5.1.1Nuclear Physics
The Nucleus — Cambridge IGCSE Co-ordinated Sciences 0654 - Physics Revision Notes & Topical Questions
5 min read 10 parts Lesson 2 of 10
Part 1 of 8Proton and nucleon numbers
Proton and nucleon numbers
Proton number Z (atomic number)
Proton number Z (atomic number) the number of protons in the nucleus
- Elements are ordered by proton number, so Z decides which element the atom is.
- Hydrogen has Z = 1, sodium has Z = 11 and uranium has Z = 92.
- A neutral atom has the same number of electrons as protons.
Nucleon number A (mass number)
Nucleon number A (mass number) the total number of protons and neutrons in the nucleus
Neutron number
- Neutron number number of neutrons = A - Z
- Electrons are not counted in A because their mass is negligible.
Part 2 of 8Nuclide notation
Notation
- Notation the symbol is written as AZX
- The top number A is the nucleon number.
- The bottom number Z is the proton number.
Nuclide
Nuclide a group of atoms with the same number of protons and neutrons
Reading a nuclide
- Reading a nuclide find Z, then A, then work out the neutrons as A - Z
Part 3 of 8Isotopes
Isotopes
Isotopes atoms of the same element with the same number of protons but different numbers of neutrons
- They have the same Z but different A.
- Each element can have more than one isotope.
Hydrogen isotopes
- Hydrogen isotopes hydrogen (0 neutrons), deuterium (1 neutron) and tritium (2 neutrons)
Carbon isotopes
- Carbon isotopes carbon-12 has 6 neutrons and carbon-14 has 8 neutrons
Stability
- Stability isotopes tend to be less stable because of the imbalance of protons and neutrons, so they are more likely to decay
Radioisotope
- Radioisotope an unstable isotope that emits radiation
Part 4 of 8Nuclear charge and mass
Nuclear charge
Nuclear charge the number of protons multiplied by the relative charge of a proton, so it is +Z
- Carbon has Z = 6, so its nuclear charge is +6.
Nuclear mass
Nuclear mass the number of protons and neutrons multiplied by the relative mass of each, so it equals A
- Carbon-12 has a relative mass of 12.
- A relative mass of 1 is 1.67 × 10-27 kg.
Nuclear equations
- Nuclear equations the total nucleon number and the total proton number are conserved
Part 5 of 8Nuclear fission
Fission
Fission the splitting of a large, unstable nucleus into two smaller nuclei
- A neutron collides with an unstable nucleus such as uranium-235.
- The nucleus splits into two daughter nuclei and two or three neutrons, and gamma rays are emitted.
- The released neutrons can cause further fissions, giving a chain reaction.
Energy
- Energy the products move away very quickly, so the nuclear energy is transferred to their kinetic energy
- The products have slightly less mass than the original nucleus, and the missing mass is converted to energy.
Equation
- 23592U + 10n → 9236Kr + 14156Ba + 3 10n + energy
- The nucleon numbers balance, 235 + 1 = 92 + 141 + 3.
- The proton numbers balance, 92 + 0 = 36 + 56 + 0.
Fuels
- Fuels uranium and plutonium are used in nuclear power stations
Part 6 of 8Nuclear fusion
Fusion
Fusion two light nuclei join to form a heavier nucleus
- Stars use fusion, in which hydrogen nuclei fuse to form helium and release a huge amount of energy.
Conditions
- Conditions it needs an extremely high temperature and pressure, so it is very hard to reproduce on Earth
Energy
- Energy the fused nucleus has slightly less mass than the two original nuclei, and the missing mass is converted to energy
- The energy from 1 kg of hydrogen fusing equals that from burning about 10 million kg of coal.
Equation
- 21H + 11H → 32He + energy
Part 7 of 8Fission and fusion compared
| Feature | Fission | Fusion |
|---|---|---|
| What happens | A large nucleus splits | Small nuclei join |
| Typical nuclei | Uranium-235 and plutonium-239 | Hydrogen isotopes |
| Conditions | Starts when a neutron is absorbed | Very high temperature and pressure |
| Where it occurs | Nuclear power stations | Stars such as the Sun |
Part 8 of 8More from the specification
More from the specification
Nuclear model
- Nuclear model the atom has a small, dense, positively charged nucleus surrounded by negative electrons, shown by alpha particle scattering
Structure of the nucleus
- Structure of the nucleus protons and neutrons, where proton number determines the element and nucleon number gives total mass
Alpha particles
- Alpha particles most pass through gold foil, a few are deflected strongly, which means the positive charge is concentrated in a tiny region
Nuclear equations
- Nuclear equations reactants and products must balance in nucleon and proton number, which can be used to determine the daughter nucleus
Particle
- A tiny portion of matter that is too small to see.
Reactant
- A substance used up in a chemical reaction.
Alpha particle
- A helium nucleus, He2+, used in the scattering experiment.
Key terms
| Term | Meaning |
|---|---|
| Proton number (Z) | The number of protons in the nucleus; it decides the element. |
| Nucleon number (A) | The total number of protons and neutrons in the nucleus. |
| Nuclide | A group of atoms with the same number of protons and neutrons. |
| Isotopes | Atoms of the same element with the same number of protons but different numbers of neutrons. |
| Nuclear fission | The splitting of a large unstable nucleus into two smaller nuclei. |
| Nuclear fusion | The joining of two light nuclei to form a heavier nucleus. |
| Radioisotope | An unstable isotope that emits radiation. |
Avoid these mistakes
1
Calculating neutrons as A + Z — the neutron number is A - Z
2
Thinking isotopes have different proton numbers — they have the same proton number and different neutron numbers
3
Counting electrons in the nucleon number — electrons are not in the nucleus and their mass is negligible
4
Mixing up fission and fusion — fission splits a large nucleus and fusion joins small nuclei
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