Journey Inside the Atom Class 9: Notes and Questions

Journey Inside the Atom is chapter 8 of the new Class 9 Science textbook. In this guide you will follow the story of the atom from ancient ideas to Bohr's model, learn about electrons, protons and neutrons, and practise atomic number, mass number, electron shells, valency, isotopes and isobars. Important questions with model answers come at the end.

From ancient ideas to Dalton

The idea of the atom began more than 2,000 years ago in ancient India and Greece, as a question about what matter is made of.

• Acharya Kanada (India): said that if matter (dravya) is divided again and again, we reach the smallest particle that cannot be divided further. He called these particles parmanus. They are infinitely small and beyond our senses. Parmanus combine to form dyads (groups of two) and triads (groups of three). His ideas were recorded in the Sanskrit text Vaisesika Sutras.
• Leucippus and Democritus (Greece): said matter is made of indivisible particles called atomos.

These early ideas came from imagination and logic, not from experiments.

In 1808, John Dalton gave the first scientific description of the atom. His atomic theory said all matter is made of indivisible atoms, the basic building blocks that cannot be broken into smaller parts. This became the foundation of modern atomic science.

Thomson: the electron and the plum pudding model

In 1897, J. J. Thomson passed electric current through gases at very low pressure in a cathode ray tube. He saw rays moving from the negative electrode (cathode) to the positive electrode (anode). He concluded that these cathode rays were streams of negatively charged particles, much smaller than atoms. These particles were later named electrons. The charge of an electron is about -1.602 × 10⁻¹⁹ C, taken as -1 by convention.

So atoms are not indivisible after all. To show how charges are arranged, Thomson proposed the Plum Pudding Model (also called the Watermelon Model):

• The atom is a sphere of uniform positive charge, like the red pulp of a watermelon.
• Electrons are scattered in it, like seeds in the watermelon.
• Positive and negative charges are equal in size, so the atom is electrically neutral.

Rutherford: the gold foil experiment

In 1911, Geiger and Marsden, working under Ernest Rutherford, fired alpha particles (positively charged helium nuclei) at a thin gold foil.

Rutherford proposed the Planetary Model: electrons revolve around the nucleus like planets around the Sun. The nucleus is about 10⁵ (one lakh) times smaller than the atom. An atom's diameter is about 10⁻¹⁰ m, while the nucleus is only about 10⁻¹⁵ m across. If an atom were as big as a cricket ground, the nucleus would be like a tiny grain of black pepper at its centre.

Bohr: why atoms are stable

Rutherford's model had a flaw. In classical physics, an orbiting electron should lose energy and spiral into the nucleus, making the atom unstable.

In 1913, Niels Bohr said electrons move only in fixed circular paths called stationary states, orbits or shells:

• In these shells, electrons have a definite energy and do not radiate energy.
• Shells are named K, L, M, N... or n = 1, 2, 3, 4... starting from the nucleus.
• Energy increases as you move away from the nucleus. The K-shell (n = 1) is closest and has the lowest energy.
• An electron can jump between levels by absorbing or releasing energy equal to the difference between the levels.

This explained why atoms are stable.

The neutron and the three particles

In 1932, James Chadwick discovered the neutron: no charge, and mass nearly equal to a proton. It is found in the nucleus of all atoms except hydrogen's protium isotope. The mass of an atom is the sum of the masses of its protons and neutrons; electrons are so light that their mass is treated as negligible. Neutrons also help hold the nucleus together, because protons repel each other and neutrons reduce this repulsion.

Atomic number, mass number and electron shells

• Atomic number (Z): number of protons. In a neutral atom it also equals the number of electrons. It identifies the element.
• Mass number (A): protons + neutrons (together called nucleons). So, number of neutrons = A - Z.
• Notation: mass number as a superscript and atomic number as a subscript on the left, for example ¹²₆C for Carbon-12.

Example: sodium has A = 23 and Z = 11, so neutrons = 23 - 11 = 12.

Filling shells (Bohr and Bury rules):

1. A shell can hold at most 2n² electrons: K holds 2, L holds 8, M up to 18.
2. The outermost shell cannot hold more than 8 electrons.
3. Shells fill step by step: inner shells first.

Valency, isotopes and isobars

Valency is the number of electrons an atom must gain, lose or share to complete its octet. The outermost shell is the valence shell. Atoms with a full outer shell (8 electrons, or 2 for helium) are stable and unreactive.

• 1 to 3 valence electrons: the atom usually loses them, so valency = number of valence electrons. Sodium (2, 8, 1) has valency 1.
• 5 to 7 valence electrons: it gains electrons, so valency = 8 - valence electrons. Oxygen (2, 6) has valency 2.
• Carbon (2, 4) shares 4 electrons, so its valency is 4.

Isotopes are atoms of the same element with the same atomic number but different mass numbers (different numbers of neutrons). Hydrogen has three: protium (0 neutrons), deuterium (1) and tritium (2). Isotopes have the same chemical properties, because they have the same electron arrangement, but different physical properties such as boiling point.

Uses of isotopes: Uranium-235 as nuclear fuel, Cobalt-60 in cancer treatment, Iodine-131 to treat goitre, and Carbon-14 to find the age of ancient fossils and artefacts.

Average atomic mass: chlorine has Cl-35 (75%) and Cl-37 (25%). Average mass = (35 × 75) + (37 × 25) / 100 = (2625 + 925) / 100 = 35.5 u.

Isobars are atoms of different elements with different atomic numbers but the same mass number. Calcium (Z = 20), potassium (Z = 19) and argon (Z = 18) can all have mass number 40. Being different elements, they have different chemical properties.

Remember this

• Kanada: parmanus, Vaisesika Sutras. Dalton: 1808. Thomson: electron, 1897. Rutherford: nucleus, 1911. Bohr: shells, 1913. Chadwick: neutron, 1932.
• A = protons + neutrons; neutrons = A - Z.
• Shell capacity 2n², but never more than 8 in the outermost shell.
• Isotopes: same Z, different A. Isobars: same A, different Z.

Important questions with answers

1. Why was Dalton's theory different from ancient ideas of the atom? Dalton's theory was based on the scientific experiments of his time. Ancient ideas were based on imagination and logical reasoning.

2. What are cathode rays made of? Streams of negatively charged particles called electrons.

3. What did the few alpha particles that bounced back tell Rutherford? That all the positive charge and mass of the atom are concentrated in a tiny nucleus.

4. What problem in Rutherford's model did Bohr solve? An orbiting electron should lose energy and fall into the nucleus. Bohr said electrons move in stationary states where their energy stays constant, so the atom is stable.

5. An atom has 20 protons and 21 neutrons. Find its mass number. A = 20 + 21 = 41.

6. Write the electronic configuration of chlorine (Z = 17) and find its valency. 2, 8, 7. It has 7 valence electrons, so it gains 1: valency = 8 - 7 = 1.

7. What is the valency of magnesium (2, 8, 2)? It has 2 valence electrons and loses them, so its valency is 2.

8. What is the valency of the element with atomic number 10? It is neon (2, 8). Its outer shell is full, so its valency is 0.

9. How many neutrons are in Cl-37? (Z = 17) Neutrons = A - Z = 37 - 17 = 20.

10. Which subatomic particle is missing in the nucleus of a common hydrogen atom? The neutron (in protium).

Common mistakes to avoid

• Mixing up isotopes and isobars. Check what stays the same: Z for isotopes, A for isobars.
• Writing a configuration like 2, 8, 9. The outermost shell never holds more than 8 electrons.
• Adding electrons to the mass number. A counts only protons and neutrons.
• Forgetting that atoms with 5 to 7 valence electrons have valency 8 minus valence electrons, not the number itself.

When you are ready to test yourself on every part of this chapter, study this chapter with Joy on Learnijoy.