)**

1. Since it has 1 valence electron, its valency is 1.

Answer (v): 1

**Part (vi)**

1. Number of protons equals the total number of electrons in a neutral atom (3).

Answer (vi): 3

**Part (vii)**

1. Atomic number is equal to the number of protons (3).

Answer (vii): 3

**Answer:** Values determined from the given electron distribution in Fig. 8.17.

> Common mistake: Confusing total electrons with valence electrons.

### Question 10

*3 marks · Short answer*

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?

**Solution**

1. According to classical physics, an accelerating charged particle moving in a circular path loses energy, which would make electrons spiral into the nucleus in Rutherford's model.
2. Bohr resolved this by postulating that electrons revolve only in fixed stationary states or orbits where they do not lose energy.

**Answer:** Rutherford's model could not explain atomic stability due to energy loss in circular motion, whereas Bohr introduced stationary states where electrons do not lose energy.

> Common mistake: Stating Bohr also used classical electromagnetic theory.

### Question 11

*3 marks · Numerical*

An atom $^{70}\text{X}$ has 31 electrons. How many neutrons are there in its nucleus?

**Solution**

1. Given: Mass number (A) = 70, Number of electrons = 31.
2. For a neutral atom, number of protons = number of electrons = 31.
3. Formula: Number of neutrons = Mass number (A) - Number of protons (Z).
4. Substitution: Number of neutrons = 70 - 31 = 39.
5. Result: 39 neutrons.

**Answer:** 39 neutrons

> Common mistake: Subtracting atomic number from electrons instead of mass number.

### Question 12

*3 marks · Numerical*

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

**Part (i)**

1. Given: Number of protons = 79, Mass number (A) = 197.
2. Formula: Number of neutrons = Mass number - Number of protons.
3. Substitution: Neutrons = 197 - 79 = 118.
4. Result: 118 neutrons.

Answer (i): 118

**Part (ii)**

1. In a neutral atom, the number of electrons equals the number of protons.
2. Result: 79 electrons.

Answer (ii): 79

**Answer:** Number of neutrons = 118, Number of electrons = 79

> Common mistake: Mixing up protons and mass number during subtraction.

### Question 13

*1 mark · Fill in the blank*

Complete the Table 8.5:

**Part (i)**

1. For row 1: Protons = Atomic number = 5, Electrons = 5, Mass number = 5 + 6 = 11, Element = Boron.
2. For row 2: Protons = Electrons = 7, Atomic number = 7, Neutrons = 14 - 7 = 7, Element = Nitrogen.
3. For row 3: Atomic number = Protons = 12, Electrons = 12, Neutrons = 24 - 12 = 12, Element = Magnesium.
4. For row 4: Protons = Atomic number = 15, Electrons = 15, Mass number = 15 + 16 = 31, Element = Phosphorus.
5. For row 5: Protons = Mass number - Neutrons = 1 - 0 = 1, Atomic number = 1, Electrons = 1, Element = Hydrogen.

Answer (i): Calculated values for each row.

**Answer:** The completed table is: Row 1: Z=5, A=11, n=6, p=5, e=5, Boron; Row 2: Z=7, A=14, n=7, p=7, e=7, Nitrogen; Row 3: Z=12, A=24, n=12, p=12, e=12, Magnesium; Row 4: Z=15, A=31, n=16, p=15, e=15, Phosphorus; Row 5: Z=1, A=1, n=0, p=1, e=1, Hydrogen.

> Common mistake: Confusing atomic number with mass number or forgetting that electrons equal protons in a neutral atom.

### Question 14

*3 marks · Short answer*

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?

**Part (i)**

1. Protons = Mass number - Neutrons = 35 - 18 = 17.
2. Since the atom is neutral, electrons = protons = 17.

Answer (i): 17 protons and 17 electrons.

**Part (ii)**

1. Atomic number (Z) = Number of protons = 17.

Answer (ii): 17

**Part (iii)**

1. The element with atomic number 17 is Chlorine.

Answer (iii): Chlorine (Cl)

**Part (iv)**

1. Electronic configuration for Z=17 is 2, 8, 7.

Answer (iv): 2, 8, 7

**Part (v)**

1. The number of electrons in the outermost shell is 7.

Answer (v): 7

**Part (vi)**

1. New mass number = 35 + 2 = 37.

Answer (vi): 37

**Part (vii)**

1. Since they have the same atomic number (17) but different mass numbers (35 and 37), they are isotopes.

Answer (vii): Isotopes

**Answer:** Element X is Chlorine (Cl) with 17 protons, 17 electrons, and 7 valence electrons.

> Common mistake: Incorrectly calculating the number of protons by adding neutrons to mass number instead of subtracting.

### Question 15

*3 marks · Short answer*

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

**Part (i)**

1. Atomic number depends only on the number of protons, which remains 12.

Answer (i): No change (remains 12).

**Part (ii)**

1. Atomic mass includes the mass of electrons; since the new particles are 500 times heavier, the total atomic mass will increase.

Answer (ii): Increases.

**Part (iii)**

1. Mass number is the sum of protons and neutrons, which remains 12 + 12 = 24.

Answer (iii): No change (remains 24).

**Part (iv)**

1. Overall charge depends on the number and charge of protons and electrons; since the charge of the new particles is the same as electrons, the atom remains neutral.

Answer (iv): No change (remains neutral).

**Answer:** The atomic number, mass number, and overall charge remain unchanged, while the atomic mass increases.

> Common mistake: Assuming that mass number changes because the mass of the atom changes; mass number is defined only by nucleons.

## Frequently asked questions

### How many total questions and exercises are included in this Class 9 Science chapter on the journey inside the atom?

This chapter in the new NCERT book for the 2026-27 session contains a total of 33 questions divided across two main sections. There are 18 questions in the Pause and Ponder section and 15 questions in the Revise, Reflect, Refine section. You can find SwaVid's free PDF and step-by-step solutions for all of them on this page only.

### What specific topics do the practice questions cover in these exercises?

The questions cover essential concepts such as the discovery of electrons, atomic mass, subatomic particles, atomic number and mass number, and electronic configuration. They also address advanced topics like average atomic mass, Bohr's atomic model statements, and the comparison between Rutherford and Bohr atomic models. SwaVid provides detailed explanations for all these topics in the free PDF available on this page only.

### Which question types are considered the most challenging in this chapter and how should I approach them?

Numerical problems and assertion-reason questions involving the calculation of neutrons, mass numbers, and average atomic mass are often the hardest for students. To approach them, you should carefully revise formulas related to subatomic particles and nuclear charge before solving. SwaVid's step-by-step solutions on this page only break down these difficult formats clearly.

### How should I structure my answers to secure full marks in Class 9 Science exams?

To score full marks, write concise answers with proper scientific terms, clear derivations for numerical problems, and well-labeled diagrams where required. Mentioning specific atomic models and their chronological order accurately will also help impress the examiner. You can study the solved examples in SwaVid's free PDF on this page only to understand the ideal answer format.

### Is the free PDF for these NCERT solutions available for download?

Yes, the complete set of solutions aligned with the new NCERT book is available as a downloadable resource. SwaVid's free PDF and step-by-step solutions are on this page only to help you prepare effectively for your school assessments.

## Related pages

- [Class 9 Science chapters](https://www.swavid.com/science/class/9)

Solutions written by SwaVid, a personal AI tutor for Class 6 to 10 Maths and Science. Practise this chapter free: https://www.swavid.com/start/student