Journey Inside the Atom - Chapter 8 Class 9 Exploration

Question 1
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.
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Correct: (ii) and (iii).
(i) Incorrect
— the experiment revealed the
nucleus
, not neutrons (neutrons were found later by Chadwick).
(ii) Correct
— results disproved the plum pudding model and led to the idea of a central nucleus.
(iii) Correct
— large deflection of a few α-particles showed mass and positive charge are packed into a tiny centre.
(iv) Incorrect
— the deflection was due to the positive nucleus, not because it showed electrons moving around.
Statement (gold foil experiment)
Verdict & reason
(i)
The experiment clearly showed the existence of neutrons in the nucleus.
Incorrect
— it revealed the
nucleus
. The neutron was found later, by Chadwick in 1932.
(ii)
The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom.
Correct
— a spread-out positive charge could never bounce an α-particle back.
(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.
Correct
— only a small, dense, heavy centre can turn α-particles through large angles.
(iv)
The way alpha particles were deflected showed that electrons move around the nucleus.
Incorrect
— the deflection was caused by the
positive nucleus
, not by the electrons.
Back to: 8.2.2 Testing Thomson’s model: The gold foil experiment

Question 2
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.
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(i) Incorrect
— in Bohr’s model, electrons in fixed orbits do
not
lose energy, so they do not fall in.
(ii) Incorrect
— electrons cannot exist anywhere; they have only
fixed, definite energies
.
(iii) Correct
— electrons revolve in orbits of fixed energy without losing energy.
(iv) Incorrect
— electrons cannot be found
between
energy levels.
Statement (Bohr’s model)
Verdict & reason
(i)
Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus.
Incorrect
— in a
stationary state
an electron does not lose energy, so it never falls in.
(ii)
Electrons can exist anywhere around the nucleus with no fixed energy.
Incorrect
— electrons exist only in
allowed shells
, each with a definite energy.
(iii)
Electrons revolve around the nucleus in orbits of fixed energy without losing energy.
Correct
— this is exactly Bohr’s postulate of stationary states.
(iv)
Electrons can be found between energy levels as they move around the nucleus.
Incorrect
— electrons revolve
only
in the allowed shells, never in between them.
Back to: 8.2.3 Bohr’s model of the atom

Question 3
The composition of the nuclei of three atomic species X, Y, and Z are given as follows.
Nucleus
X
Y
Z
Number of protons
18
17
17
Number of neutrons
19
18
20
Explain the relation between the following: (i) Y and Z (ii) Z and X.
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X: Z=18, A=18+19=37. Y: Z=17, A=17+18=35. Z: Z=17, A=17+20=37.
(i) Y and Z
— both have 17 protons (same atomic number) but different mass numbers (35 and 37) → they are
isotopes
.
(ii) Z and X
— different atomic numbers (17 and 18) but the same mass number (37) → they are
isobars
.
Same A versus Same Z
Same protons means
isotopes
Same mass number means
isobars
Back to: 8.9.2 Isobars

Question 4
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?
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The positive charge is
not
spread out; it is
concentrated
in a very small central region — the
nucleus
.
The nucleus is
tiny, dense
and carries almost all the
mass
and all the
positive charge
.
Only a hard, heavy, concentrated charge could make α-particles bounce straight back.
The Bounce-back
Conclusion
Positive charge is
concentrated
Nucleus is tiny and dense
It holds most of the mass
Back to: A. Rutherford’s model of an atom

Question 5
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.
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1. Dalton
(iv) — atoms are indivisible particles.
2. Thomson
(ii) — plum-pudding: electrons embedded in a positive sphere.
3. Rutherford
(iii) — atom has a dense central nucleus.
4. Bohr
(i) — electrons move in fixed orbits of definite energy.
Order:
(iv) → (ii) → (iii) → (i)
.
Order of Atomic Models
1. Dalton: indivisible atom 2. Thomson: plum pudding 3. Rutherford: nuclear model 4. Bohr: fixed energy levels
Back to: 8.2 A Short Historical Journey Through Atomic Models

Question 6
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
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The nucleus is
positively charged
and electrons are
negatively charged
.
The
electrostatic force of attraction
between them pulls the electrons toward the nucleus.
This attraction keeps electrons bound in their shells instead of flying away.
Why Electrons Stay
Nucleus (plus) attracts
electron (minus)
Electron moves in a fixed
shell
Attraction keeps it bound
Back to: 8.2.3 Bohr’s model of the atom

Question 7
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.
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Answer: (ii)
Both A and R are
true
.
But R (electrons = protons, i.e. neutrality) is
not
the correct explanation of A (why discovering particles helped understand structure).
Option
Verdict & why
(i)
Both A and R are true, and R is the correct explanation of A.
Incorrect
— R is true, but electron = proton count does not
explain
the discovery of subatomic particles.
(ii)
Both A and R are true, but R is not the correct explanation of A.
Correct answer
— A is true, R is true, but R is an independent fact, not the reason for A.
(iii)
A is true, but R is false.
Incorrect
— R is true: in a neutral atom, electrons equal protons.
(iv)
A is false, but R is true.
Incorrect
— A is true: discovering the electron, proton and neutron built our picture of the atom.
Back to: 8.5 Atomic Number

Question 8
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.
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(i) Protons = atomic number =
12
.
(ii) Neutrons = A − Z = 24 − 12 =
12
.
(iii) Electrons = protons =
12
.
Arrangement (electronic configuration) =
2, 8, 2
(K=2, L=8, M=2).
Magnesium (Z 12, A 24)
12 protons, 12 neutrons and 12 electrons; electronic configuration 2, 8, 2.
Back to: 8.6 Mass Number

Question 9
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
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Count the electrons in the shell diagram → that equals the
total electrons
, the
protons
and the
atomic number
(neutral atom).
Electrons in the
outermost shell
=
valence electrons
.
Valency
= electrons needed to reach an octet (or lost if fewer than 4 in the outer shell).
Match the atomic number to Table 8.4 to read the
name
and
symbol
.
Read each diagram: total electrons → Z → name & symbol; outer electrons → valence electrons & valency.
Reading a Shell Diagram
Count electrons equals atomic number Protons equal electrons (neutral atom) Outer electrons are valence electrons Valency from the octet rule
Back to: 8.8 Combining Capacity of an Atom: Valency

Question 10
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?
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Rutherford:
an orbiting electron accelerates, should continuously
lose energy
, spiral in and make the atom collapse.
Bohr:
introduced
stationary states
— in a fixed shell the electron’s energy stays
constant
, so it does not lose energy or spiral in.
Hence Bohr’s model explains the
stability
of the atom.
Rutherford versus Bohr
Rutherford: orbiting
electron loses energy
Bohr: stationary state,
energy constant
So only Bohr explains
stability
Back to: 8.2.3 Bohr’s model of the atom

Question 11
An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
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Electrons = protons =
31
(neutral atom), so Z = 31.
Mass number A = 70.
Neutrons = A − Z = 70 − 31 =
39
.
Finding Neutrons
Neutrons equal mass number minus protons; protons equal electrons for a neutral atom.
Back to: 8.6 Mass Number

Question 12
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
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(i) Neutrons = A − Z = 197 − 79 =
118
.
(ii) Electrons = protons =
79
(neutral atom). This element is gold, Au.
Neutrons and Electrons
Neutrons equal A minus Z equals 197 minus 79 equals 118; electrons equal protons equals 79.
Back to: 8.6 Mass Number

Question 13
Complete the Table 8.5:
Atomic number
Mass number
Number of neutrons
Number of protons
Number of electrons
Name of the elements
5
–
6
–
–
–
–
14
–
–
7
Nitrogen
–
24
–
12
–
–
15
–
16
–
–
–
–
1
0
–
–
–
Table 8.5
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Atomic number
Mass number
Neutrons
Protons
Electrons
Element
5
11
6
5
5
Boron
7
14
7
7
7
Nitrogen
12
24
12
12
12
Magnesium
15
31
16
15
15
Phosphorus
1
1
0
1
1
Hydrogen
Use Z = protons = electrons, A = protons + neutrons, and neutrons = A − Z.
Using the Table
Atomic number equals number of protons Mass number equals protons plus neutrons Neutrons equal A minus Z Protons equal electrons (neutral atom)
Back to: 8.6 Mass Number

Question 14
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?
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(i) Protons = A − neutrons = 35 − 18 = 17; electrons =
17
.
(ii) Atomic number =
17
.
(iii) Element X =
Chlorine (Cl)
.
(iv) Electronic configuration =
2, 8, 7
.
(v) Valence electrons =
7
.
(vi) New mass number (2 neutrons added) = 35 + 2 =
37
.
(vii) X and the new atom have the same Z but different A → they are
isotopes
.
Element X (A 35, n 18)
Protons equal 35 minus 18
equals 17
So it is chlorine (Z 17)
Config 2, 8, 7 means 7
valence electrons
Back to: 8.9.1 Isotopes

Question 15
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
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(i)
Atomic number
— unchanged (still 12; depends only on protons).
(ii)
Atomic mass
—
increases
(the new particles are much heavier than electrons).
(iii)
Mass number
— unchanged (counts only nucleons: protons + neutrons).
(iv)
Overall charge
— unchanged, still neutral (the charge of each particle is the same as an electron).
Heavier Electrons
Charge unchanged: same Z,
same overall charge
Mass number counts only
nucleons
But the atomic mass
increases
Back to: 8.3 What Components Contribute to the Mass of an Atom?

Project 1
Create an ‘Atomic Prediction Board’ game based on atomic number, mass number, electrons, protons, neutrons and valency. Students predict elements using atomic clues.
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Make cards with clues (e.g. “Z = 11, 12 neutrons, soft metal”). Players use
A=p+n
, Z = protons = electrons, and the 2n² rule to name the element (here, sodium). Award points for correct predictions and quick reasoning.

Project 2
Prepare a report on how the properties of atoms impact us in everyday life across healthcare, energy, agriculture and technology.
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Cover examples such as isotopes in medicine (
60
Co,
131
I), nuclear energy (
235
U), carbon dating (
14
C) in archaeology, and semiconductors (silicon) in technology. Link each to the atomic property that makes it useful.

Project 3
Create a role-play, stage play or story about the ‘Journey Inside the Atom’ and the scientists who contributed to identifying atomic structure.
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Give roles to Dalton, Thomson, Rutherford, Bohr and Chadwick. Each scientist presents their model and its key evidence, and the next improves on it — showing how science advances step by step.

Project 4
Use selected software or digital tools to create animations or simulations of various atomic models, and share them in class.
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Use free tools (e.g. PhET simulations, presentation animations) to show electrons filling shells, alpha-scattering, or model evolution. Present the animation and explain what each frame shows.

Project 5
Watch a film or documentary about the structure of the atom and write a report answering the given questions.
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Name the film/documentary and its main idea.
What it taught you about the atom and the atomic model(s).
Which scientists were mentioned and their contributions.
The most interesting part and one question you still have.

Project 6
Draw a bar graph showing the number of electrons in each energy level for any three elements.
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Pick three elements (e.g. Na 2,8,1; Cl 2,8,7; Ar 2,8,8). Plot shells (K, L, M) on the x-axis and electron count on the y-axis, drawing grouped bars for the three elements.

Project 7
To learn more about atoms, explore the given simulation links.
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Rutherford Scattering — https://phet.colorado.edu/en/simulations/rutherford-scattering
Isotopes and Atomic Mass — https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass
Try each simulation and note one new thing you learned from it.
🔑 The Quest Continues…
Is it possible to completely understand everything inside an atom?
Bohr’s model too was later found to be incomplete.
Electrons are now described as ‘electron clouds’.
The journey into the atom is far from over.
Exciting discoveries still lie ahead!