NCERT Solutions for Class 9 Science Exploration Chapter 8

An atom is the basic unit of matter made of subatomic particles called electrons, protons, and neutrons.
Atomic structure explains how these particles are arranged, how elements are identified, and why atoms combine to form substances.

NCERT Solutions for Class 9 Science Exploration Chapter 8 help students solve Journey Inside the Atom, a Chemistry chapter from the 2026-27 Class 9 Science textbook. This chapter explains how the idea of the atom developed from ancient atomic theories to Dalton’s theory, Thomson’s model, Rutherford’s model, Bohr’s model, and the modern understanding of subatomic particles. Students should focus on atomic number, mass number, electronic configuration, valency, isotopes, isobars, and the reasons why atomic models changed with new experimental evidence. These NCERT Class 9 Science Solutions cover Pause and Ponder questions and all Class 9 Science Chapter 8 exercise solutions in a direct answer format.

Key Takeaways

• Atomic structure: Atoms contain electrons, protons, and neutrons.
• Atomic models: Thomson, Rutherford, and Bohr proposed different models based on evidence available at their time.
• Element identity: Atomic number decides the identity of an element.
• Valency: Valency depends on the number of electrons an atom gains, loses, or shares to become stable.

NCERT Solutions for Class 9 Science Exploration Chapter 8 Structure 2026

NCERT Solutions for Class 9 Science Exploration Chapter 8 In-Text Questions

Class 9 Science Exploration Chapter 8 uses atomic models, element symbols, electron distribution, and isotope examples to explain how scientists understood the structure of the atom.

Q1. What happens if a Thomson atom model has more negative charge than positive charge?

Question: Suppose you made your own atom as Thomson described, using clay for positive charge and beads for electrons. What will happen if the positive charge on the clay is lesser than the total negative charge of the beads?

Answer: The model will not represent a neutral atom. It will represent a negatively charged particle.

Explanation:
In Thomson’s model of atom Class 9, the atom is a positively charged sphere with electrons embedded in it.

For an atom to be neutral:

$Total , positive , charge = Total , negative , charge$

If the positive charge is less than the negative charge of electrons, the atom has excess negative charge. So, it becomes negatively charged.

Q2. What happens if the clay itself carries some negative charge?

Answer: The model will not represent a neutral atom unless the total positive charge still balances the total negative charge.

Explanation:
If the clay has some negative charge, the total negative charge increases.

For the model to remain neutral, the positive charge must exactly equal the combined negative charge of the clay and beads.

If not, the model becomes charged and does not represent a neutral atom.

Q3. Can an orange or lemon be a good comparison for Thomson’s model?

Answer: An orange or lemon can partly match Thomson’s model, but it is not a perfect comparison.

Explanation:
It matches Thomson’s idea because the seeds inside the soft pulp can represent electrons embedded in a positive sphere.

It falls short because seeds are usually grouped in specific regions, while Thomson’s model imagined electrons distributed throughout a uniform positive charge.

Also, an orange or lemon has peel, segments, and real structures that do not represent the atom.

Q4. Why did Thomson conclude that electrons are present in all atoms?

Answer: Thomson concluded that electrons are present in all atoms because cathode rays had the same nature regardless of the cathode material or gas used.

Explanation:
Thomson studied cathode rays in a discharge tube. He found that these rays were made of negatively charged particles.

The same type of particles appeared even when different materials and gases were used.

This showed that electrons are a fundamental part of all atoms.

Q5. What would happen if α-particles were replaced with negatively charged particles in Rutherford’s experiment?

Answer: Negatively charged particles would be attracted towards the positively charged nucleus and repelled by electrons.

Explanation:
In Rutherford model of atom Class 9, the nucleus is small, dense, and positively charged.

Alpha particles are positively charged, so they are repelled by the nucleus. If negatively charged particles were used instead, they would be attracted to the nucleus.

Their paths would still change, but the pattern would differ from alpha-particle scattering.

Q6. How did Rutherford’s result rule out Thomson’s plum pudding model?

Answer: The sharp backward deflection of a few alpha particles showed that positive charge and mass are concentrated in a tiny dense nucleus.

Explanation:
According to Thomson’s model, positive charge was spread throughout the atom. If that were true, alpha particles would pass through with only small deflections.

Rutherford observed that a few alpha particles bounced back sharply. This could happen only if they came close to a small, dense, positively charged centre.

So, Thomson’s model could not explain the gold foil experiment.

Q7. What question could you ask Rutherford about his work?

Answer: One question could be: “How did you realise that the large deflection of a few alpha particles meant the atom had a tiny dense nucleus?”

Explanation:
This question connects the observation with the scientific conclusion. Rutherford’s experiment was important because most alpha particles passed through, but a few bounced back.

That unusual result led to a major change in atomic structure Class 9 Science.

Q8. Assertion and Reason on Rutherford’s conclusion

Question:
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.

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

Explanation:
The assertion is true. Rutherford concluded that the atom has a small, dense nucleus.

The reason is also true because Thomson’s model described electrons embedded in a positive sphere.

However, Thomson’s model does not explain Rutherford’s conclusion. Rutherford’s conclusion came from the gold foil experiment.

Q9. Name a new element after yourself and justify its symbol using IUPAC rules.

Answer: If the element is named “Anirudhum”, a possible symbol could be An.

Explanation:
The first letter of an element symbol is written in uppercase. The second letter, if used, is written in lowercase.

So, An follows the rule because A is capital and n is small.

This is similar to symbols like Al for aluminium and Co for cobalt.

Q10. What problems could arise if every scientist used different symbols for the same element?

Answer: It would create confusion in scientific communication, chemical formulas, textbooks, research, and laboratory work.

Explanation:
Element symbols are internationally accepted. If different scientists used different symbols for the same element, the same substance could be written in many ways.

This could cause mistakes in experiments, medicines, chemical reactions, and scientific reports.

Standard symbols help scientists communicate clearly across languages and countries.

Q11. Find electrons, protons, and neutrons for atomic number 26 and 56 nucleons.

Answer: The atom has 26 protons, 26 electrons, and 30 neutrons.

Explanation:
Atomic number = number of protons

So:

$Protons = 26$

For a neutral atom:

$Electrons = Protons = 26$

Mass number = protons + neutrons

$56 = 26 + neutrons$

$Neutrons = 56 - 26 = 30$

Q12. Find neutrons if nucleus has 20 protons and mass number 41.

Answer: The atom has 21 neutrons.

Explanation:
Mass number:

$A = Protons + Neutrons$

$41 = 20 + Neutrons$

$Neutrons = 41 - 20 = 21$

Q13. Find mass number if atom has 18 neutrons and atomic number 17.

Answer: The mass number is 35.

Explanation:
Atomic number = number of protons

$Protons = 17$

Mass number:

$A = Protons + Neutrons$

$A = 17 + 18 = 35$

Q14. An atom $^{23}A$ has 11 electrons. Find the number of neutrons.

Answer: The atom has 12 neutrons.

Explanation:
For a neutral atom:

$Electrons = Protons$

So:

$Protons = 11$

Mass number:

$A = 23$

$Neutrons = Mass , number - Protons$

$Neutrons = 23 - 11 = 12$

Q15. Identify valence electrons in carbon, fluorine, and silicon.

Question: Identify the number of electrons in the outermost shell of $^{12}{6}C$, $^{19}{9}F$, and $^{28}_{14}Si$.

Answer: Carbon has 4 valence electrons, fluorine has 7 valence electrons, and silicon has 4 valence electrons.

Explanation:

For carbon:

Atomic number = 6

Electronic configuration = 2, 4

Valence electrons = 4

For fluorine:

Atomic number = 9

Electronic configuration = 2, 7

Valence electrons = 7

For silicon:

Atomic number = 14

Electronic configuration = 2, 8, 4

Valence electrons = 4

Q16. Write electronic configuration for atomic numbers 12, 16, and 18.

Answer: The electronic configurations are 2, 8, 2; 2, 8, 6; and 2, 8, 8.

Explanation:

The K-shell can hold 2 electrons, the L-shell can hold 8 electrons, and the M-shell fills after that for these elements.

Q17. Solve the riddle: mass number 23 and 11 protons.

Answer: The atom is sodium, and it has 12 neutrons.

Explanation:
Atomic number = number of protons

The atom has 11 protons, so its atomic number is 11.

Element with atomic number 11 is sodium.

Mass number:

$A = Protons + Neutrons$

$23 = 11 + Neutrons$

$Neutrons = 12$

Sodium is a soft metal and reacts vigorously with water.

Q18. Compare atoms with 11 protons and different neutrons.

Question: Two atoms have 11 protons each. One has 12 neutrons, and the other has 13 neutrons. How do their atomic numbers and mass numbers compare? Are they the same element?

Answer: They have the same atomic number but different mass numbers. They are isotopes of the same element.

Explanation:
Both atoms have 11 protons, so both have atomic number 11.

First atom:

$Mass , number = 11 + 12 = 23$

Second atom:

$Mass , number = 11 + 13 = 24$

They are the same element because the atomic number is the same. They are isotopes because the number of neutrons is different.

Q19. Calculate average atomic mass of bromine isotopes.

Question: Bromine has isotopes $^{79}{35}Br$ at 49.7% and $^{81}{35}Br$ at 50.3%. Calculate average atomic mass.

Answer: The average atomic mass of bromine is approximately $80.006 , u$.

Explanation:
Weighted average atomic mass:

$= \frac{79 \times 49.7}{100} + \frac{81 \times 50.3}{100}$

$= 39.263 + 40.743$

$= 80.006 , u$

So, the average atomic mass is about $80.0 , u$.

NCERT Solutions for Class 9 Science Exploration Chapter 8 Exercise Questions

The Revise, Reflect, Refine section checks atomic models, subatomic particles, electronic configuration, isotopes, isobars, valency, and atomic number calculations.

Q1. Choose the correct options for Rutherford’s gold foil experiment.

Answer: The correct options are (ii) and (iii).

Explanation:
Statement (i) is incorrect because the gold foil experiment did not show the existence of neutrons. Neutrons were discovered later by James Chadwick.

Statement (ii) is correct because Rutherford’s experiment disproved Thomson’s plum pudding model and led to the nuclear model of the atom.

Statement (iii) is correct because large deflection of a few alpha particles showed that most mass and positive charge are concentrated in a tiny centre.

Statement (iv) is incorrect because the experiment did not directly show electrons moving around the nucleus.

Q2. Check the statements according to Bohr’s atomic model.

Answer:

Q3. Explain the relation between X, Y, and Z.

Question:
X has 18 protons and 19 neutrons.
Y has 17 protons and 18 neutrons.
Z has 17 protons and 20 neutrons.

Q3(i). Relation between Y and Z

Answer: Y and Z are isotopes.

Explanation:
Y and Z both have 17 protons, so they have the same atomic number.

Y mass number:

$17 + 18 = 35$

Z mass number:

$17 + 20 = 37$

They have the same atomic number but different mass numbers, so they are isotopes.

Q3(ii). Relation between Z and X

Answer: Z and X are isobars.

Explanation:
Z mass number:

$17 + 20 = 37$

X mass number:

$18 + 19 = 37$

They have the same mass number but different atomic numbers. So, they are isobars.

Q4. What conclusion did Rutherford draw from alpha particles that bounced back?

Answer: Rutherford concluded that the atom has a very small, dense, positively charged nucleus at its centre.

Explanation:
Most alpha particles passed through the gold foil without deflection. This showed that most of the atom is empty space.

A few alpha particles were deflected strongly or bounced back. This showed that positive charge and most of the mass are concentrated in a tiny central region.

This region was called the nucleus.

Q5. Arrange the atomic models in chronological order.

Answer: The correct chronological order is:

1. Dalton’s model
2. Thomson’s model
3. Rutherford’s model
4. Bohr’s model

Explanation:
Dalton described atoms as indivisible particles.

Thomson proposed the plum pudding model with electrons embedded in a positive sphere.

Rutherford proposed the nuclear model with a small dense nucleus.

Bohr proposed fixed energy levels or shells for electrons.

Q6. Why do electrons not fly away from the atom?

Answer: Electrons do not fly away because they are attracted to the positively charged nucleus.

Explanation:
Electrons carry negative charge, while protons in the nucleus carry positive charge.

Opposite charges attract each other. This electrostatic attraction keeps electrons bound to the atom.

In Bohr’s model, electrons move in fixed energy shells and do not lose energy while moving in those shells.

Q7. Assertion and Reason on subatomic particles

Question:
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.

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

Explanation:
The assertion is true because the discovery of electrons, protons, and neutrons helped scientists understand atomic structure.

The reason is also true for a neutral atom.

However, the equality of electrons and protons does not explain why discovery of subatomic particles helped atomic theory. So, R is not the correct explanation of A.

Q8. Find protons, neutrons, electrons, and electron arrangement in magnesium.

Question: Magnesium has mass number 24 and atomic number 12.

Answer: Magnesium has 12 protons, 12 electrons, and 12 neutrons. Its electronic configuration is 2, 8, 2.

Explanation:
Atomic number = number of protons

$Protons = 12$

For a neutral atom:

$Electrons = 12$

Mass number:

$A = Protons + Neutrons$

$24 = 12 + Neutrons$

$Neutrons = 12$

Electronic configuration:

K-shell = 2

L-shell = 8

M-shell = 2

So, magnesium has electronic configuration 2, 8, 2.

Q9. Find element details from atomic structures in Fig. 8.17.

Answer: The answer depends on the number of electrons shown in each diagram. Use the completed format below.

Explanation:
To identify the element, count total electrons in the diagram. For a neutral atom:

$Atomic , number = Protons = Electrons$

Valence electrons are the electrons in the outermost shell. Valency is usually the number of electrons lost, gained, or shared to complete the octet.

Q10. Why did Rutherford’s model fail but Bohr’s model explain stability?

Answer: Rutherford’s model failed because it could not explain why revolving electrons do not lose energy and fall into the nucleus. Bohr’s model succeeded by proposing fixed energy shells.

Explanation:
In Rutherford’s model, electrons revolve around the nucleus like planets. A charged particle moving in a circular path should lose energy.

If electrons lost energy, they would spiral into the nucleus and the atom would collapse.

Bohr proposed that electrons move only in fixed shells with definite energy. While moving in these allowed shells, electrons do not lose energy.

This explained the stability of atoms.

Q11. An atom $^{70}X$ has 31 electrons. How many neutrons are there?

Answer: The atom has 39 neutrons.

Explanation:
For a neutral atom:

$Electrons = Protons = 31$

Mass number:

$A = 70$

$Neutrons = A - Protons$

$Neutrons = 70 - 31 = 39$

Q12. An atom has 79 protons and mass number 197. Find neutrons and electrons.

Answer: The atom has 118 neutrons and 79 electrons.

Explanation:
Given:

$Protons = 79$

$Mass , number = 197$

$Neutrons = 197 - 79 = 118$

For a neutral atom:

$Electrons = Protons = 79$

Q13. Complete the table.

Answer: The completed table is given below.

Explanation:
Atomic number gives the number of protons. In a neutral atom, electrons equal protons.

Mass number:

$A = Protons + Neutrons$

Q14. Element X has mass number 35 and 18 neutrons.

Q14(i). How many electrons and protons does element X have?

Answer: Element X has 17 protons and 17 electrons.

Explanation:
Mass number:

$A = Protons + Neutrons$

$35 = Protons + 18$

$Protons = 17$

For a neutral atom:

$Electrons = 17$

Q14(ii). What is its atomic number?

Answer: Its atomic number is 17.

Explanation:
Atomic number equals the number of protons.

Q14(iii). Identify element X.

Answer: Element X is chlorine.

Explanation:
The element with atomic number 17 is chlorine.

Q14(iv). Write its electronic configuration.

Answer: Its electronic configuration is 2, 8, 7.

Explanation:
Chlorine has 17 electrons.

K-shell = 2

L-shell = 8

M-shell = 7

Q14(v). How many valence electrons does it have?

Answer: Chlorine has 7 valence electrons.

Explanation:
The outermost shell of chlorine has 7 electrons.

Q14(vi). What will be the mass number if two neutrons are added?

Answer: The new mass number will be 37.

Explanation:
Original mass number = 35

Two neutrons are added:

$35 + 2 = 37$

Q14(vii). What is the relation of X with the new atom?

Answer: The new atom is an isotope of X.

Explanation:
Both atoms have the same number of protons but different numbers of neutrons.

So, they have the same atomic number but different mass numbers. This makes them isotopes.

Q15. What happens if electrons are replaced by heavier particles with the same charge?

Question: An atom has 12 protons and 12 neutrons. Electrons are replaced with particles that have the same charge as electrons but are 500 times heavier.

Answer: Atomic number and mass number remain the same, but atomic mass increases. Overall charge remains neutral if the number of new particles is still 12.

Explanation:

Atomic number depends only on protons. Mass number depends only on protons and neutrons.

The atom remains neutral because 12 positive protons are balanced by 12 negatively charged particles.

NCERT Solutions for Class 9 Science Exploration

Topics Covered in NCERT Solutions for Class 9 Science Exploration Chapter 8

Class 9 Science Exploration Chapter 8 covers the development of atomic theory and the structure of atoms. The chapter explains how evidence changed scientific models of the atom over time.

• Ancient ideas of parmanu and atomos
• Dalton’s atomic theory
• Cathode rays and discovery of electron
• Thomson model of atom Class 9
• Plum pudding model
• Rutherford gold foil experiment
• Rutherford model of atom Class 9
• Nucleus and empty space in atom
• Limitations of Rutherford’s model
• Discovery of proton
• Bohr model of atom Class 9
• Energy levels and shells
• Discovery of neutron
• Subatomic particles Class 9
• Symbols of elements and IUPAC rules
• Atomic number and mass number Class 9
• Electronic configuration Class 9
• Valency Class 9 Science
• Isotopes and isobars Class 9

Average atomic mass

Important Concepts in NCERT Solutions for Class 9 Science Exploration Chapter 8

Journey Inside the Atom Class 9 connects atomic models, subatomic particles, and element identity. Students should learn the difference between atomic number, mass number, valency, isotopes, and isobars clearly.

Important Formulas in NCERT Solutions for Class 9 Science Exploration Chapter 8

Class 9 Science Chapter 8 solutions use simple formulas for atomic number, mass number, neutrons, electronic configuration, and average atomic mass.