Atoms & Nuclei
Physics – INI CET – Beginner Notes
1. What is an Atom?
- The smallest part that keeps the same chemical identity.
- Think of it as a tiny solar system:
- The nucleus is the Sun.
- Electrons are the planets orbiting around it.
2. Parts of an Atom
| Part | What it is | Everyday Analogy |
|---|---|---|
| Nucleus | Holds most of the mass. | The heart of a cell. |
| Protons | Positive charge. | “+” signs on a battery. |
| Neutrons | No charge, neutral. | The “empty space” in a room. |
| Electrons | Negative charge. | Tiny balls moving around the Sun. |
- Protons + Neutrons = Nucleus
- Electrons orbit the nucleus.
3. Numbers that Identify an Atom
| Number | What it tells you | Analogy |
|---|---|---|
| Atomic Number (Z) | Count of protons. | The number on a house plate. |
| Mass Number (A) | Protons + Neutrons. | Total weight of a suitcase. |
Example:
- Hydrogen: Z = 1, A = 1
- Deuterium (heavy hydrogen): Z = 1, A = 2 (one proton + one neutron)
4. Tiny Example – Hydrogen Atom
- Proton: 1, in the center.
- Electron: 1, moves around.
- No neutrons (A = 1).
- This is the lightest element.
5. Isotopes
- Same element (same Z) but different A.
- Same chemistry, different mass.
- Example:
- Carbon‑12 (6 protons + 6 neutrons)
- Carbon‑14 (6 protons + 8 neutrons) – used in dating fossils.
6. Inside the Nucleus
- Protons repel each other (like two magnets with the same pole).
- Strong nuclear force pulls them together, stronger than the repulsion.
- This force keeps the nucleus stable, like a glue that holds a team together.
7. Nuclear Reactions – Very Quick View
| Reaction | What happens | Everyday Analogy |
|---|---|---|
| Fusion | Two light nuclei join to make a heavier one. | Two Lego bricks snapping together. |
| Fission | A heavy nucleus splits into lighter pieces. | A toy car breaking into two parts when you hit it. |
- Fusion creates energy in the Sun.
- Fission is used in nuclear power plants.
8. Summary
- Atom = tiny solar system.
- Nucleus = protons + neutrons, holds mass.
- Electrons orbit the nucleus.
- Atomic number = protons.
- Mass number = protons + neutrons.
- Isotopes differ only in neutrons.
- Strong force keeps the nucleus glued.
- Fusion/fission are ways nuclei change and release energy.
9. Quick Quiz
| Question | Answer |
|---|---|
| What is the smallest part that keeps the same identity? | Atom |
| How many protons are in a helium atom? | 2 |
| What keeps protons together inside the nucleus? | Strong nuclear force |
| Which reaction powers the Sun? | Fusion |
| What is an isotope? | Same element, different mass |
Answer key at the back of the book.