Circle of Learning • New NCERT textbook "Exploration" (2026-27) • Pause and Ponder + Revise, Reflect, Refine (all questions) + extra important questions
Pause and Ponder (In-text Questions)
Suppose you made your own Thomson 'atom' using clay for the positive charge and small beads for the electrons. 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 total negative charge would be more than the positive charge, so the model would carry a net negative charge – it would represent a negative ion, not a neutral atom.
(ii) No. In Thomson's model the sphere must carry only positive charge, equal to the total negative charge of the electrons. If the clay itself is negative, the model has extra negative charge and cannot 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
Matches: the seeds are scattered inside the pulp, like electrons embedded in a sphere of positive charge; the pulp fills the whole fruit, like the evenly spread positive charge.
Falls short: the seeds and pulp have no electric charge, so the comparison cannot show that the charges balance each other; the seeds are grouped in segments, not evenly spread; and an orange has a peel and segment walls, which Thomson's atom does not have. Like any analogy, it only shows the arrangement, not the physics.
Why did Thomson conclude that electrons are present in all atoms?
Answer
Thomson found that cathode rays were made of the same negatively charged particles (with the same charge-to-mass ratio) no matter which gas was in the tube or which metal the electrodes were made of. Since these particles came out of every kind of matter he tried, he concluded that electrons are a part of all atoms.
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 positively charged nucleus instead of being repelled. Particles passing close to a nucleus would bend towards it (some might even be captured or swing around it), and none would bounce straight back the way α-particles did. They would also be affected by the electrons in the atom. The results would be harder to interpret and would not reveal the nucleus as clearly.
Rutherford found that a few α-particles bounced back sharply. How does this single surprising result completely rule out Thomson's 'plum pudding model' of the atom?
Answer
In Thomson's model, the positive charge and mass are spread thinly over the whole atom, so the repulsion on a fast, heavy α-particle would be weak everywhere – it could deflect it only slightly, never send it back. A particle can bounce back only if it meets a very strong repulsive force from a region that is tiny, very dense and highly positively charged. This is impossible in the plum pudding model, so even a few rebounds prove it wrong and point to a nucleus.
If you could ask Rutherford one question about his work, what would it be?
Answer
Sample questions: "Why did you choose gold for the foil?" (Answer: gold can be beaten into extremely thin sheets, only a few hundred atoms thick.) / "How did you count the scattered α-particles?" / "When your model could not explain the stability of the atom, how did you feel and what did you think the answer might be?"
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.
Answer
(ii) Both A and R are true, but R is not the correct explanation of A. (A is Rutherford's conclusion from the gold foil experiment; R just describes Thomson's earlier model.)
Imagine you have discovered a new element. Name this element after yourself and justify that the symbol follows the IUPAC rules.
Answer
Example: name "Priyanium", symbol "Py".
It follows the rules: the symbol is made from the letters of the name; the first letter is a capital letter and the second is a small letter; it is only one or two letters long; and it does not match the symbol of any existing element (Pr is praseodymium, so "Py" is chosen instead).
What problems could arise if every scientist used different symbols for the same element?
Answer
Scientists in different countries would not understand each other's formulae, equations and results. The same symbol might mean different elements, leading to mistakes in experiments, medicines and industry. Sharing and checking research would become slow and confusing. A common set of symbols makes chemistry a universal language.
An atom with an atomic number of 26 has 56 nucleons. Find out its number of electrons, protons and neutrons.
Answer
Protons = Z = 26; Electrons = 26 (neutral atom); Neutrons = 56 − 26 = 30. (This is iron.)
The nucleus of an atom contains 20 protons. If its mass number is 41, find the number of neutrons in it.
Answer
Neutrons = 41 − 20 = 21
An atom has 18 neutrons and an atomic number of 17. What is its mass number?
Answer
Mass number = 17 + 18 = 35 (chlorine-35)
An atom 23A has 11 electrons. Find the number of neutrons in it.
Answer
Protons = electrons = 11; Neutrons = 23 − 11 = 12
Identify the number of electrons in the outermost shell of: (i) 126C (ii) 199F (iii) 2814Si
(i) C: 2, 4 → 4
(ii) F: 2, 7 → 7
(iii) Si: 2, 8, 4 → 4
Answer
Write the electronic configuration of the elements having atomic numbers 12, 16 and 18.
Answer
Z = 12 (magnesium): 2, 8, 2
Z = 16 (sulfur): 2, 8, 6
Z = 18 (argon): 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.
Answer
I am sodium (Na), with 23 − 11 = 12 neutrons.
Sample riddle: "I have 8 protons and 8 neutrons. You breathe me in every moment, and fire cannot burn without me. Who am I?" – Oxygen (mass number 16).
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
Atomic numbers are the same (11). Mass numbers are different (23 and 24). They are the same element (sodium) – they are isotopes.
If a bromine atom is available as two isotopes, 7935Br (49.7%) and 8135Br (50.3%), calculate the average atomic mass of bromine.
Answer
Average atomic mass = (79 × 49.7 + 81 × 50.3)/100 = (3926.3 + 4074.3)/100 = 80.006 u (≈ 80 u)
Revise, Reflect, Refine (Textbook Exercise)
Choose the correct options and explain, in the context of Rutherford's gold foil experiment.
Answer
Correct: (ii) and (iii).
(i) Incorrect – neutrons were not detected in this experiment; they were discovered later by James Chadwick (1932).
(ii) Correct – the rebounding α-particles could not be explained by the plum pudding model and led to the idea of a central nucleus.
(iii) Correct – large deflections need a strong repulsion from a tiny region holding nearly all the mass and positive charge.
(iv) Incorrect – the experiment showed where the positive charge and mass are; it did not show how electrons move. (Rutherford only proposed that electrons revolve around the nucleus.)
Which statements are correct or incorrect according to Bohr's atomic model? Give a reason for each.
Answer
(i) Electrons lose energy while moving in fixed orbits and slowly fall into the nucleus. – Incorrect. Bohr proposed that electrons in permitted orbits do not radiate energy, so the atom is stable.
(ii) Electrons can exist anywhere around the nucleus with no fixed energy. – Incorrect. Electrons can occupy only certain discrete orbits, each with a definite energy.
(iii) Electrons revolve around the nucleus in orbits of fixed energy without losing energy. – Correct. This is Bohr's main postulate.
(iv) Electrons can be found between energy levels as they move around the nucleus. – Incorrect. Electrons are found only in the allowed energy levels, not in between.
Nuclei of three atomic species: X (18 protons, 19 neutrons), Y (17 protons, 18 neutrons), Z (17 protons, 20 neutrons). Explain the relation between (i) Y and Z, (ii) Z and X.
Answer
Mass numbers: X = 37, Y = 35, Z = 37.
(i) Y and Z are isotopes – same atomic number (17, chlorine) but different mass numbers (35 and 37).
(ii) Z and X are isobars – same mass number (37) but different atomic numbers (17 and 18; chlorine-37 and argon-37).
What conclusion did Rutherford draw about the position and characteristics of the atom's positively charged part based on the few α-particles that bounced back or were deflected at large angles?
Answer
He concluded that all the positive charge and nearly all the mass of the atom are concentrated in a very small, dense region at the centre of the atom, called the nucleus. The nucleus is extremely small compared to the size of the atom (about 1/100,000 of its radius), so most of the atom is empty space.
Arrange the statements in the correct chronological order to show how atomic models evolved.
Answer
(iv) → (ii) → (iii) → (i)
Dalton (early 1800s) described atoms as indivisible particles. After the discovery of the electron, Thomson (1904) proposed the plum pudding model. Rutherford's gold foil experiment (1911) showed a dense central nucleus. Bohr (1913) improved this by proposing fixed orbits of definite energy to explain the stability of the atom.
Electrons move around the nucleus in orbits. Why do they not fly away from the atom? What keeps them attracted to the nucleus?
Answer
Electrons are negatively charged and the nucleus is positively charged (due to protons). Opposite charges attract each other, so the electrostatic force of attraction between the nucleus and the electrons pulls the electrons inward. This attraction provides the force needed to keep them moving around the nucleus, just as gravity keeps a planet in orbit around the Sun.
Assertion (A): The discovery of subatomic particles helped in understanding 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.
For an atom of magnesium with mass number 24 and atomic number 12, determine (i) protons, (ii) neutrons, (iii) electrons, and illustrate the arrangement of electrons.
(i) Protons = 12
(ii) Neutrons = 24 − 12 = 12
(iii) Electrons = 12
Answer
Arrangement: K = 2, L = 8, M = 2
⇒ 2, 8, 2
Diagram: a nucleus (12p, 12n) at the centre, with three circles around it – 2 electrons on the first, 8 on the second and 2 on the third.
Find the requested information for the elements shown in Fig. 8.17.
Answer
(a) | (b) | (c) | (d) | |
Electron arrangement in figure | 2, 1 | 2, 5 | 2, 8, 3 | 2, 7 |
(i) Name | Lithium | Nitrogen | Aluminium | Fluorine |
(ii) Symbol | Li | N | Al | F |
(iii) Total electrons | 3 | 7 | 13 | 9 |
(iv) Valence electrons | 1 | 5 | 3 | 7 |
(v) Valency | 1 | 3 | 3 | 1 |
(vi) Protons | 3 | 7 | 13 | 9 |
(vii) Atomic number | 3 | 7 | 13 | 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
In Rutherford's model, an electron moving in a circle is accelerating, and an accelerating charge should continuously radiate energy. It would lose energy, spiral inward and fall into the nucleus, so atoms should collapse – but they don't. Bohr solved this by proposing that electrons can move only in certain permitted orbits (energy levels) in which they do not radiate energy. Energy changes only when an electron jumps from one level to another. This explained why atoms are stable.
An atom 70X has 31 electrons. How many neutrons are there in its nucleus?
Answer
Protons = 31; Neutrons = 70 − 31 = 39 (this is gallium-70).
An atom has 79 protons and a mass number of 197. Calculate (i) the number of neutrons, and (ii) the number of electrons.
(i) Neutrons = 197 − 79 = 118
(ii) Electrons = 79 (neutral atom; this is gold)
Answer
Complete Table 8.5.
Answer
Atomic number | Mass number | Neutrons | Protons | Electrons | Element |
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 (protium) |
Element X has mass number 35 and contains 18 neutrons. (i) Electrons and protons? (ii) Atomic number? (iii) Identify X. (iv) Electronic configuration. (v) Valence electrons. (vi) Mass number if two neutrons are added. (vii) Relation of X with the new atom.
(i) Protons = 35 − 18 = 17; electrons = 17
(ii) Atomic number = 17
(iii) X is chlorine (Cl)
(iv) 2, 8, 7
(v) 7 valence electrons
(vi) New mass number = 35 + 2 = 37
Answer
(vii) X (Cl-35) and the new atom (Cl-37) are isotopes – same atomic number, different mass numbers.
An atom has 12 protons and 12 neutrons. Imagine all the electrons are replaced by hypothetical particles with the same charge as electrons but 500 times heavier. What effect will this have on the atom's (i) atomic number, (ii) atomic mass, (iii) mass number, (iv) overall charge?
Answer
(i) No change – atomic number depends only on protons (still 12).
(ii) Increases. An electron's mass is about 1/1836 u, so each new particle has a mass of about 500/1836 ≈ 0.27 u. Twelve of them add about 3.3 u, so the atomic mass would rise from about 24 u to about 27 u.
(iii) No change – mass number counts only protons and neutrons (still 24).
(iv) No change – the atom stays neutral, because the new particles carry the same negative charge as electrons and their number equals the number of protons.
Extra Important Questions
Multiple Choice Questions (1 mark)
1. The neutron was discovered by
(a) Thomson (b) Rutherford (c) Chadwick (d) Bohr
Answer: (c) Chadwick
2. An α-particle is
(a) an electron (b) a proton (c) a helium nucleus (d) a neutron
Answer: (c) a helium nucleus (2 protons + 2 neutrons)
3. The maximum number of electrons in the L-shell is
(a) 2 (b) 8 (c) 18 (d) 32
Answer: (b) 8
4. 40Ca and 40Ar are
(a) isotopes (b) isobars (c) isotones (d) same element
Answer: (b) isobars
Short Answer Questions
What is the 2n² rule?
Answer
The maximum number of electrons that a shell can hold is 2n², where n is the shell number: K (2), L (8), M (18), N (32). The outermost shell can hold at most 8 electrons.
Why do isotopes have the same chemical properties?
Answer
Chemical properties depend on the number and arrangement of electrons. Isotopes have the same atomic number, hence the same electronic configuration.
Give two uses of isotopes.
Answer
Uranium-235 is a fuel in nuclear reactors; cobalt-60 is used in cancer treatment; iodine-131 is used to treat goitre (any two).