5.1.2 The Nucleus
Structure of the Atom
An atom consists of a central nucleus surrounded by electrons.
- The nucleus contains protons and neutrons.
- Electrons orbit the nucleus in energy levels or shells.
- The atom is electrically neutral because the number of protons equals the number of electrons.
Representation of the Atomic Nucleus
The nucleus of an element is represented as AZX.
- X → chemical symbol
- A → mass number (nucleon number) = protons + neutrons
- Z → atomic number = number of protons
Example Calculation
For 21284Po:
- Protons,
Z = 84 - Neutrons,
n = 212 − 84 = 128 - Electrons,
84(since atom is neutral)
Forces and Interactions
- Strong nuclear force binds protons and neutrons in the nucleus.
- Electrostatic attraction exists between positive protons and negative electrons.
Nuclide
Each unique nuclear species (with a specific number of protons and neutrons) is called a nuclide.
Charge and Mass of Subatomic Particles
| Particle | Charge (C) | Relative Charge | Mass (kg) | Relative Mass |
|---|---|---|---|---|
| Proton | +1.6 × 10⁻¹⁹ | +1 | 1.67 × 10⁻²⁷ | 1 |
| Neutron | 0 | 0 | 1.67 × 10⁻²⁷ | 1 |
| Electron | −1.6 × 10⁻¹⁹ | −1 | 9.11 × 10⁻³¹ | 1/1840 |
Nuclear Fission
Nuclear fission is the process in which a heavy nucleus splits into two smaller nuclei, releasing energy.
- Occurs in nuclear reactors.
- Isotopes used: Uranium-235 and Plutonium-239.
- When struck by a neutron, the nucleus splits into smaller nuclei and releases more neutrons.
- This creates a chain reaction.
Controlling the Chain Reaction
- Control rods made of boron absorb excess neutrons.
- Raising control rods → increases free neutrons → more fission.
- Lowering control rods → decreases free neutrons → slows reaction.
- This maintains a controlled release of energy and prevents explosion.
Nuclear Fusion
Nuclear fusion involves combining two small nuclei to form a larger one with a release of energy.
- Fusion occurs naturally in the Sun and other stars.
- Deuterium and tritium nuclei combine to form helium and release energy.
- To overcome electrostatic repulsion, the nuclei must move extremely fast.
- Requires very high temperature and pressure.
Comparison Between Fission and Fusion
| Process | Description | Examples | Energy Released |
|---|---|---|---|
| Fission | Splitting of heavy nucleus into smaller ones | U-235, Pu-239 | Large, used in nuclear reactors |
| Fusion | Joining of light nuclei into a heavier one | H-2 + H-3 → He | Even larger, source of energy in stars |
Exam Focus
- Be able to calculate protons, neutrons, and electrons using A and Z values.
- State that atoms are electrically neutral overall.
- Explain how the strong nuclear force overcomes proton repulsion.
- Describe differences between nuclear fission and fusion, including energy and control methods.
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