Download Journey Inside the Atom Class 9 Science Chapter 8 Questions and Answers PDF

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NCERT Solution for Class 9 Science Journey Inside the Atom (In-text Exercise)

Refer to the detailed NCERT Solutions Class 9 Science Chapter 8 Journey Inside the Atom. These solutions help students understand the concepts easily. The NCERT Solutions for Class 9 Science are prepared by experts in a very clear and comprehensive manner.

Class 9 Science Chapter 8 Journey Inside the Atom (Think it Over Questions)

Refer to the detailed Think it over questions of NCERT Class 9 Science Chapter 8 Journey Inside the Atom.

Page 140

Question: Are atoms the smallest indivisible particles?

Answer:

No, they are not the smallest particles because inside atoms protons, electrons and neutrons reside which are even smaller. Hence, an atom is not the smallest indivisible particle.

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

Answer:

Electrons reside in fixed energy levels around the nucleus due to high stability of these energy levels they do not fall into the nucleus.

Question: Why did scientists keep modifying atomic models?

Answer:

Due to continuous experiments and the inability of previous models to explain all the properties and behaviour of the atom scientists keep modifying atomic models.

Class 9 Science Chapter 8 Journey Inside the Atom (Pause and Pounder)

Below are detailed pause and ponder questions from the journey inside the matter. They are prepared by experts to help students understand the concepts easily.

Page: 143

Question 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?

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

Answer:

(i) Atoms are neutral if the positive charge on the clay is less than the negative charge then after balancing the atom would be negatively changed.

(ii) Atoms are neutral, meaning the number of electrons and protons are equal. If the clay carries negative charge then atoms become negative due to addition of that charge in it. Hence, it will not represent the neutral atom.

Question 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:

Orange and lemon can partially represent Thomson model of atoms in orange and lemon pulp represents a sphere which is positively charged and seeds represent electrons which are negatively charged. But there are noticeable differences in the JJ Thomson atomic model and representing lemon and orange as that model because seeds are not uniformly distributed, pulp do not carry positive charge and in real atoms and electrons are very small however in lemon and orange seeds are very large.

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

Answer:

On the basis of cathode ray tube experiment JJ Thomson concluded that electrons are present in all atoms. From his cathode ray tube experiment he noticed that the same type of negatively charged particles are produced in the discharge tube no matter what the gas used and what is the material of the electrode. From this observation he concluded that electrons are present in every atom.

Page -144

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

Answer:

In Rutherford gold foil experiment if α particles are replaced by negatively charged particles then due to positive charge of the nucleus they will be attracted towards the nucleus leading to the change in the path of particles and in place of passing straight through it these particles might even move toward the nucleus.

Question 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:

In plum pudding model JJ Thomson explained that positive charge is spread evenly across the atom, if it is so then α-particles passing through the atom should experience only small deflections. Later Rutherford observed that some particles are bouncing back sharply, it can only happen when whole mass and charge is spread in a very small region. Leading to rule out Thomson’s plum pudding model of the atom.

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

Answer:

It would be about how he felt when he proved that an atom has a tiny dense nucleus.

Page - 145

Question 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:

Both Assertion and Reasons are correct but Reason does not gives explanation of assertion.

Rutherford concluded that most of the mass of an atom is concentrated in a small region at the centre called the nucleus because of the failure of the JJ Thomson model to explain the bouncing back of α-particles.

Page - 149

Question 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:

If I discover a new element, I would name it Garimium, its symbol would be Gr. This name is unique and follows all the IUPAC rules.

Question 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 then it will create confusion and difficulty to communicate. A common system is important due to uniformity and ensures that symbols written anywhere in the world mean the same thing to everyone.

Page- 150

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

Answer:

Atomic Number- 26

Number of nucleons - 56

Number of protons = Atomic number

Number of protons = 26

For a neutral atom no. of electrons and no of protons are equal

Number of electrons = 26

Number of neutrons= 56-26

Number of neutrons=30

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

Answer:

Mass number = Protons+Neutrons

No. of protons=20

Mass no. =41

Neutrons = Mass no. - Protons

Neutrons=41-20

No. of Neutrons= 21

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

Answer:

No. of Neutrons=18

Atomic number=17

Mass no. = Protons + Neutrons

Atomic no. = No. of protons

No. of protons = 17

Mass no. = 17+18

Mass no. =35

Question 13: An atom ${ }^{23} \mathrm{~A}$ has 11 electrons. Find the number of neutrons in it

Answer:

Mass no. = 23

No. of electrons = 11

Atoms are neutral so no. of protons and no. of electrons must be equal.

No. of protons = 11

No. of neutrons = Mass no. - No. of protons

No. of neutrons = 23-11

No. of neutrons = 12

Page -152

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

(i) ${ }_6^{12} \mathrm{C}$

(ii) ${ }_9^{19} \mathrm{~F}$

(iii) ${ }_{14}^{28} \mathrm{Si}$

Answer:

(i) No. of Electrons = 6

Electronic configuration = 2, 4

No. of electrons in outermost cell = 4

(ii) No. of Electrons = 9

Electronic configuration = 2, 7

No. of electrons in outermost cell = 7

(iii) No. of Electrons = 14

Electronic configuration = 2, 8, 4

No. of electrons in outermost cell = 4
Question 15: Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.

Answer:

Atomic No. = 12

No. of Electrons = 12

Electronic configuration = 2, 8, 2

Atomic No. = 16

No. of Electrons = 16

Electronic configuration = 2, 8, 6

Atomic No. = 18

No. of Electrons = 18

Electronic configuration = 2, 8, 8

Question 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:

Mass number = 23

No. of protons = 11

No. of protons = Atomic number

Atomic number = 11

Atomic no. of atoms is 11 hence it is sodium. Sodium is a soft metal and reacts vigorously with water.

No. of neutrons = Mass no. - No. of protons

No. of neutrons = 23 -11

No. of neutrons = 12

Page -156

Question 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:

No. of protons = 11

No. of neurons in 1st = 12

No. of neurons in 2nd = 13

Both have 11 protons so atomic number is same = 11

Atomic mass of 1st = 11 + 12 =23

Atomic mass of 2nd = 11 + 13 =24

Both elements are the same because of the same atomic number but due to different mass numbers there are different isotopes of the same element.

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

Answer:

Isotope 1

${ }_{35}^{79} \mathrm{Br}(49.7 \%)$

Mass = 79u

Abundance = 49.7%

Isotope 2

${ }_{35}^{81} \mathrm{Br}(50.3 \%)$

Mass = 81u

Abundance = 50.3%

Average atomic mass = ((Mass of Isotope 1 x Abundance) + (Mass of Isotope 2 x Abundance))/100

Average atomic mass = ((79 x 49.7) + (81 x 50.3))/100

= 8000.6/100

=80.006u

Class 9 Science Chapter 8 Journey Inside the Atom Question Answer (Revise, Reflect, Refine)

Below are the detailed solutions for end chapter exercise questions of NCERT Class 9 Science Chapter 8 Journey Inside the Atom.

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:

(i) The experiment clearly showed the existence of neutrons in the nucleus is incorrect. In the Rutherford experiment alpha particles are used to study the structure of atoms. This experiment provides information about distribution of positive charge and mass. The existence of neutrons in the nucleus is explained by JJ Thomson. Hence, statement (i) is incorrect.

(ii) The results disproved the plum pudding model and led to the idea of a nucleus at the centre of the atom is the correct statement because in his experiment most alpha particles passed straight, few were deflected and very few were bounced back. That shows the presence of positive charge is concentrated at a tiny central place. Hence, statement (ii) is correct.

(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 is correct because only a small number of alpha particles shows large deflection which indicates the presence of positive charge at a small central place. Hence, statement (iii) is correct.

(iv) The way alpha particles were deflected showed that electrons move around the nucleus is incorrect because the Rutherford model only explained about the presence of positive charge and dense nucleus at the centre of the atom. Hence, statement (iv) is incorrect.

Hence, the correct options are (ii) and (iii).

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.

Answer:

(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus is an incorrect statement because according to Bohr's model electrons move in fixed paths called energy levels without losing energy. Hence, statement (i) is incorrect.

(ii) Electrons can exist anywhere around the nucleus with no fixed energy is incorrect because according to Bohr's model electrons can occupy only some orbitals with fixed energy, they cannot exist anywhere around the nucleus. Hence, statement (ii) is incorrect.

(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy is correct and one of the main postulates of this model. Hence, statement (iii) is correct.

(iv) Electrons can be found between energy levels as they move around the nucleus is incorrect because electrons cannot exist between to energy levels they can only jump from one orbital to another by absorbing or releasing energy. Hence, statement (iv) is incorrect.

Hence, the correct answer is option (iii)

Question 3: The composition of the nuclei of three atomic species X, Y, and Z are given as follows.

Explain the relation between the following:

(i) Y and Z

(ii) Z and X

Answer:

(i) Number of protons in Y= 17

No. of protons in Z = 17

No. of neutrons in Y = 18

No. of neutrons in Z = 20

Atomic mass of Y = 17 + 18

Atomic mass of Y = 35

Atomic mass of Z = 17 + 20

Atomic mass of Z = 37

Y and Z have the same number of protons but different numbers of neutrons. The atomic number is the same because both have the same number of protons but atomic masses are different. Hence, Y and Z are isotopes.

(ii) Number of protons in Z= 17

No. of protons in X = 18

No. of neutrons in Z = 20

No. of neutrons in X = 19

Atomic mass of Z = 17 + 20

Atomic mass of Y = 37

Atomic mass of X = 18 + 19

Atomic mass of Z = 37

X and Z have the same different number of protons but same atomic mass. The atomic mass is the same but the atomic number is different. Hence, X and Z 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:

On the basis of experiment Rutherford performed on gold foil using alpha particles he concluded that:

(i) The positive charge part of the atom is very small and dense.

(ii) Positive charge is located at the centre of the atom

(iii) Almost the whole mass of the atom is concentrated at the tiny central region called nucleus.

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 ʻplum puddingʼ 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:

First Dalton described atoms as indivisible particles then Thomson plum pudding model shows electrons embedded in a sphere of positive charge then Rutherford model shows that atoms have a dense central nucleus and at the end Bohr’s model proposed that electrons move in fixed orbits around the nucleus, each with a definite energy.

Hence, the correct chronological order of the given statements is (iv)-(ii)-(iii)-(i)

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 move around the nucleus because they are held by a strong electrostatic force of attraction between negatively charged electrons and positively charged nuclei. Electrons always move in fixed orbitals this electrostatic force provides centripetal force that leads electrons bound to the atoms and prevents them from flying away.

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:

 The discovery of subatomic particles helped in understanding the atomic structure. Hence the assertion is true.

The number of electrons is equal to the number of protons in an atom. Hence, the reason is true

But reason does not explain how discovery of subatomic particles helped in understanding the atomic structure.

Hence both Assertion and Reasons are true but reason does not explain assertion.

Hence, the correct answer is option (ii).

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.

Answer:

Mass number of Magnesium = 24

Atomic number of magnesium = 12

(i) Number of protons

Atomic number = No. of protons

No. of protons = 12

(iii) No. of Neutrons

No. of neutrons = Mass number - Atomic number

No. of neutrons = 24-12

No. of neutrons = 12

(iii) No. of electrons

No of electrons = No. of protons

No. of electrons = 12

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

Answer:

(a) Total Number of electrons in (a) are 3 (2 inner and 1 outer)

Total Number of electrons = No. of protons = Atomic Number

Atomic number of this element is 3

(i) Name of the element - Lithium

(ii) Symbol of the element - Li

(iii) Total number of electrons - 3

(iv) Number of valence electrons- 1 

(v) Valency of the element - 1 it can lose 1 electron to achieve stable configuration

(vi) Number of protons - 3

(vii) Atomic number - 3

(b) Total Number of electrons in (b) are 7 ( 2 inner and 5 outer)

Total Number of electrons = No. of protons = Atomic Number

Atomic number of this element is 7

(i) Name of the element - Nitrogen

(ii) Symbol of the element - N

(iii) Total number of electrons - 7

(iv) Number of valence electrons- 5 

(v) Valency of the element - 3 it need 3 electron to achieve stable configuration

(vi) Number of protons - 7

(vii) Atomic number - 7

(c) Total Number of electrons in (c) are 13 ( 2 inner 8 middle and 3 outer)

Total Number of electrons = No. of protons = Atomic Number

Atomic number of this element is 13

(i) Name of the element - Aluminium

(ii) Symbol of the element - Al

(iii) Total number of electrons - 13

(iv) Number of valence electrons- 3 

(v) Valency of the element - 3 it can loose 3 electron to achieve stable configuration

(vi) Number of protons - 13

(vii) Atomic number - 13

(d) Total Number of electrons in (d) are 9 ( 2 inner and 7 outer)

Total Number of electrons = No. of protons = Atomic Number

Atomic number of this element is 9

(i) Name of the element - Fluorine

(ii) Symbol of the element - F

(iii) Total number of electrons - 9

(iv) Number of valence electrons- 7 

(v) Valency of the element - 1 (It need 1 electron to achieve stable configuration)

(vi) Number of protons - 9

(vii) Atomic number - 9

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:

Rutherford's model is based on electromagnetic theory which assumes that electrons moving around the nucleus continuously radiate energy due to which electrons lose their energy and collapse inside the nucleus due to this explanation this model failed to explain atomic stability.

Bohr model explains this using quantum rules which explains that electrons move in a fixed orbits which do not radiate energy.

Question 11: An atom ${ }^{70} \mathrm{X}$ has 31 electrons. How many neutrons are there in its nucleus?

Answer:

Mass number = 70

No. of electrons = 31

In neutral atom number of electrons are equal to number of protons

No. of protons = 31

Number of neutrons = Mass number - No. of protons

Number of neutrons = 70 - 31

Number of neutrons = 39

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.

Answer:

Number of protons = 79

Mass number = 197

(i) Number of Neutrons = Mass Number - Number of Protons

Number of Neutrons = 197-79

Number of Neutrons =118

(ii) In a neutral atom Number of electrons and number of protons are equal

Number of electrons = 79

Question 13: Complete the Table 8.5:

Answer:

Row 1

Atomic number = 5

No. of neutrons = 6

No. of protons and no. of electrons is equal to atomic number

No. of protons = 5

No. of electrons = 5

Mass number = Protons + Neutrons = 5+6 = 11

Name of element = Boron

Row 2

Mass number = 14

No. of electrons = 7

Name of element = Nitrogen

Atomic number and protons = Number of electrons =7

Neutrons = Mass number - Protons

=14 - 7

= 7

Row 3

Mass number = 24

No. of Protons = 12

Atomic number and electrons = No. of protons

Atomic number = 12

No. of electrons = 12

No. of Neutrons = Mass number - No. of protons

No. of Neutrons = 24 - 12

No. of Neutrons = 12

Name of element with atomic number 12 is Magnesium

Row 4

Atomic number = 15

No. of neutrons = 16

No. of protons and no. of electrons is equal to atomic number

No. of protons = 15

No. of electrons = 15

Mass number = Protons + Neutrons = 15+16 = 31

Name of element with atomic number 15 is Phosphorus

Row 5

Mass number = 1

Number of neutrons = 0

No. of protons = Mass number - No. of neutrons

No. of protons = 1 - 0

No. of protons = 1

Atomic number and no. of electrons = Protons =1

Name of element with atomic number 1 is Hydrogen

Question 14: Aman was discussing the structure of atoms 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?

Answer:

Mass number of element X = 35

No. of neutrons = 18

(i) Mass number = No. of protons + No. of Neutrons

No. of protons = Mass number - No. of neutrons

No. of protons = 35-18

No. of protons = 17

Atom is neutral so no. of protons and electrons are equal

No. of electrons = 17

(ii) Atomic number = No. Of protons

Atomic Number = 17

(iii) Element X is Chlorine

(iv) Electronic configuration = 2,8,7

(v) Valence electrons are the no. of electrons in the outermost shell. In chlorine there are 7 electrons in its last shell

Valence electrons = 7

(vi) Mass Number if 2 neutrons are added

No. of neutrons = 17+ 2

No. of neutrons =20

Mass No. = Protons + Neutrons

= 17+20

= 37

(vi) The new element and element X will be isotopes because they have the same atomic number but different mass number.

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

Answer:

No. of Protons = 12

No. of Neutrons = 12

Atomic and mass number mainly depends on nucleus

(i) Atomic number

Atomic number depends upon the number of protons as there is no change in number of protons hence, atomic number remains same.

(ii) Atomic mass

Atomic mass depends upon the mass of protons, neutrons and electrons, as a new particle is 500 times heavier than electrons then total mass will increase.

(iii) Mass Number

Mass number = No. of Protons + No. of Neutrons

Mass no. = 12 + 12

=24

It remains same

(iv) Overall Charge

Overall charge would be same because particle have same electrons and protons

Topics Covered in Class 9 Science Chapter 8

Given below are all the topics and subtopics that are covered in Class 9 Science Chapter 8 Journey Inside the Atom:

8.1 Rediscovering the Roots of Atomic Theory

8.2 A Short Historical Journey Through Atomic Models

• 8.2.1 Thomson’s model of an atom
• 8.2.2 Testing Thomson’s model: The gold foil experiment
• 8.2.3 Bohr’s model of the atom

8.3 What Components Contribute to the Mass of an Atom?

• 8.3.1 Discovery of the Neutron

8.4 Symbols of Elements

8.5 Atomic Number

8.6 Mass Number

8.7 How Are Electrons Distributed in Different Energy Levels?

• 8.7.1 Building up atoms

8.8 Combining Capacity of an Atom: Valency

8.9 A Deeper Look into Atomic Structure

• 8.9.1 Isotopes
• 8.9.2 Isobars

Also Check

• NCERT Notes for Class 9 Science
• NCERT Syllabus for Class 9 Science

NCERT Solutions for Class 9 Science

Refer to the links given below for detailed solutions of NCERT Class 9 Science. These solutions are prepared by subject experts to help you understand the chapters well and prepare effectively for exams.