NCERT Solutions for Class 9 Science Chapter 8 Journey Inside the Atom (PDF) – 2027

No. An atom can be broken into smaller particles, so it is not indivisible.

• Radioactivity — certain elements were found to emit invisible particles and energy on their own. Something was coming out of the atom.
• Cathode rays (Thomson, 1897) — a stream of negatively charged particles, much lighter than the atom, came out of the cathode whatever metal the cathode was made of and whatever gas filled the tube. These were electrons, present in every atom.

Because, according to Bohr’s model, electrons revolve only in certain fixed shells (stationary states), and an electron moving in such a shell does not lose energy.

The attraction of the protons keeps the electron bound to the atom, but since its energy stays constant it does not spiral inward and fall into the nucleus. It can move to another shell only by absorbing or releasing a fixed amount of energy equal to the difference between the energies of the two shells.

Because each new experiment produced a result the old model could not explain, and a model that cannot explain the evidence has to be changed.

The picture shows two chains that start at very different places and end at the same place — atoms.

• Non-living chain: house → room → wall → brick → silicates → atoms.
• Living chain: human body → organ → tissue → cell → protein → atoms.

Yes. Both living beings and non-living things are ultimately made of atoms — such as C, H, O, N, P and Si shown at the centre. Only the kinds of atoms and the way they are arranged are different.

It can be divided further. An atom is made of three subatomic particles:

Atoms are made of electrons, protons and neutrons, arranged in two distinct regions.

• The nucleus at the centre holds the protons and neutrons (together called nucleons). It carries all the positive charge and almost all the mass. Its diameter is about 10^–15 m.
• The electrons move around the nucleus in shells. They occupy nearly the whole volume of the atom (diameter ≈ 10^–10 m) but contribute almost no mass.

Not like the diagrams in this chapter — there are no coloured spheres and no drawn orbits.

We can actually get pictures of atoms today. A Scanning Tunnelling Microscope (STM) maps a surface atom by atom; a Transmission Electron Microscope (TEM) shows how atoms are arranged inside a very thin sample. In an STM image (Fig. 8.15) atoms appear as a regular array of rounded bumps — no colour, no sharp edges, no visible internal parts.

The number of protons in the nucleus — the atomic number, Z.

Z is the element’s identity card. Change Z and you have a different element, no matter what else stays the same:

• Hydrogen: 1 proton, Z = 1.
• Helium: 2 protons, Z = 2.
• Sodium: 11 protons, Z = 11.

(i) The model would have a net negative charge, because the total negative charge of the beads would be greater than the positive charge of the clay. So it would not represent a neutral atom. For example, clay +8 and 10 beads of –1 each give a net charge of +8 – 10 = –2.

(ii) No. If the clay itself carries negative charge, there is no positive charge left to balance the negative beads, so the total charge can only be negative. The model would not be a neutral atom — in fact it would no longer follow Thomson’s idea at all.

Yes, partly. An orange or a lemon is a fair comparison for Thomson’s model, but only up to a point.

Where it matches: the seeds (electrons) are embedded inside the soft pulp (the positive charge) and are spread through the whole, roughly spherical fruit.

Where it falls short: the pulp is divided into segments and covered by a peel, while Thomson’s positive charge is uniform with no boundary; the seeds are fixed, but electrons move; and the seeds are large compared with the fruit, while electrons are extremely small and light compared with the atom.

Because the cathode rays came out identical no matter what the tube was made of or filled with.

Thomson repeated the discharge-tube experiment while changing two things:

• the metal of the cathode, and
• the gas filling the tube at low pressure.

In every case the rays behaved the same way in electric and magnetic fields — same sign of charge, same charge-to-mass ratio. The particles were always negatively charged and always far lighter than an atom.

About 10⁶ atoms — one million atoms stacked one on top of another.

Formula: number of atoms across the thickness = thickness of sheet ÷ diameter of one atom.

Prediction: more deflection and less transmission. With a thicker foil you would see

• fewer α-particles passing straight through undeflected;
• more α-particles deflected, and more of them deflected through large angles;
• a larger fraction bouncing back towards the source;
• many particles arriving at odd angles because they were deflected more than once on the way through.

Reason: a thicker foil has more layers of gold atoms, so each α-particle has more chances of passing close to a nucleus and being deflected.

The negatively charged particles would be attracted by the positively charged nucleus instead of being repelled.

• Most of them would still pass straight through the foil, because most of the atom is empty space.
• A particle passing close to a nucleus would be pulled towards it and would swing around it, instead of being pushed away from it.
• So the clear result of the α-particle experiment — a few particles being pushed back by a small positive centre — would not be seen in the same way, and it would have been much harder to conclude that the nucleus is positively charged.

In Thomson’s model the positive charge is spread thinly and evenly through the whole atom, so it can push an α-particle only slightly — it could never turn a fast, heavy α-particle back.

Turning a particle back needs a very strong repulsive force, which is possible only if all the positive charge (and most of the mass) is packed into a very small region. So even a few α-particles bouncing back prove that the positive charge is concentrated in a tiny nucleus, and the plum pudding model must be wrong.

“Sir, your model says the electrons revolve around the nucleus like planets around the Sun. But a revolving electron is accelerating, and an accelerating charge should radiate energy and spiral into the nucleus within a fraction of a second. Since atoms clearly do not collapse, what do you think is holding the electron up?”

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

Reason: A is true — the gold foil experiment showed that the positive charge and most of the mass are in a tiny nucleus. R is also true — it correctly describes Thomson’s model. But R describes the older model that this experiment proved wrong, so it does not explain A.

By a postulate: the allowed circular paths are stationary states, and in a stationary state an electron’s energy stays constant even though it is moving.

Set the two models side by side.

Because the letters were borrowed from X-ray spectroscopy, where the naming had already started at K.

• The physicist Charles Barkla, studying X-rays given out by elements, found a series of X-ray lines and called the first one he identified the K series.
• He deliberately did not begin at A. He left room in case a series with even higher energy — earlier in the alphabet — was discovered later.
• No such series was ever found, so the letters below K were never used.
• Bohr simply adopted Barkla’s existing notation for the atomic shells: K, L, M, N, …

Every object would shrink enormously, but its mass would stay the same.

Almost all the volume of an atom is empty space; the nucleus is about 10⁵ times smaller in diameter than the atom. Without the empty space, every length would become about 10⁵ times smaller — a person 1.6 m tall would be only about 16 µm (0.016 mm) tall. Because the mass would not change, all objects would become extremely dense.

They do repel one another — but a much stronger attraction, the nuclear force, holds the nucleus together, and the neutrons are what make that possible.

Neutrons help in two ways:

• Being uncharged, they sit between the protons and increase the distance between them, which weakens the electrostatic repulsion.
• They add to the nuclear force — the short-range attraction that acts between all nucleons (proton–proton, proton–neutron and neutron–neutron alike) and binds them tightly.

Dhruva is India’s largest research reactor, at the Bhabha Atomic Research Centre (BARC), Trombay, Mumbai. It does not produce electricity; it is used as a strong source of neutrons. Scientists use its neutrons (i) in neutron-scattering experiments to study materials such as superconductors, battery electrodes, drug molecules and alloys, (ii) to make radioactive isotopes for medicine and industry, and (iii) to test materials for nuclear power reactors.

I name the element Anandium, after myself, with the symbol An.

Justification against the IUPAC norms:

Chemistry would lose its common language. Problems that could arise:

• The same formula could mean different substances, causing confusion and mistakes.
• Scientists in different places could not read, check or repeat each other’s work.
• Calculations (for example, of mass) would go wrong if one element were mistaken for another.
• Labels on chemicals and medicines could be misread, which is unsafe.

Yes. The atomic number Z uniquely identifies an element, and two atoms with different Z are always different elements.

The atomic number is the number of protons in the nucleus, and each element has its own number of protons — hydrogen 1, helium 2, lithium 3, sodium 11. If the number of protons changes, the element itself changes.

A lithium atom has 3 protons and 4 neutrons in its nucleus. Its atomic number is 3.

The enlarged nucleus in Fig. 8.10 shows 7 particles — 3 of one colour and 4 of the other — and the atom shows 3 electrons. A neutral atom has as many protons as electrons, so there are 3 protons and the other 4 particles are neutrons (Table 8.3 gives the same numbers).

Electrons 26, protons 26, neutrons 30. The element is iron, ⁵⁶₂₆Fe.

Formulae: Z = number of protons = number of electrons (neutral atom); nucleons = A = p⁺ + n⁰, so n⁰ = A – Z.

21 neutrons.

Formula: Mass number A = number of protons + number of neutrons, so n⁰ = A – p⁺.

Mass number = 35.

Formula: A = number of protons + number of neutrons, and number of protons = Z.

12 neutrons.

Reading the symbol: in ^A_ZX the upper number is the mass number. Here the upper number is 23, so A = 23. The atom is neutral, so the number of protons equals the number of electrons.

Both electrons go into the K-shell: the configuration of helium is simply 2.

Rule: a shell can hold at most 2n² electrons, and shells are filled from the innermost outward.

(i) 4 (ii) 7 (iii) 4.

Method: the lower number in ^A_ZX is the atomic number Z, which equals the number of electrons in a neutral atom. Fill the shells K, L, M in turn, using the maximum 2n² and the rule that the outermost shell may not hold more than 8.

Z = 12 → 2, 8, 2 | Z = 16 → 2, 8, 6 | Z = 18 → 2, 8, 8.

I am sodium, ²³₁₁Na, and I have 12 neutrons.

Sample riddle: “My atomic number is 17 and I have 18 neutrons. I am a greenish-yellow gas and my outermost shell has 7 electrons. Who am I, and what is my mass number?” Answer: chlorine; A = 17 + 18 = 35.

Nitrogen: 3. Magnesium: 2.

Rule: the combining capacity of an element is the number of hydrogen or chlorine atoms that combine with one of its atoms — because hydrogen and chlorine each have a combining capacity of one.

No. They stay as they are. Such atoms are already stable, so they are almost completely unreactive.

An atom with 8 electrons in its outermost shell (an octet) — or 2 in the case of helium — is already stable. So it does not lose, gain or share electrons. Such elements, for example neon (2, 8) and argon (2, 8, 8), are largely unreactive, and their valency is 0.

Here is Table 8.4 with the extra column. Rule: if the valence shell has fewer than 4 electrons the atom loses them; if more than 4 it gains the electrons needed to reach 8; with exactly 4 it shares. Valency is that number of electrons lost, gained or shared.

One electron each. Protium, deuterium and tritium all have exactly 1 electron.

Reasoning: all three are hydrogen, so Z = 1 for each. A neutral atom has as many electrons as protons, so each has 1 electron, sitting in the K-shell.

Same atomic number, different mass numbers — they are the same element (sodium), and they are isotopes of each other.

Average atomic mass of bromine ≈ 80.01 u, i.e. about 80 u.

Formula (weighted average):

Substituting:

So the average atomic mass of bromine is about 80 u.

Correct options: (ii) and (iii). Incorrect: (i) and (iv).

Only (iii) is correct. (i), (ii) and (iv) are incorrect.

(i) Y and Z are isotopes. (ii) Z and X are isobars.

First work out Z (atomic number) and A (mass number) for each species, using A = p⁺ + n⁰.

(i) Y and Z have the same atomic number (17) but different mass numbers (35 and 37), so they are isotopes (both are chlorine).
(ii) Z and X have the same mass number (37) but different atomic numbers (17 and 18), so they are isobars — atoms of different elements (chlorine and argon).

Rutherford concluded that the positive charge is not spread through the atom but is concentrated in an extremely small region at the centre, which he called the nucleus.

Position. At the centre of the atom, occupying only a minute fraction of its volume. The atom’s diameter is about 10^–10 m; the nucleus is about 10^–15 m across — some 10⁵ times smaller. Everything outside is essentially empty space through which the electrons move.

Characteristics.

• It carries all the positive charge of the atom.
• It contains almost all the mass — the electrons outside are so light that their mass can be ignored.
• It is therefore extremely dense.

Correct chronological order: (iv) → (ii) → (iii) → (i), that is Dalton → Thomson → Rutherford → Bohr.

They do not fly away because of the electrostatic force of attraction between the negatively charged electrons and the positively charged nucleus.

• The nucleus contains protons, each carrying charge +1. Each electron carries –1. Unlike charges attract.
• This attraction is directed continuously towards the centre, so it keeps pulling the electron back in as the electron moves. It is the force that bends the electron’s path into a closed shell instead of a straight line.
• The role it plays is the same as that of gravity for a planet going round the Sun: an inward force that keeps a moving body in a closed path.

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

Reason: A is true — the discovery of electrons, protons and neutrons made it possible to work out how an atom is built. R is also true for a neutral atom. But R is only one fact about atoms; it does not explain why the discovery of subatomic particles helped us understand atomic structure.

(i) 12 protons (ii) 12 neutrons (iii) 12 electrons. The electronic configuration is 2, 8, 2.

Counting the electrons drawn on each shell in Fig. 8.17 gives the configurations 2,1 · 2,5 · 2,8,3 · 2,7, which identify the four elements.

Rutherford’s model: an electron revolving in a circle is accelerating, so it should keep losing energy, spiral inward and fall into the nucleus. The atom would collapse — so the model could not explain stability.

Bohr’s model: electrons can revolve only in certain fixed shells (stationary states) K, L, M, N, each with a definite energy, and an electron moving in such a shell does not lose energy. So it does not fall into the nucleus, and the atom is stable.

39 neutrons.

Reading the symbol: the superscript in ⁷⁰X is the mass number, so A = 70. The atom is neutral, so protons = electrons.

(i) 118 neutrons (ii) 79 electrons. The element is gold, ¹⁹⁷₇₉Au.

Every blank follows from two relations: Z = number of protons = number of electrons (neutral atom) and A = number of protons + number of neutrons.

(i) Protons = A – neutrons = 35 – 18 = 17; electrons = protons = 17 (neutral atom).
(ii) Atomic number Z = number of protons = 17.
(iii) Z = 17, so X is chlorine (Cl), ³⁵₁₇Cl.
(iv) Electronic configuration: K = 2, L = 8, M = 7, i.e. 2, 8, 7.
(v) 7 valence electrons (in the M-shell).
(vi) Neutrons = 18 + 2 = 20, so new mass number = 17 + 20 = 37.
(vii) Same atomic number (17) but different mass numbers (35 and 37), so X and the new atom are isotopes (of chlorine).

(i) Atomic number — no change; it stays 12, because it is the number of protons, and the nucleus is not changed.

(ii) Atomic mass — increases. The mass of ordinary electrons is negligible, but 12 particles each 500 times heavier than an electron add noticeably to the mass (about 3.3 u in all), so the atom becomes heavier.

(iii) Mass number — no change; it stays 12 + 12 = 24, because it counts only protons and neutrons.

(iv) Overall charge — no change; the atom stays neutral, because the 12 new particles carry the same charge as 12 electrons (total –12), which balances the +12 of the protons.

How to build it. Make 18 cards, one for each of the first eighteen elements in Table 8.4. On the back of each card write four clues that get easier as you go down.

What a good report must contain: for each field, name the atomic property involved, give a concrete Indian example, and explain the link between them. Do not just list applications — show which property makes each one possible.

The play works best if each scientist is put on stage to defend a claim and is then challenged by the next. Here is a scene plan you can build on.

Start with two free, ready-made PhET simulations — they are free, they run in a browser, and they let you change the variables that matter.

• Rutherford Scattering (phet.colorado.edu/en/simulations/rutherford-scattering) — switch between the plum pudding atom and the nuclear atom and watch the difference in the alpha particle tracks. Increase the number of protons and see the deflections grow.
• Isotopes and Atomic Mass (phet.colorado.edu/en/simulations/isotopes-and-atomic-mass) — build isotopes by adding neutrons and watch the average atomic mass change as you adjust the abundances. Try to reproduce chlorine’s 35.5 u from 75% and 25%.

• What I watched: a documentary on the discovery of the atomic nucleus. Its main idea was that our picture of the atom changed each time an experiment gave an unexpected result.
• What it taught me: the atom is mostly empty space with a tiny, dense, positive nucleus at the centre; electrons move in energy levels around it; the modern model describes electrons as clouds.
• Scientists: Thomson (electron, plum pudding model), Rutherford (nucleus, gold foil experiment), Bohr (fixed energy levels), Chadwick (neutron).
• Most interesting part and my question: α-particles bouncing back from a thin gold foil. If electrons are clouds, what exactly is ‘moving’ around the nucleus?

Method: put the three elements on the horizontal axis and the number of electrons on the vertical axis, then draw one bar per shell for each element — three bars in a group, for K, L and M. Keep the same colour for the same shell throughout.

Take sodium (2, 8, 1), silicon (2, 8, 4) and argon (2, 8, 8):

• https://phet.colorado.edu/en/simulations/rutherford-scattering — compare the paths of α-particles through a plum pudding atom and a nuclear atom.
• https://phet.colorado.edu/en/simulations/isotopes-and-atomic-mass — build isotopes and see how their abundances decide the average atomic mass.

Probably not completely. Every atomic model — Dalton’s, Thomson’s, Rutherford’s and Bohr’s — was improved when new experiments gave new results. Today we describe electrons as clouds: we can predict where an electron is most likely to be, but not exactly where it is. So our understanding keeps getting better, but there is still much to discover — the journey inside the atom is not over.