Class 9 Science

Chapter 8 Solutions — Journey Inside the Atom

Open Solutions PDF →Also seeTextbook page→

Open Solutions PDFReads in your browser→Also seeTextbook page→

Get the full experience in the app

Free practice, offline PDFs & solutions.

Already installed? Open in the app

Solutions

Overview

Step-by-step NCERT solutions for Journey Inside the Atom (Chapter 8, NCERT Class 9 Science) — every question and answer worked out in full, not just the final result. You can also read the Journey Inside the Atom textbook chapter.

Solved

What these solutions cover

All 15 questions in Journey Inside the Atom are solved in the PDF. Here's what's inside, exercise by exercise:

Revise, Reflect, Refine

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…
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)…
3.
The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X: protons = 18, neutrons = 19. Y: protons = 17, neutrons = 18. Z: protons = 17, neutrons = 20. Explain the relation between the following:
- (i) Y and Z,
- (ii) Z and X.
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?
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…
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
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…
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.
9.
Find the following information for the elements shown in Fig. 8.17 (four atomic diagrams labelled a, b, c, d):
- (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.
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?
An atom ⁷⁰X has 31 electrons. How many neutrons are there in its nucleus?
12.
An atom has 79 protons and a mass number of 197. Calculate
- (i) the number of neutrons, and
- (ii) the number of electrons.
Complete the Table 8.5 (columns: Atomic number | Mass number | Number of neutrons | Number of protons | Number of electrons | Name of element). Given rows: Row 1: Z=5, neutrons=6. Row 2: A=14, electrons=7, name=Nitrogen. Row 3: A=24, protons=12. Row 4: Z=15, neutrons=16. Row 5: A=1, neutrons=0.
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…
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.

Sample

Worked examples

3 questions from Journey Inside the Atom, solved step by step.

Revise, Reflect, Refine

The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X: protons = 18, neutrons = 19. Y: protons = 17, neutrons = 18. Z: protons = 17, neutrons = 20. Explain the relation between the following: (i) Y and Z, (ii) Z and X.

Calculate the mass number (A = protons + neutrons) for each species:

X: A = 18 + 19 = 37

Y: A = 17 + 18 = 35

Z: A = 17 + 20 = 37

(i) Relation between Y and Z:

Y has 17 protons and Z has 17 protons — the same atomic number (same element, Chlorine).

But Y has mass number 35 and Z has mass number 37 — different mass numbers (different numbers of neutrons).

Y and Z are ISOTOPES — atoms of the same element with the same number of protons but different numbers of neutrons.

(ii) Relation between Z and X:

Z has 17 protons and X has 18 protons — different atomic numbers (different elements).

But Z has mass number 37 and X also has mass number 37 — the same mass number.

Z and X are ISOBARS — atoms of different elements that have the same mass number.

Answer: (i) Y and Z are isotopes (same proton number 17, different mass numbers 35 and 37). (ii) Z and X are isobars (same mass number 37, different proton numbers 17 and 18).

Revise, Reflect, Refine

Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.

Electrons are negatively charged (-1 charge each) and the nucleus contains protons, which are positively charged (+1 charge each).

Opposite charges attract each other according to Coulomb's law. The electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus acts as a centripetal force, pulling the electrons continuously towards the nucleus.

This inward electrostatic attraction balances the tendency of the electrons to fly outward (due to their circular motion). As long as the electron remains in its fixed orbit, these forces are balanced and the electron neither flies away nor falls into the nucleus.

According to Bohr's model, electrons occupy only those orbits where this balance is maintained (stationary states). In these orbits the electrons do not radiate energy, so they remain stable and do not spiral inward.

Answer: Electrons do not fly away because the electrostatic (Coulomb) force of attraction between the negatively charged electron and the positively charged nucleus acts as a centripetal force, holding the electron in its orbit. This attractive force prevents the electron from leaving the atom.

Revise, Reflect, Refine

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.

Given: Mass number (A) = 24, Atomic number (Z) = 12

(i) Number of protons = Atomic number = 12

(ii) Number of neutrons = Mass number - Atomic number = 24 - 12 = 12

(iii) Number of electrons = Number of protons (neutral atom) = 12

Electron arrangement in Magnesium (Z = 12):

Shell K (n=1): maximum 2 electrons → 2 electrons

Shell L (n=2): maximum 8 electrons → 8 electrons

Shell M (n=3): remaining electrons → 12 - 2 - 8 = 2 electrons

Electronic configuration of Mg: 2, 8, 2

The 2 electrons in the outermost shell (M) are the valence electrons.

Answer: (i) Protons = 12, (ii) Neutrons = 12, (iii) Electrons = 12. Electronic configuration: K=2, L=8, M=2 (written as 2,8,2).

All 15 questions in Journey Inside the Atom are solved step by step in the Solutions PDF above.

Keep solving

Chapter 8 Solutions — Journey Inside the Atom

Overview

What these solutions cover

Revise, Reflect, Refine

Worked examples

The composition of the nuclei of three atomic species X, Y, and Z are given as follows. X: protons = 18, neutrons = 19. Y: protons = 17, neutrons = 18. Z: protons = 17, neutrons = 20. Explain the relation between the following: (i) Y and Z, (ii) Z and X.

Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.

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.

More solutions in Exploration

More NCERT Solutions for Class 9