NCERT Solutions Class 9 Science Ch 8 Atom 2026-27

No. Atoms are made up of smaller subatomic particles: electrons, protons and neutrons. Experiments such as J. J. Thomson’s cathode ray experiment and Rutherford’s gold foil experiment showed that atoms can be divided into smaller parts and have an internal structure.

According to Bohr’s model, electrons revolve in fixed orbits (energy levels) around the nucleus. An electron has a definite energy in each orbit and does not lose energy while moving in it, so it does not spiral into the nucleus in spite of the attraction between the electrons and the protons.

New experimental results could not be explained by the earlier models. For example, Thomson’s model could not explain the results of Rutherford’s gold foil experiment, so a new model was proposed. Atomic models were revised so that they could explain the observations and evidence better.

• (i) If the positive charge is less than the total negative charge, the model will have a net negative charge, so it will not be neutral.
• (ii) If the clay itself carries some negative charge, the total negative charge increases even more, so the model again will not represent a neutral atom.

In both cases the model is not neutral, because the total positive and negative charges are not equal.

Where it matches: the soft pulp can stand for the positively charged sphere, and the seeds embedded in it can stand for the electrons. This matches Thomson’s idea that electrons are embedded in a positively charged medium.

Where it falls short:

• In an atom, the positive charge is spread uniformly and the electrons are distributed evenly, but the seeds in a fruit are few and not evenly spread.
• The pulp of the fruit does not actually carry a positive charge.

So the fruit is only a partial model of Thomson’s atom.

In his cathode ray experiments, Thomson found that the cathode rays behaved in the same way whatever gas was in the discharge tube and whatever material the electrodes were made of. This showed that the particles in the rays (electrons) are a common part of all atoms.

Negatively charged particles would be attracted by the positively charged nucleus (alpha particles were repelled by it). So they would be bent towards the nucleus instead of being pushed back, and they would not show the large-angle bouncing back seen with α-particles. The experiment would therefore not lead to the same conclusions about the structure of the atom.

In Thomson’s model the positive charge is spread uniformly through the whole atom. If this were true, the α-particles would be deflected only slightly and none would bounce back.

The sharp bouncing back showed that a very small region of the atom contains a large amount of positive charge and mass. So the positive charge is concentrated in a tiny, dense nucleus, and Thomson’s model was ruled out.

This is an open question. A good example: “If the electrons are attracted by the nucleus, how are they arranged around it so that they do not fall into it?” This is the question that led to Bohr’s model.

Correct option: (b)

Both A and R are true. But R describes Thomson’s model, which Rutherford’s result went against, so R does not explain A.

This is an open question. Example: suppose the new element is named Rudrium and given the symbol Rd.

The IUPAC rules say that the symbol of an element is made from its name and has one or two letters. The first letter is written in capital and the second letter (if any) in small. The symbol Rd follows these rules: it comes from the name Rudrium, with R in capital and d in small letter.

There would be confusion in scientific communication. Scientists from different places could not easily understand each other’s work, and it would be hard to write chemical formulae and share knowledge across the world. This is why standard symbols given by IUPAC are needed.

1. ProtonsNumber of protons = atomic number = 26
2. ElectronsFor a neutral atom, electrons = protons = 26
3. NeutronsNeutrons = mass number − atomic number = 56 − 26 = 30

Protons = 26, electrons = 26, neutrons = 30

Number of neutrons = 21

Mass number = 35

1. Atomic numberFor a neutral atom, protons = electrons = 11, so the atomic number = 11.
2. NeutronsNeutrons = 23 − 11 = 12

Number of neutrons = 12

The element with atomic number 11 is sodium (Na), a soft metal that reacts vigorously with water.

A riddle of your own: I am an atom with atomic number 17. I am a pale yellow-green gas used to disinfect water. Who am I, and what is my electronic configuration? Answer: chlorine (Cl), 2, 8, 7.

The atomic numbers are the same but the mass numbers are different. So they are atoms of the same element (sodium) and are called isotopes.

The average atomic mass of bromine ≈ 80.01 u

Correct option: (c) Statements b and c are correct.

• (a) Incorrect: the experiment did not show neutrons. They were discovered later by Chadwick.
• (b) Correct: the results disproved Thomson’s model and led to the idea of a small, dense, positive nucleus with mostly empty space around it.
• (c) Correct: the large deflection of a few particles showed that most of the mass and all the positive charge is in a very small centre.
• (d) Incorrect: the experiment gave no direct proof that electrons move around the nucleus. That idea came later with Bohr’s model.

(i) Y and Z: both have 17 protons, so the same atomic number, but different numbers of neutrons (18 and 20), so different mass numbers (35 and 37). They are isotopes of the same element (chlorine).

(ii) Z and X: Z has 17 protons and X has 18 protons. The atomic numbers are different, so they are atoms of different elements.

Rutherford concluded that the positively charged part of the atom is located at the centre, in a very small, dense region called the nucleus.

Since only a few alpha particles were deflected or bounced back, the nucleus must be extremely small compared with the whole atom, but it holds most of the positive charge and most of the mass of the atom.

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

1. Dalton: atoms are indivisible particles.
2. Thomson: the ‘plum pudding’ model, with electrons embedded in a positively charged sphere.
3. Rutherford: atoms have a small, dense, positively charged nucleus at the centre.
4. Bohr: electrons move in fixed orbits around the nucleus, each with a definite energy.

The negatively charged electrons are held by the strong electrostatic force of attraction between them and the positively charged nucleus. This attraction between opposite charges keeps the electrons moving in their fixed orbits and stops them from escaping the atom.

Correct option: (b)

A is true because the discovery of electrons, protons and neutrons showed the structure inside an atom. R is also true because a neutral atom has equal numbers of electrons and protons. But R does not explain how the discovery helped in understanding the structure.

Arrangement of electrons: 2, 8, 2. That is, 2 electrons in the K shell, 8 in the L shell and 2 in the M shell (2 valence electrons).

Count the electrons in each diagram. The number of electrons equals the number of protons (the atomic number).

Rutherford’s model: by the laws of classical physics, an electron moving in a circular orbit should keep radiating energy. It would then lose energy, slow down and spiral into the nucleus, so the atom would be unstable. But atoms are stable, so the model failed.

Bohr’s model: Bohr proposed that electrons move in fixed energy levels (stationary orbits) where they do not radiate energy. Energy is absorbed or emitted only when an electron jumps from one orbit to another. So the electrons do not fall into the nucleus, and the atom is stable.

1. ProtonsFor a neutral atom, protons = electrons = 31
2. NeutronsNeutrons = 70 − 31 = 39

Number of neutrons = 39

1. (i) Neutrons197 − 79 = 118
2. (ii) ElectronsFor a neutral atom, electrons = protons = 79

Neutrons = 118, electrons = 79

Use: protons = electrons = atomic number (Z), and mass number (A) = protons + neutrons.