Journey inside the Atom NCERT Solution - Class 9 Science

NCERT Solutions for Chapter 8: Journey Inside the Atom

Here are the NCERT Solutions for Chapter 8: Journey Inside the Atom. These solutions use the exact question statements from your sources with simplified, easy-to-understand answers.

Revise, Reflect, Refine (Exercise Questions - Page 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: This experiment didn't find neutrons; they were discovered much later by James Chadwick.
• (ii) Correct: It proved Thomson’s "plum pudding" idea was wrong because it showed the atom has a solid center.
• (iii) Correct: Because some particles bounced back, Rutherford knew the positive charge and mass must be squeezed into a tiny spot called the nucleus.
• (iv) Incorrect: The experiment proved there was a nucleus, but it didn't actually show how electrons move.

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: Bohr stated that electrons stay in their orbits without losing any energy.
• (ii) Incorrect: Electrons can only stay in specific paths (energy levels), not just anywhere.
• (iii) Correct: Electrons move in set orbits with a specific amount of energy and don't lose it.
• (iv) Incorrect: Electrons "jump" between levels; they can never stop or stay in the space between them.

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

Answer:

• (i) Y and Z: They have the same number of protons (17) but different neutrons. This makes them isotopes.
• (ii) Z and X: They have different protons but the same total mass (Protons + Neutrons = 37 for both). This makes them 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:

He concluded that the atom's positive charge and almost all of its mass are concentrated in a very small, dense area at the center called 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... (ii) Thomson's model... (iii) Rutherford's model... (iv) Dalton's model...

Answer:

The correct order is:

• 1. (iv) Dalton’s model (Atoms are indivisible)
• 2. (ii) Thomson’s model (Plum pudding)
• 3. (iii) Rutherford’s model (Nucleus)
• 4. (i) Bohr’s model (Fixed orbits).

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:

The nucleus is positive and electrons are negative. The electrostatic force of attraction (opposite charges attracting) acts like an invisible string that keeps the electrons pulled toward the center.

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.

Answer:

(ii) Both A and R are true, but R is not the correct explanation of A. Knowing that electrons equal protons makes an atom neutral, but it doesn't explain the whole "structure" (how they are arranged).

Question 8. 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:

• (i) Protons: 12 (same as atomic number)
• (ii) Neutrons: 12 (Mass 24 - Protons 12)
• (iii) Electrons: 12 (same as protons)
• Electronic Configuration: 2, 8, 2.

Question 9. Find the following information for the elements (Lithium, Nitrogen, Aluminium, Fluorine):

(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) Lithium (Li): 3 electrons, 1 valence electron, Valency = 1, 3 protons, Atomic No. = 3.
• (b) Nitrogen (N): 7 electrons, 5 valence electrons, Valency = 3, 7 protons, Atomic No. = 7.
• (c) Aluminium (Al): 13 electrons, 3 valence electrons, Valency = 3, 13 protons, Atomic No. = 13.
• (d) Fluorine (F): 9 electrons, 7 valence electrons, Valency = 1, 9 protons, Atomic No. = 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 suggested electrons would lose energy while moving and eventually crash into the nucleus. Bohr fixed this by saying electrons move in special shells where they don't lose energy, keeping the atom stable.

Question 11. An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?

Answer:

Since it has 31 electrons, it has 31 protons. Neutrons = Mass (70) - Protons (31) = 39 neutrons.

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:

• (i) Neutrons: 197 - 79 = 118.
• (ii) Electrons: Same as protons = 79.

Question 13. Complete the Table 8.5 (Boron, Nitrogen, Magnesium, Phosphorus, Hydrogen):

Answer:

• Boron: At. No 5, Mass 11, Neutrons 6, Protons 5, Electrons 5.
• Nitrogen: At. No 7, Mass 14, Neutrons 7, Protons 7, Electrons 7.
• Magnesium: At. No 12, Mass 24, Neutrons 12, Protons 12, Electrons 12.
• Phosphorus: At. No 15, Mass 31, Neutrons 16, Protons 15, Electrons 15.
• Hydrogen: At. No 1, Mass 1, Neutrons 0, Protons 1, Electrons 1.

Question 14. Element X has a mass number of 35 and contains 18 neutrons. Answer the following:

Answer:

• (i) Electrons/Protons: 17 (35 - 18)
• (ii) Atomic Number: 17
• (iii) Element: Chlorine (Cl)
• (iv) Configuration: 2, 8, 7
• (v) Valence electrons: 7
• (vi) New mass if 2 neutrons added: 37
• (vii) Relation: They are isotopes.

Question 15. Imagine that all the electrons are replaced with hypothetical particles that have the same charge but are 500 times heavier. What effect will this have?

Answer:

• (i) Atomic number: Stays the same (only depends on protons).
• (ii) Atomic mass: Increases (the new particles add significant weight).
• (iii) Mass number: Stays the same (only counts protons and neutrons).
• (iv) Overall charge: Stays neutral (charges still balance).

In-Text Questions (Pause & Ponder)

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) The atom will become negatively charged because negative charge is more than positive charge.

(ii) No, the model will not represent a neutral atom because extra negative charge is present.

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:

Yes, it matches Thomson’s idea because seeds are spread inside the pulp like electrons in positive charge. But it falls short because seeds are not negative charges and pulp is not positive charge.

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

Answer:

Thomson found the same negatively charged particles in experiments with different gases. So, he concluded that electrons are present in all atoms.

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

Answer:

Negatively charged particles would be attracted towards the positive nucleus. So, their path would bend towards the nucleus instead of being repelled.

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:

If positive charge was spread evenly as Thomson said, alpha particles would not bounce back. Sharp bouncing showed that positive charge is concentrated in a small dense nucleus.

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

Answer:

I would ask: How did you feel when some alpha particles bounced back instead of passing through the gold foil?

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:

The correct option is (ii) Both A and R are true, but R is not the correct explanation of A.

Rutherford’s conclusion came from the gold foil experiment, not from Thomson’s model.

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 it Vijayium and give it the symbol Vi. It follows IUPAC style because the first letter is capital and the second letter is small.

Question 9. What problems could arise if every scientist used different symbols for the same element?

Answer:

It would create confusion in reading chemical formulas, writing reactions and sharing scientific information.

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
• Protons = 26
• Electrons = 26
• 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 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:

• Mass number = Protons + Neutrons
• Mass number = 17 + 18 = 35

Question 13. An atom 23A has 11 electrons. Find the number of neutrons in it.

Answer:

• Electrons = 11, so protons = 11.
• Mass number = 23
• Neutrons = 23 - 11 = 12

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

(i) 12 6 C

(ii) 19 9 F

(iii) 28 14 Si

Answer:

(i) Carbon: Electronic configuration = 2, 4. Outermost electrons = 4.

(ii) Fluorine: Electronic configuration = 2, 7. Outermost electrons = 7.

(iii) Silicon: Electronic configuration = 2, 8, 4. Outermost electrons = 4.

Question 15. Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.

Answer:

• Atomic number 12: 2, 8, 2
• Atomic number 16: 2, 8, 6
• 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:

• The atom is sodium.
• Number of neutrons = 23 - 11 = 12
• Riddle: I have atomic number 17 and valency 1. I am used in common salt. Who am I?

Answer: Chlorine.

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:

Both atoms have the same atomic number 11 because both have 11 protons.

• First atom mass number = 11 + 12 = 23
• Second atom mass number = 11 + 13 = 24

They are the same element but different isotopes.

Question 18. If a bromine atom is available in the form of, say two isotopes, 79 35 Br (49.7%) and 81 35 Br (50.3%), calculate the average atomic mass of the bromine atom.

Answer:

Average atomic mass = (79 × 49.7 + 81 × 50.3) / 100
= (3926.3 + 4074.3) / 100
= 8000.6 / 100
= 80.006 u