NCERT Class 9 Science (Exploration) Solutions
Chapter 8: Journey Inside the Atom
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Class 9 Science Exploration Chapter 8 Question Answer
Class 9 Science Ch 8 Journey Inside the Atom Question Answer
Journey Inside the Atom Class 9 Questions and Answers (Exercise)
Revise, Reflect, Refine (NCERT Textbook Page No. 158)
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.
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.
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 |
(i) Y and Z
They have the same number of protons (17) but different numbers of neutrons. So, they are isotopes.
(ii) Z and X
They have different atomic numbers but the same mass number. So, they are isobars.
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:
Based on the observation that a few alpha particles bounced back or were deflected at large angles, Rutherford drew the following conclusions about the positively charged part of the atom:
The positive charge of the atom is not spread throughout the volume of the atom (as Thomson suggested) but is concentrated in an extremely small region at the centre.
This small, central, positively charged region is called the nucleus.
The nucleus is extremely dense – it contains most of the mass of the atom packed into a very tiny volume.
The nucleus is very small compared to the overall size of the atom. The diameter of the atom is approximately 10-10 m, while the diameter of the nucleus is approximately 10-15 m – the nucleus is about 105 (one lakh) times smaller than the atom.
Since most of the alpha particles passed straight through without deflection, most of an atom is space.
Only when an alpha particle came very close to the nucleus was it repelled strongly enough to bounce back or deflect at a large angle.
Explain and arrange the following statements in the correct chronological order to show how atomic models have evolved.
(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:
The correct chronological order (from oldest to newest) is: (iv) → (ii) → (iii) → (i).
Explanation: First, Dalton proposed that atoms are the smallest, indivisible particles i.e. (iv). Years later, Thomson discovered electrons and gave the ‘plum pudding model’ to show charges i.e. (ii). After that, Rutherford conducted his gold foil experiment and discovered the central nucleus, i.e. (iii). Finally, to fix the stability problems in Rutherford’s model, Bohr introduced the concept of fixed energy shells i.e., (i).
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.
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.
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:
Given for Magnesium: Atomic Number = 12,
Mass Number = 24.
(i) Protons:
The atomic number is always equal to the number of protons. Thus, it has 12 protons.
(ii) Neutrons:
Neutrons = Mass Number – Protons. So,
24 – 12 = 12 neutrons.
(iii) Electrons:
In a neutral atom, the number of electrons equals the number of protons. Thus, it has 12 electrons.
Arrangement of electrons:
The 12 electrons will fill the shells step-by-step. The first K-shell takes 2 electrons, the L-shell takes 8 electrons, and the remaining 2 electrons go to the M-shell. So, the electronic configuration is 2, 8, 2.
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
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:
The key difference lies in how each model describes the electron’s motion:
Rutherford’s model and why it failed:
- In Rutherford’s model, electrons orbit the nucleus freely, similar to planets around the Sun.
- However, according to the laws of physics (as understood from Chapter 4 – describing circular motion), a particle moving in a circular path is constantly changing direction, which means it is accelerating.
- A negatively charged particle that is accelerating should continuously emit energy (in the form of radiation).
- If electrons kept losing energy, they would slow down, spiral inward toward the nucleus, and eventually fall into it.
- If this happened, atoms would collapse in a very short time and would not exist in a stable form.
- Rutherford’s model had no explanation for why this does not happen. It could not explain atomic stability.
Bohr’s model and why it succeeded:
- Bohr introduced the concept of stationary states (fixed orbits/shells).
- He proposed that when an electron is in a fixed allowed orbit, its energy remains constant – it does not emit or lose energy while in that orbit.
- This was a postulate (an assumed rule) that Bohr introduced specifically to address Rutherford’s limitation.
- By placing electrons in fixed energy levels where energy is constant, Bohr’s model successfully explained why electrons do not spiral inward and why atoms are stable.
- Bohr’s model could also explain many experimental observations, like the spectrum of hydrogen.
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
An atom has 79 protons and a mass number of 197.
Calculate
(i) the number of neutrons, and
(ii) the number of electrons.
Answer:
Here, number of protons = atomic number (Z) = 79.
Mass number (A) = 197.
(i) Number of neutrons = Mass number – Number of protons = 197 – 79 = 118.
(ii) Since the atom is electrically neutral, number of electrons = number of protons = 79.
This element is Gold, (Au).
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 |
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
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) Atomic number:
It will remain unchanged (12). The atomic number only depends on the number of protons inside the nucleus, which has not changed.
(ii) Atomic mass:
It will increase. Normally, electrons are so light that their weight is ignored. However, these new particles are 500 times heavier, meaning their combined weight will now add a very noticeable amount to the overall mass of the atom.
(iii) Mass number:
It will remain unchanged (24). The mass number is strictly defined as the sum of protons and neutrons in the nucleus (12 + 12), regardless of what is orbiting outside.
(iv) Overall charge:
It will remain unchanged (neutral). Since the new particles carry the exact same negative charge as normal electrons, they perfectly balance the positive charge of the 12 protons just like before.
Class 9 Science Chapter 8 Journey Inside the Atom Question Answer (InText)
Think It Over (NCERT Textbook Page No. 140)
Are atoms the smallest indivisible particles?
Answer:
No, Atoms are not the smallest indivisible particles. While Dalton’s atomic theory once described atoms as indivisible, modern science has shown that atoms are made up of even smaller subatomic particles: protons, neutrons, and electrons. These particles themselves can be further studied, with protons and neutrons composed of quarks.
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.
Why did scientists keep modifying atomic models?
Answer:
Scientists continuously modified atomic models because new experimental evidence often contradicted existing theories, requiring updated models that could more accurately explain observed phenomena. As technology improved, scientists were able to uncover new subatomic particles—such as electrons, protons, and neutrons—and understand the structure of the nucleus, rendering older, simpler models obsolete.
Pause and Ponder (NCERT Textbook Page No. 143)
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 less 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) The atom would not be electrically neutral. It would carry a net negative charge. This would not represent a real neutral atom since in a real atom, the total positive charge always equals the total negative charge, making the atom electrically neutral.
(ii) No, the model would not represent a neutral atom. If the clay (which is supposed to represent the positive sphere) carries a negative charge, then both the clay and the beads (electrons) would be negatively charged. The total charge of the model would be entirely negative. There would be no positive charge to balance the negative charge of the electrons, so the model would not represent a neutral atom.
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.
Why did Thomson conclude that electrons are present in all atoms?
Answer:
Thomson concluded that electrons are present in all atoms because cathode rays were 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)
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.
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.
If you could ask Rutherford one question about his work, what would it be?
Answer:
A thoughtful question could be: “If the nucleus is so small and yet contains all the positive charge and most of the mass of the atom, what force is responsible for holding the positively charged protons together inside the nucleus without them flying apart due to their mutual repulsion?”
This question relates directly to the limitation of Rutherford’s model and the concept of nuclear force, which is relevant to the chapter.
Pause and Ponder (NCERT Textbook Page No. 145)
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)
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.
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, it would create a lot of confusion across the world. Scientists from different countries would not be able to read or understand each other’s research, chemical formulas, or discoveries. Sharing scientific knowledge and studying chemistry together would become extremely difficult because there would be no common language to unite them.
Pause and Ponder (NCERT Textbook Page No. 150)
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
The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Answer:
Given:
Number of protons = 20
∴ Mass No. (A) = No. of protons + No. of neutrons = 41
or Number of Neutrons = 41 – Number of protons
= 41 – 20
= 21
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
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)
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.
Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Answer:
1. Atomic number = 12 (Magnesium, Mg)
Electrons = 12
Distribution: 2 (K), 8 (L), 2 (M)
Electronic configuration: 2, 8, 2
Atomic number 16 (Sulphur, S)
Electrons = 16
Electronic distribution: 2 (K), 8 (L), 6 (M)
Electronic configuration: 2, 8, 6
3. Atomic number 18 (Argon, Ar)
Electrons = 18
Distribution: 2 (K), 8 (L), 8 (M)
Electronic configuration: 2, 8, 8
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.
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)
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:
Both atoms have 11 protons, so both have atomic number = 11.
Since the atomic number is the same, both atoms belong to the same element – Sodium (Na).
Mass number of first atom = 11 + 12 = 23.
Mass number of second atom = 11 + 13 = 24.
So their atomic numbers are the same but their mass numbers are different.
These two atoms are isotopes of sodium – they are the same element but with different mass numbers.
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:
The average atomic mass is calculated by multiplying the mass of each isotope by its percentage of occurrence in nature, and then adding those values together.
Average atomic mass = (79 × ) + (81 × )
= (79 × 0.497) + (81 × 0.503)
= 39.263 + 40.743 = 80.006 u
Therefore, the average atomic mass of the bromine atom is approximately 80.006 u.
What if…? (NCERT Textbook Page No. 147)
What if 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)
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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