Step-by-step answers to all 15 "Revise, Reflect, Refine" questions of Chapter 8, Journey Inside the Atom (NCERT Class 9 Science, Exploration, 2026-27): Thomson, Rutherford and Bohr models, protons, neutrons and electrons, atomic and mass number, electronic configuration, valency, isotopes and isobars. All 15 questions are answered, with the key answer highlighted.
Atomic number Z = number of protons (= number of electrons in a neutral atom). Mass number A = protons + neutrons, so neutrons = A − Z. Shells fill as K (max 2), L (max 8), M (8 for the first 18 elements); the outermost shell holds the valence electrons. Valency = valence electrons (if 1–4) or 8 − valence electrons (if 5–8).
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.
Solution
Correct: (ii) and (iii).
(ii) Correct. In Thomson's plum pudding model the positive charge is spread out, so alpha particles should pass with only tiny deflections. A few bouncing back proved that the positive charge is concentrated in a small nucleus at the centre.
(iii) Correct. Only a very small, heavy and positively charged centre could repel the fast, heavy alpha particles through large angles, and since very few were deflected, this centre must be tiny.
(i) Incorrect. The experiment said nothing about neutrons; they were discovered later (by James Chadwick, 1932).
(iv) Incorrect. The deflections were caused by the nucleus; they gave no information on how electrons move. Rutherford only proposed that electrons revolve around the nucleus.
Which of the following statements are correct or incorrect according to 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.
Solution
(i) Incorrect. Bohr proposed that an electron in a permitted orbit does not radiate (lose) energy, so it does not fall into the nucleus.
(ii) Incorrect. Electrons occupy only certain fixed orbits (energy levels), each with a definite energy.
(iii) Correct. This is the main idea of Bohr's model: stationary orbits (K, L, M, N…) of fixed energy.
(iv) Incorrect. Electrons can only be in the allowed energy levels; they jump from one level to another (by absorbing or releasing energy) but are never found in between.
The composition of the nuclei of three atomic species X, Y, and Z are given as follows: X has 18 protons and 19 neutrons; Y has 17 protons and 18 neutrons; Z has 17 protons and 20 neutrons. Explain the relation between the following: (i) Y and Z (ii) Z and X
Solution
X
Y
Z
Protons (atomic number)
18
17
17
Neutrons
19
18
20
Mass number
37
35
37
(i) Y and Z are isotopes: same atomic number (17, both are chlorine) but different mass numbers (35 and 37), because they have different numbers of neutrons.
(ii) Z and X are isobars: same mass number (37) but different atomic numbers (17 and 18), so they are atoms of different elements (chlorine and argon).
(i) Isotopes (same Z = 17, different A) (ii) Isobars (same A = 37, different Z)
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?
Solution
Rutherford concluded that:
The positive charge of the atom is concentrated in a very small region at its centre, called the nucleus, not spread throughout the atom.
Nearly all the mass of the atom is in this nucleus (only a heavy, dense centre could turn back the fast alpha particles).
The nucleus is extremely small compared with the size of the atom, because only very few alpha particles were deflected strongly; most passed straight through, showing that most of the atom is empty space.
The positive charge and almost all the mass of an atom are concentrated in a tiny, dense nucleus at its centre; the rest of the atom is mostly empty space.
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.
Solution
Order: (iv) → (ii) → (iii) → (i)
Dalton (1808): atoms are tiny, indivisible particles.
Thomson (1904): after he discovered the electron, the atom could not be indivisible; he pictured electrons embedded in a sphere of positive charge.
Rutherford (1911): the gold foil experiment showed the positive charge and mass are in a tiny central nucleus, with electrons revolving around it.
Bohr (1913): to explain why the atom is stable, electrons move only in fixed orbits of definite energy without losing energy.
Each model was replaced when new experimental evidence could not be explained by it.
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? Explain what keeps them attracted to the nucleus.
Solution
Electrons are negatively charged and the nucleus is positively charged (because of its protons). Opposite charges attract, so there is a strong electrostatic force of attraction between the nucleus and each electron. This attraction pulls the moving electron towards the centre and keeps it in its orbit (just as the gravitational pull of the Sun keeps the planets in their orbits), so the electron does not fly away. Its motion, in turn, stops it from falling into the nucleus.
The electrostatic attraction between the positively charged nucleus and the negatively charged electrons holds the electrons in their orbits.
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.
Solution
Answer: (ii)
A is true: the discovery of electrons, protons and neutrons led to the models of Thomson, Rutherford and Bohr. R is true: a neutral atom has equal numbers of electrons and protons. But R only explains why an atom is electrically neutral; it does not explain why discovering subatomic particles helped us understand atomic structure.
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.
Solution
(i) Protons = atomic number = 12
(ii) Neutrons = mass number − atomic number = 24 − 12 = 12
(iii) Electrons = protons (neutral atom) = 12
Electronic configuration: K = 2, L = 8, M = 2, i.e. 2, 8, 2.
Bohr model of a magnesium atom: K = 2, L = 8, M = 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
Solution
Count the electrons in each shell of Fig. 8.17: (a) 2, 1 (b) 2, 5 (c) 2, 8, 3 (d) 2, 7.
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?
Solution
Rutherford's model: an electron moving in a circle is continuously accelerating (its direction changes). A charged particle that accelerates should give out energy as radiation. So the electron would keep losing energy, spiral inwards and fall into the nucleus in a tiny fraction of a second, and the atom would collapse. But atoms are stable, so the model could not explain this.
Bohr's model: Bohr proposed that electrons revolve only in certain permitted orbits (energy levels) of fixed energy, and while in these orbits they do not radiate energy. Energy is absorbed or given out only when an electron jumps between levels. So the electron does not spiral into the nucleus, and the atom stays stable.
In Rutherford's model, revolving electrons would radiate energy and spiral into the nucleus, so the atom would be unstable; Bohr's fixed orbits, in which electrons do not lose energy, explained the stability.
Aman was discussing the structure of the 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?
Solution
(i) Protons = 35 − 18 = 17; electrons = 17.
(ii) Atomic number = 17.
(iii) X is chlorine (Cl).
(iv) 2, 8, 7 (K = 2, L = 8, M = 7)
Electron arrangement in a chlorine atom (³⁵Cl): 2, 8, 7
(v) 7 valence electrons (so its valency is 8 − 7 = 1).
(vi) Mass number =35+2=37.
(vii) The new atom has the same atomic number (17) but a different mass number (37): it is an isotope of X (³⁵Cl and ³⁷Cl).
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
Solution
(i) No change: still 12. The atomic number is the number of protons, which is unchanged.
(ii) Increases. An electron's mass is only about 18361 u (≈ 0.00055 u), so normally the 12 electrons add almost nothing. Each new particle would be about 500 × 0.00055 ≈ 0.27 u, so the 12 of them add about 3.3 u: the atomic mass rises from about 24 u to about 27 u.
(iii) No change: still 24. Mass number counts only protons and neutrons, and they are unchanged.
(iv) No change: still neutral. The new particles have the same charge as electrons, so 12 negative charges still balance the 12 protons.
(i) Unchanged (12) (ii) Increases, by about 3.3 u to ≈ 27 u (iii) Unchanged (24) (iv) Unchanged: the atom stays neutral
