5.1.2 The nucleus
- Syllabus
- 0625–2026–2027
- Topic
- 5.1.2
- Level
- —
Learning objectives
5.1.2.1Composition of the nucleus in terms of• Describe the composition of the nucleus in terms of protons and neutrons5.1.2.2Relative charges of protons, neutrons• State the relative charges of protons, neutrons and electrons as +1, 0 and –1 respectively5.1.2.3Terms proton number (atomic number) Z• Define the terms proton number (atomic number) Z and nucleon number (mass number) A and be able to calculate the number of neutrons in a nucleus5.1.2.4Nuclide notation A ZX• Use the nuclide notation A ZX5.1.2.5What is meant by an isotope and state• Explain what is meant by an isotope and state that an element may have more than one isotope5.1.2.6Processes of nuclear fission and• Describe the processes of nuclear fission and nuclear fusion as the splitting or joining of nuclei, to include the nuclide equation and qualitative description of mass and energy changes without values5.1.2.7Relationship between the proton number• Know the relationship between the proton number and the relative charge on a nucleus5.1.2.8Relationship between the nucleon• Know the relationship between the nucleon number and the relative mass of a nucleus
Identify what is inside a nucleus
The nucleus of an atom is made of protons and neutrons. Together, protons and neutrons are called nucleons.
| Particle | Location | Role in the nucleus |
|---|---|---|
| proton | inside the nucleus | nucleon that contributes positive charge and mass |
| neutron | inside the nucleus | uncharged nucleon that contributes mass |
| electron | outside the nucleus | not part of the nucleus |
A nucleus contains protons and neutrons, not electrons. The number of protons and neutrons need not be equal.
Recall the relative charges of particles
| Particle | Relative charge |
|---|---|
| proton | +1 |
| neutron | 0 |
| electron | −1 |
A proton and an electron have equal charge magnitude but opposite signs. A neutron is uncharged; it is not a negatively charged particle.
The charge of a collection of particles is the sum of their relative charges. Protons add positive charge, electrons add negative charge and neutrons add none.
Relative charge has no unit and does not state particle mass. Keep charge and mass comparisons separate.
Use proton number and nucleon number
| Quantity | Symbol | Meaning |
|---|---|---|
| proton number (atomic number) | Z | number of protons in the nucleus |
| nucleon number (mass number) | A | total number of protons and neutrons in the nucleus |
| neutron number | — | A − Z |
Read Z to get the number of protons. Subtract Z from A to get neutrons. For a neutral atom only, the number of electrons equals Z.
For chlorine-35 with Z = 17: protons = 17, neutrons = 35 − 17 = 18, and a neutral atom has 17 electrons.
A is not the neutron number. Do not subtract in the opposite direction, and do not assume an ion has Z electrons.
Read and write nuclide notation
Nuclide notation is written with the nucleon number A at the upper left and proton number Z at the lower left of the element symbol X: ᴬ_ZX.
| Part | Position | Meaning |
|---|---|---|
| A | upper left | protons + neutrons |
| Z | lower left | protons and element identity |
| X | centre/right | chemical symbol of the element |
A platinum nucleus with 78 protons and 118 neutrons has A = 196 and is written ¹⁹⁶₇₈Pt.
Check that A is at least Z and that A − Z gives a whole, non-negative neutron number. The element symbol must match Z.
Recognise isotopes of one element
Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. An element may have more than one isotope.
| Feature | Isotopes of the same element |
|---|---|
| proton number Z | same |
| number of protons | same |
| number of neutrons | different |
| nucleon number A | different |
| element symbol | same |
Carbon-12 and carbon-14 both have Z = 6, so both contain 6 protons. Carbon-14 has two more neutrons and a greater nucleon number.
Isotopes are not required to be radioactive or unstable. Different proton numbers mean different elements, not isotopes of one element.
Compare nuclear fission and fusion
| Process | Nuclear change | Typical setting |
|---|---|---|
| fission | a heavy nucleus splits into two or more smaller nuclei, often after absorbing a neutron; neutrons may be released | nuclear reactor |
| fusion | two light nuclei join to form a heavier nucleus; very high temperature helps them overcome electrostatic repulsion | stars |
Both processes can release energy. In each energy-releasing reaction, the total mass of the products is slightly smaller than the total mass before the reaction; the mass decrease corresponds to released energy.
In a nuclide equation, balance both totals across the arrow: upper nucleon numbers must sum to the same value on both sides, and lower proton numbers must also sum to the same value.
Fission pattern: ¹₀n + ²³⁵₉₂U → two smaller nuclei + further ¹₀n particles + energy. The exact product nuclei can vary, but both number sums must balance.
Fusion example: ²₁H + ³₁H → ⁴₂He + ¹₀n + energy. Upper numbers balance 5 = 4 + 1 and lower numbers balance 2 = 2 + 0.
Fission is nuclear splitting and fusion is nuclear joining; neither is electron transfer or a chemical reaction. Released energy does not mean nucleon or proton totals fail to balance.
Relate proton number to nuclear charge
A nucleus contains Z protons and each proton has relative charge +1. Neutrons have charge 0, so the relative charge of the nucleus is +Z.
| Proton number Z | Relative nuclear charge |
|---|---|
| 1 | +1 |
| 6 | +6 |
| 79 | +79 |
For a neutral atom, Z electrons outside the nucleus supply total charge −Z and balance the nuclear charge +Z.
Nuclear charge depends on proton number, not nucleon number. Neutrons increase mass but do not increase charge.
Relate nucleon number to nuclear mass
Protons and neutrons each have relative mass approximately 1, while electron mass is negligible at this scale. A nucleus with nucleon number A therefore has relative mass approximately A.
| Quantity | Nuclear relationship |
|---|---|
| number of protons + neutrons | A |
| relative nuclear mass | approximately A |
| relative nuclear charge | +Z, not +A |
A helium-3 nucleus has relative mass about 3; a helium-4 nucleus has relative mass about 4. Their charge is the same because both have Z = 2.
A is a particle count and an approximate relative mass, not a mass in kilograms. It does not give nuclear charge or electron number.