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Journey Inside the Atom

CBSE · Class 9 · Science

NCERT Solutions for Journey Inside the Atom — CBSE Class 9 Science.

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60 Questions Solved · 3 Sections

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Think It Over

1Are atoms the smallest indivisible particles?Show solution
No. The chapter says atoms were once thought to be indivisible, but later experiments showed that atoms are made of smaller subatomic particles like electrons, protons, and neutrons. So atoms are not the smallest indivisible particles.

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2Why do electrons not fall into the nucleus even though they are attracted to protons in it?Show solution
In Bohr's model, electrons move in fixed energy levels or shells called stationary states. While moving in one allowed shell, an electron does not lose energy, so it does not spiral into the positively charged nucleus. Rutherford's model could not explain this stability, but Bohr's model did.

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3Why did scientists keep modifying atomic models?Show solution
Scientists kept modifying atomic models because new experiments gave new evidence that earlier models could not explain. For example, the gold foil experiment showed that Thomson's model was incorrect, and Rutherford's model still could not explain atomic stability. So models were improved step by step as science advanced.

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Pause and Ponder

1(i)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:Show solution
If the positive charge in the clay is less than the total negative charge of the beads, the model would not be neutral; it would have a net negative charge. If the clay itself carries a little negative charge, that would reduce the positive charge further, so it would again fail to represent a neutral atom. A neutral atom needs total positive and negative charges to be equal.

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2Could 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?Show solution
Yes, an orange or a lemon can be a rough comparison because, like Thomson's idea, it has seeds embedded inside soft pulp. The pulp can represent the positive charge and the seeds the electrons. But it falls short because in Thomson's model the electrons are spread throughout the positive sphere, not just placed in one region, and the comparison does not explain the exact balance of charge in an atom.

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3Why did Thomson conclude that electrons are present in all atoms?Show solution
Thomson concluded that electrons are present in all atoms because the nature of cathode rays was independent of the material of the cathode and the gas in the tube. This showed that the negatively charged particles were a fundamental component of all atoms, not just of one particular substance.

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4What do you think would happen if α\alpha-particles were replaced with negatively charged particles in Rutherford's gold foil experiment?Show solution
If negatively charged particles were used instead of alpha particles, they would be repelled by the electrons in the atom and attracted toward the positive nucleus. Their path would be affected very differently from alpha particles. Since Rutherford's experiment depended on the behavior of positively charged alpha particles, replacing them with negative particles would not give the same scattering observations.

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5Rutherford found that a few α\alpha-particles bounced back sharply. How does this single surprising result completely rule out Thomson's 'plum pudding model' of the atom?Show solution
A few alpha particles bouncing back showed that the atom could not be a uniform sphere of positive charge as Thomson proposed. If positive charge were spread evenly, no alpha particle would have met a very concentrated, massive centre and bounced back sharply. The result proved that most of the atom is empty space and that nearly all the positive charge and mass are concentrated in a tiny nucleus. That contradicts the plum pudding model completely.

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6If you could ask Rutherford one question about his work, what would it be?Show solution
A good question would be: How did you infer that the atom has a tiny, dense nucleus from the scattering of only a few alpha particles? This would help understand the reasoning behind Rutherford's conclusion.

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7Assertion (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:Show solution
Both statements are true. Rutherford did conclude that most of the mass is concentrated in a small central nucleus, and Thomson's model did describe electrons embedded in a uniformly distributed positive sphere. However, R is not the correct explanation of A; Rutherford's conclusion came from the gold foil experiment, not from Thomson's description. So the correct option is (ii).

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8Imagine 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.Show solution
One possible name is Chatgptium with symbol Cg. This follows IUPAC-style rules because the symbol has two letters, the first letter is capital and the second is lowercase. If I chose a symbol, I would make sure it uses the element name properly and follows the convention of writing symbols with a capital first letter and a lowercase second letter.

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9What problems could arise if every scientist used different symbols for the same element?Show solution
If different scientists used different symbols for the same element, it would create confusion and make communication difficult. Scientists around the world would not know whether the same symbol referred to the same element, so it would be hard to compare results, write formulas, and share research clearly.

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10An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.Show solution
Atomic number Z=26Z = 26, so the atom has 26 protons. Since the atom is neutral, it also has 26 electrons. The mass number (nucleons) is 56, so

neutrons=56−26=30.\text{neutrons} = 56 - 26 = 30.

So the atom has 26 electrons, 26 protons, and 30 neutrons.

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11The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.Show solution
Mass number A=41A = 41 and protons =20= 20.

Using

neutrons=A−Z=41−20=21,\text{neutrons} = A - Z = 41 - 20 = 21,

the number of neutrons is 21.

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12An atom has 18 neutrons and an atomic number of 17. What is its mass number?Show solution
Atomic number Z=17Z = 17, so protons = 17. Neutrons = 18.

Mass number

A=p+n=17+18=35.A = p + n = 17 + 18 = 35.

So the mass number is 35.

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13An atom 23A^{23}\mathrm{A} has 11 electrons. Find the number of neutrons in it.Show solution
The atom has mass number A=23A = 23 and 11 electrons. Since the atom is neutral, it has 11 protons. Then

neutrons=A−Z=23−11=12.\text{neutrons} = A - Z = 23 - 11 = 12.

So the number of neutrons is 12.

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14Identify the number of electrons in the outermost shell of the following elements:Show solution
Use the electronic configurations from the chapter:
- Carbon, Z=6Z=6: 2,42,4 so outermost shell has 4 electrons.
- Fluorine, Z=9Z=9: 2,72,7 so outermost shell has 7 electrons.
- Silicon, Z=14Z=14: 2,8,42,8,4 so outermost shell has 4 electrons.

So the numbers are 4, 7, 4.

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15Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.Show solution
From the table in the chapter:
- Atomic number 12 (magnesium) has electronic configuration 2, 8, 2.
- Atomic number 16 (sulfur) has electronic configuration 2, 8, 6.
- Atomic number 18 (argon) has electronic configuration 2, 8, 8.

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16Solve 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.Show solution
An atom with 11 protons has atomic number 11, which is sodium (Na). Its mass number is 23, so

neutrons=23−11=12.\text{neutrons} = 23 - 11 = 12.

So the atom is sodium and it has 12 neutrons. A suitable riddle of this type could be made using mass number, atomic number, and a property of the element.

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17Two 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?Show solution
Both atoms have the same number of protons, so their atomic number is the same: 11. But their mass numbers are different:
- First atom: 11+12=2311 + 12 = 23
- Second atom: 11+13=2411 + 13 = 24

Since they have the same atomic number but different mass numbers, they are isotopes of the same element, not different elements.

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18If a bromine atom is available in the form of, say two isotopes, 3579Br^{79}_{35}\mathrm{Br} (49.7%) and 3581Br^{81}_{35}\mathrm{Br} (50.3%), calculate the average atomic mass of the bromine atom.Show solution
Use the weighted average:

Average atomic mass=79×49.7100+81×50.3100\text{Average atomic mass} = 79\times \frac{49.7}{100} + 81\times \frac{50.3}{100}

=79(0.497)+81(0.503)= 79(0.497) + 81(0.503)

=39.263+40.743= 39.263 + 40.743

=80.006 u≈80.0 u= 80.006\,\text{u} \approx 80.0\,\text{u}

So the average atomic mass is about 80.0 u. Using rounding, this is commonly written as 79.99 u or 80.0 u.

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1Choose the correct option:Show solution
The question asks for the correct option to the Rutherford gold foil assertion-reason item. Rutherford concluded that most of the mass of an atom is concentrated in a small central nucleus, and the statement about Thomson's model is also true. However, the reason is not the correct explanation of the assertion. Therefore the correct option is (ii). If the printed option list is the standard one from the chapter, mark (ii) as correct.

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2Which of the following statements are correct or incorrect according to the Bohr's atomic model? Give a reason for each statement.Show solution
According to Bohr's atomic model:

- (i) Incorrect — electrons do not lose energy while moving in fixed orbits, so they do not slowly fall into the nucleus.
- (ii) Incorrect — electrons cannot exist anywhere around the nucleus with no fixed energy; they occupy fixed energy levels.
- (iii) Correct — electrons revolve around the nucleus in orbits of fixed energy without losing energy.
- (iv) Incorrect — electrons are not found between energy levels; they can exist only in the allowed shells.

So only statement (iii) is correct.

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3(i)Explain the relation between the following:Show solution
The relation is:

- Y and Z: They have the same number of protons, 17, but different numbers of neutrons, 18 and 20. So they are isotopes of the same element.
- Z and X: X has 18 protons and Z has 17 protons, but both have the same mass number:
- X: 18+19=3718 + 19 = 37
- Z: 17+20=3717 + 20 = 37
So they are isobars.

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5Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved over time.Show solution
The correct chronological order is:

1. Dalton's model — atoms were indivisible particles.
2. Thomson's model — atom as a sphere of positive charge with electrons embedded in it.
3. Rutherford's model — atom has a dense central nucleus and electrons around it.
4. Bohr's model — electrons move in fixed orbits with definite energy.

So the order is (iv) → (ii) → (iii) → (i).

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Revise, Reflect, Refine

1(i)The experiment clearly showed the existence of neutrons in the nucleus.Show solution
Rutherford's gold foil experiment showed the presence of a small, dense nucleus and that most of the atom is empty space. It did not show the existence of neutrons; neutrons were discovered later by James Chadwick in 1932.

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1(iv)The way alpha particles were deflected showed that electrons move around the nucleus.Show solution
The scattering pattern showed the existence of a small nucleus and that most of the atom is empty space. It did not directly show that electrons move around the nucleus; that was part of Rutherford's model, but the experiment itself did not prove it.

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2(i)Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.Show solution
In Bohr's model, electrons move in fixed energy orbits and do not lose energy while in those allowed shells. So they do not slowly fall into the nucleus.

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2(ii)Electrons can exist anywhere around the nucleus with no fixed energy.Show solution
According to Bohr's model, electrons cannot exist anywhere around the nucleus. They can exist only in fixed shells or energy levels.

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2(iii)Electrons revolve around the nucleus in orbits of fixed energy without losing energy.Show solution
Bohr stated that electrons revolve around the nucleus in allowed fixed shells or stationary states and do not lose energy while staying in those orbits.

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2(iv)Electrons can be found between energy levels as they move around the nucleus.
3(i)Y and Z
3(ii)Z and X
5(i)Bohr's model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.
5(ii)Thomson's model depicted the atom as a 'plum pudding' with electrons embedded in a sphere of positive charge.
5(iii)Rutherford's model proposed that atoms have a dense central nucleus.
5(iv)Dalton's model described atoms as indivisible particles.
6Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
7Assertion (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:
8Magnesium 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.
9Find the following information for the elements shown in Fig. 8.17:
10Both 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?
11An atom 70X^{70}\text{X} has 31 electrons. How many neutrons are there in its nucleus?
12An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
13Complete the Table 8.5:
14(i)How many electrons and protons does element X have?
14(ii)What is its atomic number?
14(iii)Identify the element X.
14(iv)Write its electronic configuration.
14(v)How many valence electrons does it have?
14(vi)What will be the mass number if two neutrons are added to its nucleus?
14(vii)What will be the relation of X with the new atom?
15(i)Atomic number
15(ii)Atomic mass
15(iii)Mass number
15(iv)Overall charge
1(i)Choose the correct options and explain the reason for the correct and incorrect options in the context of Ernest Rutherford's gold foil experiment:
3(i)The composition of the nuclei of three atomic species X, Y, and Z are given as follows.
14(i)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:
15In 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:

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Frequently Asked Questions

What are the important topics in Journey Inside the Atom for CBSE Class 9 Science?
Key topics in Journey Inside the Atom include Force and Laws of Motion — Complete Concept Map, Correct vs Incorrect Understanding of Newton's First Law, Chapter Overview — Force and Laws of Motion. These are the concepts CBSE Class 9 examiners draw on most — study them first, then practise related questions.
How to score full marks in Journey Inside the Atom — CBSE Class 9 Science?
Understand the core concepts first, then work through the 111 practice questions available for this chapter. Revise formulas and definitions regularly, and use flashcards for quick recall before the exam.
Where can I get free NCERT Solutions for Journey Inside the Atom Class 9 Science?
This page has free step-by-step NCERT Solutions for every exercise question in Journey Inside the Atom (CBSE Class 9 Science) — written the way examiners award marks: given, formula, working, answer.

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