This note covers the structure of the atom, how the nucleus was discovered, how nuclides and isotopes are described, and how energy is released by nuclear fission and fusion. These ideas underpin radioactivity, nuclear power and the way stars shine.
Key ideas
The nuclear atom. An atom has a tiny, dense, positively charged nucleus at its centre, surrounded by negatively charged electrons. Almost all the mass is in the nucleus. Most of the atom is empty space.
The nucleus contains two types of particle, called nucleons.
- Proton: relative charge +1, relative mass 1.
- Neutron: relative charge 0, relative mass 1.
- Electron: relative charge −1, mass about 1/2000 of a proton, so it is negligible.
A neutral atom has equal numbers of protons and electrons, so the charges cancel. If an atom loses electrons it becomes a positive ion. If it gains electrons it becomes a negative ion.
Evidence for the nucleus: alpha-particle scattering. In this experiment, a narrow beam of alpha particles is fired at a very thin gold foil.
- Most alpha particles pass straight through. So most of the atom is empty space.
- A small number are deflected through large angles. The nucleus has a positive charge, which repels the positive alpha particles.
- Very few bounce almost straight back. The positive charge and most of the mass are concentrated in a very small nucleus.
Nuclide notation. A nuclide is a particular type of nucleus. It is written with its symbol, its proton number and its nucleon number:
AZX
- Proton number Z (atomic number): the number of protons. It defines the element.
- Nucleon number A (mass number): the total number of protons and neutrons.
- Number of neutrons = A − Z.
Isotopes. Isotopes are nuclei of the same element (same proton number) with different numbers of neutrons, so different nucleon numbers. They have the same chemical properties, because chemistry depends on the electrons. Some isotopes are unstable and radioactive. Carbon-12 and carbon-14 are an example.
Nuclear fission. Fission is the splitting of a large nucleus, such as uranium-235, into two smaller nuclei. It starts when the nucleus absorbs a neutron. The fission also releases two or three more neutrons, and a large amount of energy, mostly as kinetic energy of the fragments. The new neutrons can cause further fissions. This is a chain reaction. In a nuclear power station, the chain reaction is controlled so that the energy is released steadily and used to heat water to make steam for turbines.
Nuclear fusion. Fusion is the joining of two light nuclei, such as isotopes of hydrogen, to form a heavier nucleus such as helium. Energy is released. Fusion needs a very high temperature and pressure, because the positively charged nuclei repel each other and must be moving fast enough to get close. These conditions exist in the core of stars, where fusion releases the energy that the stars radiate.
Definitions and formulae
| Term or formula | Meaning / when to use |
|---|---|
| Proton number Z | Number of protons in the nucleus. Identifies the element. |
| Nucleon number A | Number of protons plus neutrons. |
| Neutrons = A − Z | Use to find the number of neutrons from the nuclide notation. |
| Isotopes | Same Z, different A (different numbers of neutrons). |
| Fission | A large nucleus splits into smaller nuclei, releasing energy and neutrons. |
| Fusion | Light nuclei join to form a heavier nucleus, releasing energy. |
| Chain reaction | Neutrons released by one fission cause further fissions. |
Worked examples
Example 1 A uranium-235 nucleus has 92 protons. How many neutrons does it have?
The nucleon number is A = 235 and the proton number is Z = 92.
Neutrons = A − Z = 235 − 92 = 143.
Answer: 143 neutrons
Example 2 Carbon-12 and carbon-14 are both isotopes of carbon, which has proton number 6. Compare their nuclei and their neutral atoms.
Both have 6 protons, because they are both carbon.
Carbon-12: neutrons = 12 − 6 = 6. Carbon-14: neutrons = 14 − 6 = 8.
A neutral atom has the same number of electrons as protons, so each neutral atom has 6 electrons.
Answer: both have 6 protons and 6 electrons. Carbon-12 has 6 neutrons and carbon-14 has 8 neutrons.
Example 3 A magnesium ion has the notation 2412Mg and a charge of +2. State the numbers of protons, neutrons and electrons.
Protons = Z = 12.
Neutrons = 24 − 12 = 12.
A neutral atom would have 12 electrons. A charge of +2 means two electrons have been lost, so there are 12 − 2 = 10 electrons.
Answer: 12 protons, 12 neutrons, 10 electrons
Common errors
- Saying isotopes have different numbers of protons. They have different numbers of neutrons.
- Using the nucleon number as the number of neutrons. Subtract the proton number first.
- Explaining the alpha scattering result by saying the nucleus is "negative". It is positive, which is why it repels alpha particles.
- Saying fusion splits nuclei or fission joins them. Fission splits, fusion joins.
- Forgetting that the electron number changes in an ion, but the nucleus does not.
Exam tip: For the scattering experiment, link each observation to a conclusion: "most pass straight through, so the atom is mostly empty space".
Check yourself
- A copper nucleus is written 6329Cu. How many protons and neutrons does it contain?
Answer
Protons = 29. Neutrons = 63 − 29 = 34. - Why do most alpha particles go straight through the gold foil?
Answer
Most of an atom is empty space, and the nucleus is very small, so most alpha particles never come close to a nucleus. - Why does fusion need a very high temperature?
Answer
Nuclei are positively charged and repel one another. They need very high speeds, so very high temperature, to get close enough to join. - What is the job of the neutrons released in a fission reaction?
Answer
They can be absorbed by other nuclei and cause them to split, which keeps the chain reaction going.