Class 9 Science NCERT Solution chapter 8 Journey Inside the Atom

Revise, Reflect, Refine — Complete Solutions

🟦 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.

🟩 ANSWER

Correct options: (ii) and (iii)

(i) Incorrect: Rutherford’s experiment did not establish the existence of neutrons. James Chadwick discovered the neutron later, in 1932.

(ii) Correct: Thomson’s model predicted that positive charge was spread uniformly throughout the atom. The sharp deflection of some alpha particles could not be explained by this model. Therefore, the experiment disproved the plum-pudding model and led to the idea of a central nucleus.

(iii) Correct: Only a very small number of alpha particles were deflected through large angles or bounced back. This showed that almost all the positive charge and most of the atomic mass are concentrated in a very small, dense central region called the nucleus.

(iv) Incorrect: The experiment provided evidence for the nucleus, but the deflection of alpha particles did not directly prove that electrons move around it.

🟦 QUESTION 2

Which of the following statements are correct or incorrect according to 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.

🟩 ANSWER

(i) Incorrect

According to Bohr’s model, electrons moving in permitted fixed orbits do not lose energy and therefore do not fall into the nucleus.

(ii) Incorrect

Electrons cannot exist randomly anywhere around the nucleus. They occupy specific permitted shells or energy levels.

(iii) Correct

Electrons revolve in fixed energy levels called stationary states, shells or orbits without radiating or losing energy.

(iv) Incorrect

Electrons cannot remain between two permitted energy levels. They move from one shell to another only by absorbing or releasing a fixed amount of energy.

🟦 QUESTION 3

The composition of the nuclei of three atomic species X, Y and Z is given below:

Atomic species	X	Y	Z
Number of protons	18	17	17
Number of neutrons	19	18	20

Explain the relationship between:

(i) Y and Z
(ii) Z and X

🟩 ANSWER

The mass number is calculated as: $A=\text{Number of protons}+\text{Number of neutrons}$ A=Number of protons+Number of neutrons

For X: $A_X=18+19=37$ AX=18+19=37

For Y: $A_Y=17+18=35$ AY=17+18=35

For Z: $A_Z=17+20=37$ AZ=17+20=37

(i) Relationship between Y and Z

Y and Z have:

The same number of protons = 17
Different numbers of neutrons
Different mass numbers: 35 and 37

Therefore, Y and Z are isotopes of the same element.

(ii) Relationship between Z and X

Z and X have:

The same mass number = 37
Different atomic numbers: 17 and 18

Therefore, Z and X are 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?

🟩 ANSWER

Rutherford concluded that:

The positive charge of an atom is not spread uniformly throughout it.
It is concentrated in an extremely small region at the centre, called the nucleus.
The nucleus is positively charged.
It is very dense and contains most of the mass of the atom.
The nucleus occupies only a tiny fraction of the total volume of the atom.
Most of the atom is empty space.

A few alpha particles bounced back because they came very close to the small, dense and positively charged nucleus and experienced strong repulsion.

🟦 QUESTION 5

Explain and arrange the following statements in the correct chronological order to show how atomic models evolved over time:

(i) Bohr’s model proposed that electrons move in fixed orbits around the nucleus, each with 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.

🟩 ANSWER

The correct chronological order is: $\boxed{(iv)\rightarrow(ii)\rightarrow(iii)\rightarrow(i)}$ (iv)→(ii)→(iii)→(i)

1. Dalton’s atomic model

Dalton described atoms as tiny, solid and indivisible particles. His model did not include any internal structure.

2. Thomson’s atomic model

After discovering electrons, Thomson proposed that an atom is a positively charged sphere with negatively charged electrons embedded in it.

3. Rutherford’s atomic model

The gold foil experiment showed that an atom contains a tiny, dense and positively charged nucleus at its centre. Electrons were proposed to revolve around this nucleus.

4. Bohr’s atomic model

Bohr proposed that electrons revolve around the nucleus only in fixed shells or energy levels without losing energy.

🟦 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.

🟩 ANSWER

Electrons carry a negative charge, while the nucleus is positively charged because it contains protons.

Opposite electric charges attract each other. Therefore, the electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus keeps the electrons bound to the atom.

According to Bohr’s model, electrons move in permitted fixed energy levels around the nucleus and do not continuously lose energy while moving in them.

Thus, electrons neither fly away nor normally collapse into the nucleus.

🟦 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.

🟩 ANSWER

Correct option: (ii)

Both the Assertion and Reason are true, but the Reason is not the correct explanation of the Assertion.

The discovery of electrons, protons and neutrons revealed that atoms have an internal structure.

In a neutral atom, the number of electrons equals the number of protons, but this fact alone does not explain how the discovery of subatomic particles helped scientists understand the complete structure of the atom.

🟦 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

Also illustrate the arrangement of electrons in a magnesium atom.

🟩 ANSWER

Given: $A=24,\qquad Z=12$ A=24,Z=12

(i) Number of protons

$\text{Protons}=Z=12$ Protons=Z=12

(ii) Number of neutrons

$\text{Neutrons}=A-Z$ Neutrons=A−Z $=24-12=12$ =24−12=12

(iii) Number of electrons

A magnesium atom is electrically neutral. $\text{Electrons}=\text{Protons}=12$ Electrons=Protons=12

Electronic configuration

The 12 electrons are distributed as:

K shell = 2 electrons
L shell = 8 electrons
M shell = 2 electrons

$\boxed{2,8,2}$ 2,8,2

Final Answer:

Protons = 12
Neutrons = 12
Electrons = 12
Electronic configuration = 2, 8, 2

🟦 QUESTION 9

Find the following information for each element shown in Fig. 8.17:

(i) Name of the element
(ii) Symbol
(iii) Total number of electrons
(iv) Number of valence electrons
(v) Valency
(vi) Number of protons
(vii) Atomic number

🟩 ANSWER

The shell diagrams in Fig. 8.17 on page 159 represent lithium, nitrogen, magnesium and oxygen.

Diagram	Element	Symbol	Electronic configuration	Total electrons	Valence electrons	Valency	Protons	Atomic number
(a)	Lithium	Li	2, 1	3	1	1	3	3
(b)	Nitrogen	N	2, 5	7	5	3	7	7
(c)	Magnesium	Mg	2, 8, 2	12	2	2	12	12
(d)	Oxygen	O	2, 6	8	6	2	8	8

How valency is determined

Lithium loses 1 electron; therefore, its valency is 1.
Nitrogen gains or shares 3 electrons to complete its octet; therefore, its valency is 3.
Magnesium loses 2 electrons; therefore, its valency is 2.
Oxygen gains or shares 2 electrons; therefore, its valency is 2.

🟦 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?

🟩 ANSWER

Limitation of Rutherford’s model

According to classical physics, a charged particle moving in a circular path is continuously accelerating.

Therefore, an electron revolving around the nucleus should continuously radiate energy. As it loses energy, it should move closer to the nucleus in a spiral path and eventually fall into it.

If this happened, atoms would collapse and matter would not remain stable. Rutherford’s model could not explain why this does not occur.

Bohr’s explanation

Bohr proposed that:

Electrons move only in certain permitted fixed energy levels or stationary orbits.
An electron does not lose energy while moving in a permitted orbit.
Energy is absorbed or released only when the electron jumps from one energy level to another.

Therefore, Bohr’s model successfully explained the stability of atoms.

🟦 QUESTION 11

An atom 70X{}^{70}X70X has 31 electrons. How many neutrons are present in its nucleus?

🟩 ANSWER

Since the atom is neutral: $\text{Number of protons}=\text{Number of electrons}=31$ Number of protons=Number of electrons=31

Therefore, its atomic number is: $Z=31$ Z=31

Mass number: $A=70$ A=70

Number of neutrons: $n=A-Z$ n=A−Z $n=70-31$ n=70−31 $\boxed{n=39}$ n=39

Final Answer: The atom contains 39 neutrons.

🟦 QUESTION 12

An atom has 79 protons and a mass number of 197. Calculate:

(i) The number of neutrons
(ii) The number of electrons

🟩 ANSWER

Given: $Z=79,\qquad A=197$ Z=79,A=197

(i) Number of neutrons

$n=A-Z$ n=A−Z $n=197-79$ n=197−79 $\boxed{n=118}$ n=118

(ii) Number of electrons

For a neutral atom: $\text{Electrons}=\text{Protons}$ Electrons=Protons $\boxed{\text{Electrons}=79}$ Electrons=79

Final Answer:

Neutrons = 118
Electrons = 79

🟦 QUESTION 13

Complete Table 8.5:

Atomic number	Mass number	Number of neutrons	Number of protons	Number of electrons	Name of element
5	—	6	—	—	—
—	14	—	—	7	Nitrogen
—	24	—	12	—	—
15	—	16	—	—	—
—	1	0	—	—	—

🟩 ANSWER

For a neutral atom: $Z=\text{Number of protons}=\text{Number of electrons}$ Z=Number of protons=Number of electrons

and $A=\text{Protons}+\text{Neutrons}$ A=Protons+Neutrons

The completed table is:

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

🟦 QUESTION 14

Aman was discussing the structure of an 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 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 relationship of X with the new atom?

🟩 ANSWER

Given: $A=35,\qquad n=18$ A=35,n=18

(i) Number of protons and electrons

$p=A-n$ p=A−n $p=35-18=17$ p=35−18=17

Since the atom is neutral: $e=p=17$ e=p=17

Therefore:

Protons = 17
Electrons = 17

(ii) Atomic number

$Z=\text{Number of protons}=17$ Z=Number of protons=17

(iii) Identification of element X

The element with atomic number 17 is chlorine.

Its symbol is Cl.

(iv) Electronic configuration

Chlorine has 17 electrons: $\boxed{2,8,7}$ 2,8,7

(v) Valence electrons

The outermost shell contains: $\boxed{7\text{ valence electrons}}$ 7 valence electrons

(vi) New mass number after adding two neutrons

$A_{\text{new}}=35+2=37$ Anew=35+2=37

(vii) Relationship between X and the new atom

Both atoms have the same atomic number, 17, but different mass numbers, 35 and 37.

Therefore, they are isotopes of chlorine.

🟦 QUESTION 15

In an atom, there are 12 protons and 12 neutrons in the nucleus. Imagine that all the electrons are replaced with 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

🟩 ANSWER

The original atom has:

Protons = 12
Neutrons = 12
Electrons = 12

Its mass number is: $A=12+12=24$ A=12+12=24

(i) Atomic number

Atomic number depends only on the number of protons.

Since the number of protons remains 12: $\boxed{\text{Atomic number remains }12}$ Atomic number remains 12

(ii) Atomic mass

The replacement particles are 500 times heavier than ordinary electrons.

Therefore, the total actual mass of the atom will increase, because the particles outside the nucleus now contribute much more mass than normal electrons. $\boxed{\text{Atomic mass increases}}$ Atomic mass increases

(iii) Mass number

Mass number is defined as: $A=\text{Number of protons}+\text{Number of neutrons}$ A=Number of protons+Number of neutrons

The numbers of protons and neutrons have not changed. $\boxed{\text{Mass number remains }24}$ Mass number remains 24

(iv) Overall charge

Each hypothetical particle has the same negative charge as an electron.

There are still 12 negatively charged particles balancing 12 positively charged protons. $\boxed{\text{Overall charge remains zero}}$ Overall charge remains zero

Therefore, the atom remains electrically neutral.

Final Answer

Property	Effect
Atomic number	Remains 12
Atomic mass	Increases
Mass number	Remains 24
Overall charge	Remains zero or neutral