General Science Notes Chapter 5 Inside the Atom Class 8 Maharashtra Board

Notes For All Chapters – General Science Class 8
1. Introduction

• Everything around us is made of matter.
• Matter is made of tiny particles called atoms.
• Atoms are the smallest unit of an element and cannot be seen with the naked eye.

2. What is an Atom?

• An atom is the smallest unit of matter that retains the properties of an element.
• Atoms combine to form molecules, which make up all substances.

Parts of an Atom:

1. Nucleus – The central part of the atom.
2. Electrons – Negatively charged particles that revolve around the nucleus.
3. Protons – Positively charged particles inside the nucleus.
4. Neutrons – Neutral particles inside the nucleus.

3. Subatomic Particles

• Protons and neutrons are inside the nucleus.
• Electrons move around the nucleus in shells (orbits).

4. Atomic Number and Mass Number

Atomic Number (Z):

• The number of protons in an atom.
• Example: Hydrogen (H) has 1 proton, so its atomic number is 1.

Atomic Mass Number (A):

• The total number of protons + neutrons in an atom.
• Formula:Mass Number (A) = Protons + Neutrons
• Example: Carbon (C) has 6 protons and 6 neutrons, so its mass number is 12.

5. Models of the Atom

(A) Dalton’s Atomic Theory (1803)

• John Dalton said that:
  1. Atoms are tiny, indivisible particles.
  2. Atoms of the same element are identical.
  3. Atoms of different elements are different.
  4. Atoms combine to form compounds.

(B) Thomson’s Atomic Model (1897)

• J.J. Thomson discovered the electron.
• He proposed the Plum Pudding Model:
  • The atom is a positively charged sphere with electrons embedded like “raisins in a pudding”.
  • This model was incorrect because it could not explain the nucleus.

(C) Rutherford’s Atomic Model (1911)

• Ernest Rutherford performed the Gold Foil Experiment:
  • He bombarded a thin gold foil with alpha particles (positively charged particles).
  • Some particles passed through, but a few bounced back.
• Conclusions:
  1. Atoms have a small, dense nucleus in the center.
  2. The nucleus contains protons and neutrons.
  3. Electrons revolve around the nucleus in empty space.

(D) Bohr’s Atomic Model (1913)

• Niels Bohr improved Rutherford’s model by introducing energy levels (shells).
• Bohr’s Postulates:
  1. Electrons revolve in fixed orbits around the nucleus.
  2. Each orbit has a fixed energy and electrons do not lose energy while moving in an orbit.
  3. Electrons can jump from a lower to a higher energy level by absorbing energy and vice versa.

6. Electronic Configuration

Electron Shells:

• Electrons are arranged in shells (orbits) around the nucleus.
• The shells are named K, L, M, N,…
• The maximum number of electrons in a shell is given by 2n², where n is the shell number.

Example:

• Carbon (C) has 6 electrons → K(2), L(4).
• Oxygen (O) has 8 electrons → K(2), L(6).

7. Valency and Stability

• Valency: The number of bonds an atom can form.
• If an atom has 1 to 4 valence electrons, its valency is the same.
• If an atom has 5 to 8 valence electrons, valency = (8 – number of valence electrons).
• Example:
  • Hydrogen (H) has 1 valence electron → Valency = 1
  • Oxygen (O) has 6 valence electrons → Valency = 2

8. Isotopes

• Atoms of the same element with the same atomic number but different mass numbers.
• Example: Carbon has three isotopes: C-12, C-13, C-14.

9. Nuclear Reactor and Nuclear Energy

What is a Nuclear Reactor?

• A machine that generates electricity using nuclear fission.

How does a Nuclear Reactor work?

1. Uranium-235 or Thorium-232 is used as fuel.
2. Neutrons hit uranium atoms, splitting them into smaller atoms and releasing huge energy.
3. This energy heats water to produce steam.
4. Steam turns turbines to generate electricity.

Parts of a Nuclear Reactor:

• Fuel (Uranium-235): Provides energy.
• Moderator (Graphite/Heavy Water): Slows down neutrons.
• Control Rods (Boron/Cadmium): Absorb extra neutrons to control the reaction.
• Coolant (Water/Carbon dioxide): Removes heat from the reactor.

10. Uses of Nuclear Energy

1. Electricity production in power plants.
2. Medical treatments (e.g., Cancer treatment using radiation).
3. Preserving food by killing bacteria.
4. Finding the age of fossils using Carbon-14 dating.