Class 9 Science Chapter 8 Exploration Journey Inside the Atom Question Answer If you’re looking for Class 9 Science Chapter 8 – Exploration Journey Inside the Atom Question Answer, you’ve come to the right place. This chapter explains the structure of atoms, subatomic particles, atomic models, and important concepts in a simple and easy-to-understand manner. Practising these NCERT question answers will help you prepare effectively for school exams and strengthen your understanding of the chapter.
Revise, Reflect, Refine (NCERT Textbook Page No. 158)
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) Incorrect
The experiment did not show the existence of neutrons. Neutrons were discovered later by James Chadwick.
(ii) Correct
The experiment disproved Thomson’s plum pudding model and showed that the atom has a nucleus at the centre.
(iii) Correct
The large deflection of a few a-particles showed that most of the mass and positive charge are concentrated in a small, central region of the atom called nucleus.
(iv) Incorrect
The experiment did not give any information about the movement of electrons.
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) Incorrect
According to Bohr, electrons do not lose energy while moving in fixed orbits.
(ii) Incorrect
Electrons can exist only in specific energy levels, with fixed energy not anywhere around the nucleus.
(iii) Correct
Electrons revolve in fixed orbits with definite energy without losing energy.
(iv) Incorrect
Electrons cannot exist between energy levels; they jump from one level to another.
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
| X | Y | Z | |
| Number of protons | 18 | 17 | 17 |
| Number of neutrons | 19 | 18 | 20 |
Answer:
(i) Y and Z
They have the same number of protons (17) but different neutrons. So, they are
isotopes.
(ii) Z and X
They have different atomic numbers but same mass number. So, they 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 the atom is concentrated in a very small, dense region called the nucleus. Most of the mass of the atom is concentrated in the 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 ‘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:
Correct order:
(iv) Dalton’s model
(ii) Thomson’s model
(iii) Rutherford’s model
(i) Bohr’s model
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 are negatively charged, while the nucleus is positively charged. The electrostatic force of attraction between them keeps electrons bound to the nucleus 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:
Correct option: (ii)
Both A and R are true, but R is not the correct explanation of A.
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 Atomic protons = number 12
(ii) Number of neutrons = Mass number – Number of protons
Number of neutrons = 24 – 12 = 12
(iii) Number of electrons = Number of protons = 12
Electronic configuration = 2, 8, 2
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)
(i) Name of the element = Lithium
(ii) Symbol = Li
(iii) Total number of electrons = 3
(iv) Number of valence electrons = 1
(v) Valency of the element = 1
(vi) Number of protons 3
(vii) Atomic number = 3
(b)
(i) Name of the element = Nitrogen
(ii) Symbol = N
(iii) Total number of electrons = 7
(iv) Number of valence electrons = 5
(v) Valency of the element = 3
(vi) Number of protons = 7
(vii) Atomic number = 7
(c)
(i) Name of the element = Aluminium
(ii) Symbol = Al
(iii) Total number of electrons = 13
(iv) Number of valence electrons 3
(v) Valency of the element = 3
(vi) Number of protons = 13
(vii) Atomic number = 13
(d)
(i) Name of the element = Fluorine
(ii) Symbol = F
(iii) Total number of electrons 9
(iv) Number of valence electrons = 7
(v) Valency of the element = 1
(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 failed to explain atomic stability because, according to it, electrons moving around the nucleus should continuously lose energy and eventually fall into the nucleus, causing the atom to collapse.
Bohr’s model succeeded because it proposed that electrons move in fixed energy levels (shells) where they do not lose energy while revolving around the nucleus. This prevents them from falling into the nucleus and keeps the atom stable.
Question 11.
An atom 70X has 31 electrons. How many neutrons are there in its nucleus?
Answer:
Mass number = 70
Number of electrons = 31
Number of protons = Number of electrons
Number of neutrons = Mass number – Number of protons
Number of neutrons = 70 – 31 = 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 = 118
(ii) Number of electrons = Number of protons = 79
Question 13.
Complete the Table 8.5:
Answer:
| Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of the elements |
| 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 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?
Answer:
Mass number 35
Number of neutrons = 18
(i) Number of protons = Mass number – Number of neutrons
Number of protons =35 – 18 = 17
Number of protons = Number of electrons = 17
(ii) Atomic number = 17
(iii) Element = Chlorine (Cl)
(iv) Electronic configuration = 2, 8, 7
(v) Valence electrons = 7
(vi) New mass number = 35 + 2 = 37
(vii) Relation = 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 wifi this replacement have on the atom’s:
(i) Atomic number
(ii) Atomic mass
(iii) Mass number
(iv) Overall charge
Answer:
(i) The atomic number will remain the same (12) because it depends only on the number of protons.
(ii) The atomic mass will increase because the new particles are much heavier than electrons.
(iii) The mass number will remain the same (24) as it depends only on the number of protons and neutrons.
(iv) The overall charge will remain neutral because the number of negatively charged particles is equal to the number of protons.
Class 9 Science Chapter 8 Journey Inside the Atom Question Answer (InText)
Think It Over (NCERT Textbook Page No. 140)
Question 1.
Are atoms the smallest indivisible particles?
Answer:
Atoms are not the smallest indivisible particles. Earlier, they were considered indivisible, but later discoveries showed that they are made up of smaller particles like electrons, protons, and neutrons.
Question 2.
Why do electrons not fall into the nucleus even though they are attracted to protons in it?
Answer:
Electrons do not fall into the nucleus because they move in fixed energy levels around the nucleus, where they do not lose energy. This keeps them stable and prevents them from collapsing into the nucleus.
Question 3.
Why did scientists keep modifying atomic models?
Answer:
Scientists kept modifying atomic models because new experiments provided better evidence. Earlier models could not explain all observations, so they were improved to give a more accurate understanding of the atom.
Pause and Ponder (NCERT Textbook Page No. 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) If the positive charge on the clay is less than the total negative charge of the beads, the atom will become negatively charged and not neutral.
(ii) If the clay also carries a negative charge, the total negative charge increases, so the atom will not remain neutral.
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:
An orange or a lemon partly matches Thomson’s model because the pulp can be considered as the positively charged sphere and the seeds as electrons embedded in it.
However, it falls short because in Thomson’s model, the positive charge is uniformly spread throughout, whereas in an orange or a lemon, the seeds are not evenly distributed and are present in specific regions of the pulp.
Question 3.
Why did Thomson conclude that electrons are present in all atoms?
Answer:
Thomson concluded that electrons are present in all atoms because cathode rays was independent of the gas or material of the cathode used, which proved that electrons are a fundamental component of all atoms present in every element.
Pause and Ponder (NCERT Textbook Page No. 144)
Question 4.
What do you think would happen if ∝-particles were replaced with negatively charged particles in Rutherford’s gold foil experiments?
Answer:
If ∝-particles were replaced with negatively charged particles, they would be attracted towards the positively charged nucleus, and would not show large deflections or bounce back. Most would move closer to the nucleus instead of being repelled.
Question 5.
Rutherford found that a few Alpha particles bounced back sharply. How does this single surprising result completely rule out Thomson’s ‘plum pudding model’ of the atom?
Answer:
The bouncing back of a few ∝ – particles showed that a large amount of positive charge and mass is concentrated in a small, dense region (nucleus). This contradicts Thomson’s model, which assumed that positive charge is spread uniformly, so such strong deflection would not be possible.
Question 6.
If you could ask Rutherford one question about his work, what would it be?
Answer:
I would ask Rutherford: “How are electrons arranged around the nucleus, and what keeps them from falling into it?”
Pause and Ponder (NCERT Textbook Page No. 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:
(ii) Both A and R are true, but R is not the correct explanation of A. Rutherford’s conclusion about the nucleus was based on the gold foil experiment, not on Thomson’s model. Thomson’s model only described electrons embedded in a positive sphere and does not explain the concentration of mass at the centre.
Pause and Ponder (NCERT Textbook Page No. 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:
I would name the element Amanium (Am). The symbol follows TUPAC rules because it uses the first two letters of the name, with the first letter in uppercase and the second in lowercase.
Question 9.
What problems could arise if every scientist used different symbols for the same element?
Answer:
If different scientists used different symbols for the same element, it would lead to confusion and lack of uniformity, making scientific communication and understanding difficult.
Pause and Ponder (NCERT Textbook Page No. 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 protons 26
Number of electrons = 26
Number of neutrons = Number of nucleons – Number of protons
Number of neutrons = 56 – 26 = 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:
Number of protons = 20
Mass number = 41
Number of neutrons = Mass number – Number of protons
Number of neutrons = 41 – 20 = 21
Question 12.
An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Answer:
Number of neutrons = 18
Atomic number = 17
Number of protons = Atomic number 17
Mass number = Number of protons + Number of neutrons
Mass number = 17 + 18 = 35
Question 13.
An atom 23A has 11 electrons. Find the number of neutrons in it.
Answer:
Mass number = 23
Number of electrons = 11
Number of protons = Number of electrons = 11
Number of neutrons = Mass number – Number of protons
Number of neutrons 23 – 11 = 12
Pause and Ponder (NCERT Textbook Page No. 152)
Question 14.
Identify the number of electrons in the outermost shell of the following elements:
(i) 612C
(ii) 919F
(iii) 1428Si
Answer:
(i) 612C
Total number of electrons 6
Electronic distribution =
4 electrons in the outermost shell
(ii) 919F
Total number of electrons =9
Electronic distribution = 
7 electrons in the outermost shell
(iii) 1428Si
Total number of electrons = 14
Electronic distribution =
4 electrons in the outermost shell.
Question 15.
Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Answer:
- Electronic configuration of the element having atomic number 12 = 2, 8, 2
- Electronic configuration of the element having atomic number 16 = 2, 8, 6
- Electronic configuration of the element having atomic number 18 = 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:
Number of protons = 11 → Atomic number = 11
Element with atomic number 11 = Sodium (Na). Sodium is a soft metal and reacts vigorously with water.
Now,
Number of neutrons = Mass number = Atomic number
Number of neutrons = 23 – 11 = 12
So, the atom is Sodium (Na), and it has 12 neutrons.
Riddle (example):
I am an atom with atomic number 8.
I have 8 protons and usually 8 neutrons.
I am essential for breathing and life.
Who am I? (Answer: Oxygen)
Pause and Ponder (NCERT Textbook Page No. 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:
Since both atoms have 11 protons, their atomic number is the same (11).
Mass number = Number of protons + Number of neutrons
First atom: 11 + 12 = 23
Second atom: 11 + 13 = 24
Therefore, both atoms have the same atomic number but different mass numbers (23 and 24). Hence, they are isotopes of the same element, not different elements.
Question 18.
If a bromine atom is available in the form of, say, two isotopes, 3579Br (49.7%) and
3581Br (50.3%), calculate the average atomic mass of the bromine atom.
Answer:
Isotopes of bromine:
3579 Br = 49.7%
3581 Br = 50.3%
Average atomic mass is calculated using
weighted average:
Average atomic mass = 79×49.7100+81×50.3100
Now calculating:
79 × 49.7 = 3926.3
81 × 50.3 = 4074.3
Total = 3926.3 + 4074.3 = 8000.6
Average atomic mass 8.000.6 ÷ 100 = 80.006 u
What if…? (NCERT Textbook Page No. 147)
Question 1.
An atom had no empty space? How would this have affected the size of various objects?
Answer:
If an atom had no empty space, all matter would be extremely compact and occupy much less space. As a result, the size of all objects would decrease drastically because Atoms are mostly empty space, and removing it would greatly reduce their volume.
Think as a Scientist (NCERT Textbook Page No. 144)
Question 1.
Observe Fig. 8.4 of the gold foil experiment. Predict the observations you would expect if the gold foil in the experiment were made thicker. Also, draw a simple diagram to show the observations you expect. Hint: Compare thin foil vs thick foil. How does the thickness affect the chances of hitting a nucleus?
Answer:
If the gold foil were made thicker, more ∝ – particles would collide with atoms. As a result, fewer particles would pass straight through, and more would be deflected at different angles. Some may even bounce back more frequently because the chances of hitting a nucleus increase with thickness.
Gold foil experiment expected observation if used thick foil.
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