1. Overview
The nucleus is the tiny, dense central core of an atom that contains nearly all of its mass. This topic explores how we describe the subatomic particles within the nucleus and how changes to this core result in the release of nuclear energy through fission and fusion.
Key Definitions
- Nucleon: A particle found in the nucleus (either a proton or a neutron).
- Proton Number ($Z$): Also known as the atomic number; the number of protons in the nucleus of an atom.
- Nucleon Number ($A$): Also known as the mass number; the total number of protons and neutrons in the nucleus.
- Isotope: Atoms of the same element that have the same number of protons but a different number of neutrons.
- Nuclide: A distinct kind of atom or nucleus characterized by a specific number of protons and neutrons.
Core Content
Composition of the Nucleus
The nucleus is made up of two types of particles, collectively called nucleons:
- Protons: Positively charged particles.
- Neutrons: Neutral particles (no charge).
Electrons are not in the nucleus; they orbit the nucleus in shells.
Relative Charges
In physics, we use "relative" charges to compare subatomic particles:
- Proton: +1
- Neutron: 0
- Electron: -1
Nuclide Notation
We use a standard symbol to describe a specific nucleus:
- $Z$ (Proton Number): Tells you the identity of the element.
- $A$ (Nucleon Number): Tells you the total mass.
Calculating the Number of Neutrons
To find the number of neutrons ($N$), subtract the proton number from the nucleon number: $N = A - Z$
Worked Example: A nucleus of Sodium is written as $^{23}_{11}\text{Na}$.
- Proton Number ($Z$) = 11
- Nucleon Number ($A$) = 23
- Number of Neutrons = $23 - 11 = 12$
Isotopes
An element can have different versions called isotopes.
- They have the same number of protons (so they are the same element).
- They have different numbers of neutrons (so they have different masses).
- Example: Carbon-12 ($^12_6\text{C}$) and Carbon-14 ($^14_6\text{C}$). Both have 6 protons, but Carbon-14 has two extra neutrons.
Extended Content (Extended Curriculum Only)
Nuclear Mass and Charge
- Nuclear Charge: The relative charge of a nucleus is equal to its Proton Number ($Z$). For example, a Helium nucleus ($Z=2$) has a relative charge of +2.
- Nuclear Mass: Because protons and neutrons have almost identical masses and electrons have negligible mass, the relative mass of a nucleus is equal to its Nucleon Number ($A$).
Nuclear Fission
Fission is the splitting of a large, unstable nucleus (like Uranium-235) into two smaller "daughter" nuclei.
- Process: Usually triggered by the nucleus absorbing a neutron.
- Energy Change: The total mass of the particles after the split is slightly less than the mass before. This "lost" mass is converted into a huge amount of energy.
Nuclear Fusion
Fusion is the joining (fusing) of two small, light nuclei to form a single, larger nucleus.
- Process: Occurs in stars (like the Sun) where high temperatures and pressures force nuclei together.
- Energy Change: Similar to fission, the mass of the final large nucleus is slightly less than the combined mass of the original smaller nuclei. This mass difference is released as energy.
Fission/Fusion Equation (Qualitative)
$Nucleus_1 + Nucleus_2 \rightarrow Nucleus_{new} + Energy$
Key Equations
| Equation | Symbols | Units |
|---|---|---|
| $A = Z + N$ | $A$: Nucleon number, $Z$: Proton number, $N$: Neutrons | None (Counting numbers) |
| Relative Nuclear Charge = $+Z$ | $Z$: Proton number | None (Relative) |
| Relative Nuclear Mass $\approx A$ | $A$: Nucleon number | None (Relative) |
Common Mistakes to Avoid
- ❌ Wrong: Adding the Proton Number and Nucleon Number together to find neutrons.
- ✅ Right: Always subtract the bottom number from the top number ($A - Z$) to find neutrons.
- ❌ Wrong: Thinking the top number ($A$) represents neutrons only.
- ✅ Right: The top number is the total of protons + neutrons.
- ❌ Wrong: Assuming electrons contribute to the mass of the atom.
- ✅ Right: Electrons have negligible mass; the mass is concentrated in the nucleus (protons and neutrons).
- ❌ Wrong: Swapping the positions of $A$ and $Z$ in notation.
- ✅ Right: Remember A is the Attic (top) and Z is the Zound (ground/bottom).
Exam Tips
- Check your subtraction: Many marks are lost on simple arithmetic when calculating neutron numbers. Double-check your $A - Z$ calculation.
- Isotope Definition: If asked to define an isotope, always mention both that they have the same number of protons AND a different number of neutrons.
- Fission vs. Fusion: To remember which is which, think of "Fission" as a "fissure" (a crack/split) and "Fusion" as "fusing" things together (like welding).