Intext Question & Answer Class 9 Science Exploration Chapter 8 Journey Inside the Atom

Think It Over (Page No. 140)

1. Are atoms the smallest indivisible particles?

Answer: No, atoms are not the smallest indivisible particles. Scientists discovered that atoms are made up of even smaller subatomic particles — electrons, protons, and neutrons. The discovery of electrons by J.J. Thomson (1897), protons by Rutherford, and neutrons by James Chadwick (1932) proved that atoms can be divided further. Even today, research continues, as protons and neutrons themselves are made of still smaller particles called quarks.

2. Why do electrons not fall into the nucleus even though they are attracted to protons in it?

Answer: According to Bohr’s model, electrons move in fixed circular paths called stationary states or shells (K, L, M, N…). While revolving in these fixed orbits, electrons do not lose energy. Bohr introduced the concept that in a stationary state, the energy of an electron remains constant, so it neither spirals inward nor falls into the nucleus. This is what keeps atoms stable.

3. Why did scientists keep modifying atomic models?

Answer: Scientists kept modifying atomic models because new experimental evidence kept disproving older models. For example:

• Dalton’s model (atom as indivisible) was disproved when electrons were discovered.
• Thomson’s plum pudding model was disproved by Rutherford’s gold foil experiment, which showed a concentrated nucleus.
• Rutherford’s model couldn’t explain why electrons don’t fall into the nucleus, so Bohr modified it.
• Even Bohr’s model had limitations and was later replaced by the quantum mechanical model.

Think It Over (Page No. 143)

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?

Answer: The model would no longer represent a neutral atom. Since the negative charge (beads) would be greater than the positive charge (clay), the overall charge of the model would be negative. It would represent a negatively charged ion, not a neutral atom. In a real neutral atom, the number of protons (positive charge) must always equal the number of electrons (negative charge).

(ii) by mistake, the clay itself carries a bit of negative charge? Would your model still represent a neutral atom?

Answer: No, the model would not represent a neutral atom. In this case, both the clay and the beads carry negative charge, making the total charge of the model entirely negative. There would be no positive charge to balance the negative charge of the beads. This completely contradicts Thomson’s idea, where the sphere must be positively charged to balance the negative electrons embedded in it.

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: How it matches Thomson’s model:

• The soft pulp is like the positive charge spread throughout the atom
• The seeds are like electrons embedded inside

So, it is similar to Thomson’s “plum pudding model”

Where it falls short:

• In an orange/lemon, seeds are not evenly spread, but in Thomson’s model electrons are uniformly distributed
• The fruit structure is not exactly like atomic structure

So, it is only a partial comparison, not fully accurate.

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

Answer: Thomson performed experiments on the conduction of electric current through gases at very low pressure using a cathode ray tube. He observed cathode rays moving from the cathode (negative electrode) to the anode (positive electrode).

He concluded that electrons are present in all atoms because:

• The cathode rays were found to be streams of negatively charged particles (electrons) with mass much smaller than atoms.
• Most importantly, the nature of cathode rays was the same regardless of the material used for the cathode or the type of gas filled in the tube.
• This meant that these particles (electrons) were not specific to any one element — they were a fundamental and universal component present in the atoms of every element.

Therefore, Thomson concluded that electrons are present in all atoms, making them a basic building block of matter.

Think It Over (Page No. 144)

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

Answer: If negatively charged particles were used instead of positively charged α-particles, the results would be completely different. Since the nucleus of gold atoms is positively charged, it would attract the incoming negatively charged particles instead of repelling them. As a result, the particles would bend towards the nucleus rather than passing straight through or bouncing back. There would be very little straight-through passage. The pattern of deflection would be opposite to what Rutherford observed, because the force between the particles and the nucleus would be attractive instead of repulsive.

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: According to Thomson’s model, the positive charge was spread uniformly throughout the atom. If this were true, the repulsive force on any passing α-particle would be very weak, and the particles should only be slightly deflected — never bounced back.
However, Rutherford found that a few α-particles bounced straight back, which means they hit an extremely strong concentrated repulsive force. This was only possible if the positive charge was packed into a very tiny, dense region at the centre of the atom.
Since Thomson’s model had no such concentrated region, it could not explain this observation at all. This single result was enough to completely disprove the plum pudding model and led Rutherford to propose the concept of the nucleus.

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

Answer: “Sir Rutherford, after discovering the nucleus, were you curious about what force holds all the positively charged protons together inside it — since like charges should repel each other?”

This is an important question because Rutherford’s model showed that protons are tightly packed in the nucleus, but like charges repel each other, so the nucleus should fly apart. Yet it remains extremely stable. This curiosity later led scientists to discover the nuclear force — a powerful force that binds protons and neutrons together inside the nucleus. This question reflects the spirit of scientific thinking — always looking beyond what is already known.

Think It Over (Page No. 145)

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

Assertion (A) is true:
Rutherford concluded from the gold foil experiment that most of the mass and positive charge of an atom are concentrated in a small central region called the nucleus.

Reason (R) is also true:
Thomson’s model stated that electrons are embedded in a uniformly distributed positive charge sphere (plum pudding model).

But R is NOT the correct explanation of A:
Rutherford’s conclusion was based on the gold foil experiment results, not because of Thomson’s model.

Therefore, the correct option is (ii).

Think It Over (Page No. 149)

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: Suppose my name is Aryan Kumar. I would name the newly discovered element “Aryanium” with the symbol “Ay”.

Justification that the symbol follows IUPAC rules:

• The symbol “Ay” is taken from the first two letters of the name “Aryanium”, which follows the IUPAC norm.
• The first letter “A” is written in uppercase and the second letter “y” is in lowercase, which strictly follows IUPAC rules.
• The symbol is short, unique, and internationally recognisable.

Hence, the symbol “Ay” for Aryanium correctly follows all IUPAC rules.

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

Answer: If every scientist used different symbols for the same element, it would create several serious problems:

• Communication confusion: Scientists from different countries would not understand each other’s work, making global scientific communication very difficult.
• Errors in experiments: Wrong symbols could lead to using incorrect substances in laboratories, causing failed experiments or dangerous accidents.
• Problems in chemical formulas: Writing and understanding chemical equations would become nearly impossible without standard symbols.
• Difficulty in research: Scientific papers shared globally could not be easily understood by scientists of other countries.

This is why IUPAC establishes universal standard symbols for all elements, so that chemistry remains a truly global and unified science.

Think It Over (Page No. 150)

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

Answer: Given:

Atomic number (Z) = 26
Number of nucleons (Mass number, A) = 56

Solution:

Number of protons = Atomic number = 26
Number of electrons = Number of protons = 26 (atom is neutral)
Number of neutrons = Mass number − Number of protons
Neutrons = 56 − 26 = 30

Protons = 26, Electrons = 26, Neutrons = 30
(This element is Iron — Fe)

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

Answer: Given:

Number of protons = 20
Mass number (A) = 41

Solution:

Mass number = Number of protons + Number of neutrons
41 = 20 + Number of neutrons
Number of neutrons = 41 − 20 = 21

Number of neutrons = 21
(This element is Calcium — Ca)

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

Answer: Given:

Number of neutrons = 18
Atomic number (Z) = 17
Therefore, Number of protons = 17

Solution:

Mass number = Number of protons + Number of neutrons
Mass number = 17 + 18 = 35

Mass number = 35
(This element is Chlorine — Cl)

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

Answer: Given:

Mass number (A) = 23
Number of electrons = 11
Therefore, Number of protons = 11 (atom is neutral)

Solution:

Mass number = Number of protons + Number of neutrons
23 = 11 + Number of neutrons
Number of neutrons = 23 − 11 = 12

Number of neutrons = 12
(This element is Sodium — Na)

Think It Over (Page No. 152)

14. Identify the number of electrons in the outermost shell of the following elements:

(i) ¹²₆C
(ii) ¹⁹₉F
(iii) ²⁸₁₄Si

Answer: (i) ¹²₆C (Carbon)

• Atomic number = 6, so total electrons = 6
• Electronic configuration = 2, 4
• K shell = 2, L shell = 4
• Electrons in outermost shell = 4

Answer: (ii) ¹⁹₉F (Fluorine)

• Atomic number = 9, so total electrons = 9
• Electronic configuration = 2, 7
• K shell = 2, L shell = 7
• Electrons in outermost shell = 7

Answer: (iii) ²⁸₁₄Si (Silicon)

• Atomic number = 14, so total electrons = 14
• Electronic configuration = 2, 8, 4
• K shell = 2, L shell = 8, M shell = 4
• Electrons in outermost shell = 4

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

Answer: Atomic number 12 — Magnesium (Mg)

• Total electrons = 12
• K shell (max 2) = 2
• L shell (max 8) = 8
• M shell = 2
• Electronic configuration = 2, 8, 2

Atomic number 16 — Sulfur (S)

• Total electrons = 16
• K shell (max 2) = 2
• L shell (max 8) = 8
• M shell = 6
• Electronic configuration = 2, 8, 6

Atomic number 18 — Argon (Ar)

• Total electrons = 18
• K shell (max 2) = 2
• L shell (max 8) = 8
• M shell = 8
• Electronic configuration = 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:

Given:

• Mass number = 23
• Number of protons = 11
• Therefore, Number of electrons = 11
• Number of neutrons = Mass number − Protons = 23 − 11 = 12

The element is Sodium (Na)

• It is indeed a soft metal that reacts vigorously with water.
• Number of neutrons = 12

Think It Over (Page No. 156)

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: Given:

• Atom 1: Protons = 11, Neutrons = 12
• Atom 2: Protons = 11, Neutrons = 13

Comparing Atomic Numbers:

• Atomic number of Atom 1 = 11
• Atomic number of Atom 2 = 11
• Both have the same atomic number = 11

Comparing Mass Numbers:

• Mass number of Atom 1 = Protons + Neutrons = 11 + 12 = 23
• Mass number of Atom 2 = Protons + Neutrons = 11 + 13 = 24
• Both have different mass numbers

Are they the same element or different?

They are the same element — Sodium (Na) because they have the same atomic number (11). However, since they have different mass numbers (23 and 24), they are isotopes of each other. Isotopes are atoms of the same element that have the same atomic number but different mass numbers due to different numbers of neutrons.

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

Answer: