Particles (3.2.1)
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The alpha particle scattering experiment determined the existence of the atomic nucleus:
- In 1917, Rutherford determined that the hydrogen nucleus was a single proton.
- Chadwick discovered the neutron in 1933.
- These discoveries led to the nuclear model of the atom, which features a nucleus at the centre surrounded by electrons.
- Niels Bohr later improved this simple model by suggesting that electrons orbit the nucleus at certain distances from the nucleus in electron shells.
The atomic nucleus consists of protons and neutrons:
- Protons are positively charged and neutrons have zero charge so the overall charge of the nucleus is always positive.
- Protons and neutrons have approximately the same mass.
In the nuclear model of the atom, it was determined that:
- Electrons orbit the nucleus in fixed energy levels.
- For every atom the number of electrons is always equal to the number of protons so the overall charge is zero.
- The mass of the electron is equal to approximately that of the proton.
The term nucleon refers to particles inside the nucleus: protons and neutrons:
- The proton and the neutron have approximately the same mass.
- The proton has a charge of where is the elementary charge.
- The neutron has no charge.
An atom has no net charge and has the same number of protons as electrons.
For an element the nucleus of an atom of that element can be represented by nuclear notation.
Where:
- is the nucleon number: the total number of protons and neutrons. is also known as the mass number, since it gives the mass of the nucleus in units of the proton (or neutron) mass.
- is the proton number: the number of protons. is also known as the atomic number, since it determines the type of atom.
Isotopes are atoms of the same element that contain a different number of neutrons in their nuclei.
For example, carbon exists in three naturally occurring isotopes: carbon-12 (six neutrons), carbon-13 (seven neutrons) and carbon-14 (eight neutrons).
- Isotopes of a given element have the same number of protons and hence the same number of electrons, meaning they have the same chemical properties.
- Isotopes of a given element have different masses, meaning they have different physical properties, such as melting point and density.
When using nuclear notation, always label the element with the nucleon and proton numbers.
For example, carbon-14 has six protons and eight neutrons.
Protons are positively charged. Like charges repel, so protons in the atom should repel each other.
The electrostatic force of attraction between two like charges is inversely proportional to the square of the distance between them:
Therefore, as becomes small, such as the spacing between protons in the nucleus, the electrostatic force of repulsion becomes extremely large.
The size of the nucleus is approximately All protons in an atom are squeezed within this small space. This distance of separation corresponds to extremely large electrostatic forces between protons of about
The electrostatic repulsion between protons in the nucleus is balanced by the strong nuclear force. Colloquially referred to as the strong force:
- The strong nuclear force acts between all nucleons (protons and neutrons) equally.
- The strong force is exclusively a short-range force and becomes practically zero when nucleons are separated by distances of about where is a femtometre, or
- The strong force is repulsive at extremely short distances, meaning that the nucleus does not crush itself.
- The strong force is attractive between distances of approximately and
Ionising radiation includes three main types: alpha particles, beta particles and gamma rays, which have sufficient energy to remove electrons from atoms:
- The ionisation power of radiation is the ability to ionise atoms: a greater mass leads to a greater ionisation power.
- Penetration power of radiation is the ability to penetrate through matter: a smaller mass leads to a greater penetration power.
The table below summarises some of the properties of each type of radiation.
Alpha particles
- Made of two neutrons and two protons: a helium nucleus
- Emitted from nuclei that are too large to be stable
Beta particles
- Beta-minus particles are high-energy electrons, emitted from nuclei with too many neutrons to be stable.
- Beta-plus particles are high-energy positrons, emitted from nuclei with too many protons.
Gamma rays
- High energy electromagnetic waves
- Emitted from a nucleus that has excess energy after alpha or beta decay
Question walkthrough
Identifying Radiation Type by Absorption
Identifies the types of radiation emitted from a source by interpreting count rate drops as paper, aluminium, and lead absorbers are introduced.
On the periodic table, all elements are represented by nuclide notation:
Where:
- is the nucleon number: the total number of protons and neutrons. is also known as the mass number, since it gives the mass of the nucleus in units of the proton (or neutron) mass.
- is the proton number: the number of protons. is also known as the atomic number, since it determines the type of atom.
Radioactive decay of a nucleus alters the nucleon number and/or the proton number.
Alpha decay occurs when a nucleus has too much mass, which makes the nucleus unstable.
In large nuclei with more protons, the larger electromagnetic repulsion between the protons becomes larger compared to the strong nuclear force holding the nucleus together and makes the nucleus unstable.
The daughter nucleus will have a lower mass number by four and a lower proton number by two.
Beta minus decay releases a high speed, high energy electron.
Beta minus decay occurs in nuclei with too many neutrons: a neutron in the nucleus is converted to a proton, an electron, and an electron antineutrino.
The daughter nucleus will have an increased proton number since the beta minus particle has a −1 proton number. The lepton number must be conserved, hence an antineutrino is also emitted.
Beta plus decay releases a positron – the antiparticle of an electron – which has a positive charge.
Beta plus decay occurs in nuclei with too many protons; a proton in the nucleus is converted to a neutron, a positron and an electron neutrino.
The daughter nucleus will have a decreased proton number since the beta plus particle has a +1 proton number. The lepton number must be conserved, hence a neutrino is also emitted.
Gamma decay occurs following decay from alpha and/or beta decay, where the daughter nucleus still has too much energy to be stable. No particles are released.
The daughter nucleus will have a preserved nucleon and proton number. Only energy is expelled through gamma decay.
Decays involving beta-minus or beta-plus can be confused. Remember that lepton number is conserved: in beta-minus decay an electron, is released with an electron antineutrino, whereas in beta-plus decay a positron is released with an electron neutrino,
The Universe is composed of matter particles such as protons, neutrons, and electrons.
All matter particles have corresponding antimatter particles, which are identical in most properties but have the opposite charge:
- If a matter particle is positively charged, its antimatter counterpart is negatively charged, and vice versa.
- If a matter particle has zero charge, its antiparticle also has zero charge.
Common matter–antimatter particle pairs are shown in the figure below.
For particles other than electrons, their antiparticle counterpart has the same name, named with the prefix “anti-” and is symbolised by the same letter with a bar above it (e.g. for antiproton).
Be careful not to confuse neutral particles, like the neutron and neutrino, with their antiparticles.
Antiparticles are written with a bar over the symbol, so do not rely solely on the charge to identify a particle or antiparticle.
Antimatter particles have the same mass as their corresponding matter particles.
Charged antimatter particles have an equal and opposite charge to their corresponding matter particles.
The electron and positron have the same mass but an opposite charge:
The proton and antiproton also have the same mass but an opposite charge.
The rest mass–energy of a particle is the energy equivalent to the mass of the particle when it is at rest.
It can be calculated using Einstein’s famous equation:
Where:
- is the rest mass–energy,
- is the particle’s rest mass, and
- is the speed of light
Rest mass–energy is important for calculations related to particle interactions, in which mass can be converted to energy.
Rest mass–energy has SI units of joules (J) but is often expressed in for convenience.
The table below provides the mass in and the rest mass–energy in for a proton, neutron, electron and neutrino.
The rest mass–energy for the neutrino is which is on the scale of magnitudes smaller than the other particles in the above table.
All particles have a corresponding antiparticle. They are similar in every aspect, except that they have opposite charges. When a particle and its corresponding antiparticle collide, they annihilate each other: all of the particle’s mass is converted into energy, typically in the form of photons.
An example of the process of annihilation is electron–positron annihilation:
The electron has an associated antiparticle known as the positron . The electron and the positron are similar in every aspect, except that they have the opposite charge. When an electron and positron collide, they annihilate and all their mass is transformed into energy in the form of two identical photons.
A process opposite to annihilation can occur, in which a photon transforms into a particle-antiparticle pair: this is known as pair production.
For example, a photon may create an electron–positron pair, provided the photon has an energy that is equal to at least twice the rest-mass energy of the electron i.e.
For pair production to occur, a photon must enter into the Coulomb field of a nearby atomic nucleus: it cannot happen in free space. This is because the reaction requires energy and momentum to be conserved.
In free space, the momentum of the electron and positron combined would not equal that of the photon’s initial momentum. Therefore, the nucleus absorbs some of the photon’s initial momentum, thus obeying conservation laws in the reaction.
In different circumstances, electromagnetic radiation can be considered as either a wave or a photon. Since a photon is a particle, this is known as the wave–particle duality of EM radiation.
When EM radiation propagates through space, it should be modelled as a continuous wave:
- The wave model of EM radiation describes the diffraction and interference of waves.
When EM radiation interacts with matter, it should be modelled as a particle:
- Electrons exist in energy levels in atoms. When these electrons absorb a photon, they can transition to a higher energy level. This cannot be explained by considering EM radiation as continuous waves.
Max Planck demonstrated that photons are the smallest indivisible unit of energy of an electromagnetic wave. A quantum is the smallest indivisible unit. The photon is the quantum of energy of EM radiation.
Planck believed the particulate nature of EM radiation to be a mathematical trick that explained the black body emission spectrum. He argued light was exclusively a wave.
In 1905, Einstein further developed Planck’s ideas by demonstrating that photons are real particles and that several phenomena related to EM radiation could only be explained using the photon model.
Photons are discrete wave-packets of energy.
Photons demonstrate the discrete nature of energy, meaning that energy cannot be divided into smaller portions indefinitely.
For example, an electron in an atom can absorb a photon, thereby gaining energy. The electron cannot absorb ‘part’ of the photon. It can only gain the whole energy of the photon or none at all.
As an analogy, consider a person walking up a set of stairs. His height above the ground increases in discrete steps, and he cannot be at any height in between the steps. On the other hand, if they walk up a slope, then their height increases continuously.
- On a small scale, energy is discrete, as seen in the case of an electron absorbing a photon.
- On a large scale, energy can be seen as continuous. For example, a car gains kinetic energy as it speeds up, and the kinetic energy is observed to increase continuously.
Planck’s constant is a fundamental quantity in quantum physics. Its value is:
In 1900, Max Planck introduced Planck’s constant, a fundamental concept defining the smallest unit of energy, or packet, that constitutes electromagnetic waves. These packets of energy are photons.
Planck’s constant, is an extremely small quantity. The energy of a photon is equal to:
Therefore, the energies in of individual photons are extremely small. The electronvolt is a more convenient unit of energy for photons. An example of this is the energy in electronvolts of a red visible light photon of wavelength is , which is easier to read and use than .
It is important to note that the following units are commonly used as units of energy:
The energy of a photon of electromagnetic radiation is equal to:
Where:
- is energy, measured in joules ,
- is the Planck constant,
- is the frequency of the EM radiation in
The energy of a photon is directly proportional to its frequency, and Planck’s constant is the proportionality constant.
The wave equation for EM waves states that:
Where:
- is the speed of light in a vacuum,
- is the frequency of the wave in ,
- is the wavelength of the wave in
The wave equation can be rearranged to:
If we substitute this expression for frequency into the equation for the energy of a photon we get:
This equation allows the energy of EM radiation to be calculated if its wavelength is known.
The energy of a photon is inversely proportional to its wavelength:
- Short-wavelength EM radiation, such as X-rays, consists of high-energy photons.
- Long-wavelength EM radiation, such as radio waves, consists of low-energy photons.
Question walkthrough
Finding Photon Frequency from Energy in Terahertz
Calculate the frequency of a photon of red light in terahertz, given its energy, using the Planck relation.
Question walkthrough
Calculating Photon Energy from Wavelength
Calculate the energy of a photon of green light given its wavelength, using the Planck-wavelength relation.
The behaviour of all particles can be explained using four fundamental interactions:
- Strong interaction: Acts between quarks and is responsible for binding nucleons in the nucleus.
- Electromagnetic interaction: Acts between charged particles.
- Weak interaction: Is responsible for processes such as decay.
- Gravitational interaction: Acts between all particles with mass.
Each interaction differs in strength, range and the particles it affects:
| Interaction | Particles affected | Range |
|---|---|---|
| Strong | Hadrons and quarks | Very short () |
| Electromagnetic | Charged particles | Infinite |
| Weak | All | Very short () |
| Gravitational | Particles with mass | Infinite |
It is important to note that you are not required to know about the graviton for your exams, but it can be helpful for your holistic understanding.
Particle interactions can be explained using exchange particles. Forces arise when particles exchange virtual particles, which transfer momentum and energy between them. This exchange results in an effective force between the particles.
The analogy below is used only to illustrate how momentum transfer can produce forces between objects; it does not represent the actual mechanism of particle exchange.
A repulsive force can be illustrated by two skaters standing on ice, throwing a ball between them: the act of throwing causes the first person to move backwards to conserve momentum, and the second person to move backwards upon catching it.
Similarly, an attractive force is comparable to an ice skater launching a boomerang away from their partner. Following the path illustrated in the diagram above, this action draws both individuals towards one another as momentum is exchanged in the opposite direction.
Each fundamental force has an associated exchange particle (or gauge boson) that mediates the interaction. These exchange particles determine the range and strength of the force:
| Interaction | Exchange particle | Particle properties | Range |
|---|---|---|---|
| Strong | Gluons: between quarks Pions: residual force between nucleons |
Gluons: massless Pions: massive |
Very short |
| Electromagnetic | Virtual photon | Massless, no charge | Infinite |
| Weak | and bosons | Massive, charged | Very short |
| Gravitational | Graviton (hypothetical) | Massless, no charge | Infinite |
Exchange particles are called virtual particles because they cannot be directly detected during an interaction.
It is important to note that you are not required to know about the graviton for your exams, but it can be helpful for your holistic understanding.
Feynman diagrams are used to represent the structure of particle interactions, including which particles are involved and how forces are transmitted.
Rules for drawing Feynman diagrams:
- The vertical axis of a Feynman diagram represents time , with events:
- before the interaction at the bottom
- after the interaction at the top
- Particles are represented with straight arrows:
- Matter: Arrows point upwards (forwards in time).
- Antimatter: Arrows point downwards (backwards in time).
- Exchange particles are represented by wavy lines.
- Baryons are typically drawn on the left-hand side and leptons on the right-hand side.
- Every meeting point must show three lines joining together: an incoming particle, an exchange particle and an outgoing particle.
The diagram above represents beta-minus (β−) decay. In this process, a neutron transforms into a proton via the weak interaction, producing an electron and an electron antineutrino. At the quark level, a down quark changes into an up quark through the emission of a W− boson.
It is useful to note that antimatter particles are sometimes described as particles travelling backwards in time in Feynman diagrams, but this should not be taken literally at A-level. It is a mathematical interpretation: an antiparticle moving forward in time can be treated in the same way as the corresponding particle moving backwards in time.
In practice, the arrow on a particle line is mainly a bookkeeping tool. It helps show the difference between particles and antiparticles in interactions such as pair production and annihilation.
The electromagnetic interaction acts between particles with electric charge. It is described by the exchange of virtual photons, which transfer momentum and energy between particles.
This interaction can produce:
- attraction between opposite charges
- repulsion between like charges
Only charged particles experience the electromagnetic interaction.
The diagram shows an electromagnetic interaction between two electrons. A virtual photon is exchanged between them, transferring momentum and causing the electrons to repel each other.
It is important to note that electromagnetic interactions do not change the identity of the particles involved. The electrons before and after the interaction remain electrons.
The weak interaction is responsible for processes in which particle type changes. It is also the only interaction that can change quark flavour.
For AQA A-level physics, this interaction is limited to:
- decay
- decay
- electron capture
- electron–proton collisions
The exchange particles are:
- bosons
- bosons
Because strangeness depends on the presence of strange quarks, it is not always conserved in weak interactions. This is a direct result of quark flavour change. For example, a strange quark can change into an up quark during weak decay.
Beta decay demonstrates that the weak interaction can change particle type.
During weak interactions:
- Quarks can change flavour.
- Leptons can be produced.
- Interactions are mediated by and bosons.
In and decay, a quark changes flavour, leading to the transformation of a neutron or proton and the emission of leptons.
In electron capture, an inner-shell electron is absorbed by a proton in the nucleus. The proton changes into a neutron, and an electron neutrino is produced:
This process is mediated by the weak interaction via a boson emitted from the proton.
In an electron–proton weak interaction, a similar process occurs when a high-energy electron interacts with a proton, producing an electron neutrino and a neutron via a boson emitted by the electron.
Both processes involve quark flavour change within the proton, allowing a proton to transform into a neutron. These reactions occur only via the weak interaction, as they involve a change in particle type and the production of leptons.
Question walkthrough
Identifying particles in electron capture
Uses charge, baryon number, and lepton number conservation to identify the down quark and electron neutrino in an electron capture Feynman diagram (p+e⁻→n+ν_e).
Question walkthrough
Drawing a beta-plus decay diagram
Explains how to draw a Feynman diagram for beta-plus decay (p→n+e⁺+ν_e), covering arrow conventions for matter and antimatter particles, the W⁺ exchange boson, and the conservation laws it must satisfy.
Question walkthrough
Identifying Feynman diagram mistakes
Identifies four errors in a Feynman diagram of an electron–proton weak interaction: the quark transformation, the electron’s arrow direction, the exchange boson, and the final lepton produced.
Modern particle physics relies on large international collaborations due to the scale and cost of experiments. Particle discoveries require expertise from:
- physicists (theory and experimental design)
- engineers (design and construction of detectors and accelerators)
- computer scientists (data processing and analysis of large datasets)
- technicians (operation and maintenance of equipment)
Experiments are typically conducted in large facilities, such as particle accelerators.
According to the standard quark-lepton model, all particles can be classified as either hadrons or leptons:
- Hadrons experience the strong interaction (strong nuclear force) and are composed of smaller particles called quarks.
- Leptons do not experience the strong interaction and are fundamental (this means they cannot be broken down into anything smaller).
Other examples of hadrons include neutrons, pions and kaons.
Other examples of leptons include muons, electron neutrinos and muon neutrinos.
Hadrons are divided into two groups: baryons and mesons.
- Baryons are made of three quarks.
- Mesons are made of one quark and one antiquark.
Protons are the only stable baryons. All other baryons eventually decay into lighter particles, making protons unique among the baryons studied at A-level.
It is useful to note that some scientists hypothesise that protons will eventually decay, but this has never actually been observed, and current experiments establish the lower limit on the proton’s lifetime to be greater than years. Moreover, exotic hadrons consisting of four or more quarks have been observed in particle colliders, but they are extremely unstable and short-lived.
Baryon number is a quantum number used to describe particle interactions. Quantum numbers are quantities that must be conserved in particle reactions.
Charge is an example of a conserved quantity that we have already encountered.
Baryon number is defined as:
- baryons: +1
- antibaryons: −1
- all other particles: 0
The total baryon number before and after an interaction must remain the same. If it does not, the interaction is not possible.
Mesons are hadrons made from one quark and one antiquark. They can have positive, negative or zero charge, depending on their quark composition. Mesons have a baryon number of 0.
The two main mesons considered are pions and kaons. Both pions and kaons are unstable particles that decay into other particles. They are short-lived particles that often appear when high-energy collisions briefly create new matter.
It is important to note that kaons are strange particles. They are produced by the strong interaction but decay via the weak interaction, making them important examples in the study of conservation laws.
Strange particles are produced through the strong interaction but decay through the weak interaction. Examples of strange particles include:
- kaons
- baryons
- baryons
The discovery of strange particles led to the introduction of a quantum number called strangeness.
Strangeness is conserved in strong interactions but not necessarily in weak interactions. Strange particles are often produced in pairs, so total strangeness is conserved at the point of creation.
In weak interactions, strange particles may decay into non-strange ones. So, strangeness may change by -1, 0, or +1.
The strangeness values of the four kaons must be memorised:
| Particle | Symbol | Strangeness |
|---|---|---|
| Positive kaon | +1 | |
| Neutral kaon | +1 | |
| Antineutral kaon | −1 | |
| Negative kaon | −1 |
Leptons are fundamental particles.
Unlike hadrons, leptons are not composed of quarks, meaning they are not affected by the strong nuclear force.
Leptons experience the following forces:
- Weak nuclear force
- Gravitational force
- Electromagnetic force (if charged)
There are a total of six leptons, categorised into three different flavours (types):
- Electron
- Muon
- Tauon
There are three uncharged leptons, one for each flavour:
- Electron neutrino
- Muon neutrino
- Tauon neutrino
Neutrinos are the most abundant leptons in the Universe and possess no charge and negligible mass. They are formed during particle interactions that also involve charged leptons.
It is useful to note that physicists use the term “flavour” because it’s a lighthearted label for the different “varieties” of quarks and leptons. It does not imply any literal taste.
There are three charged leptons with charge one for each of the three flavours: the electron, muon and tauon.
The three charged leptons have almost identical properties, but the muon and tauon particles have much greater masses than the electron:
It is important to note that is the unified atomic mass unit, equal to
Muons and tauons are extremely unstable, and will quickly decay into other particles:
- The muon will decay into an electron and an electron antineutrino and a muon neutrino.
- Tauons are massive enough to decay into a variety of particles, including baryons such as protons or neutrons.
Anti-leptons have the same mass and opposite charge to their corresponding leptons.
For example, the electron has a charge of and the positron has a charge of
For the six leptons, there are six corresponding anti-leptons as shown in the table.
Lepton number is a quantum number that must be conserved in particle interactions. In particle physics, lepton number is split into two independent quantities:
Electron lepton number ():
- Electron and electron neutrino:
- Positron and antielectron neutrino:
Muon lepton number ():
- Muon and muon neutrino:
- Antimuon and antimuon neutrino:
All non-leptons have lepton number 0. Electron and muon lepton numbers must be conserved separately in any interaction.
Lepton number conservation is a key rule used to determine whether particle interactions are possible.
Muon decay is an important example because it always produces an electron and neutrinos to conserve both the electron and muon lepton numbers.
For a particle interaction to occur, the following quantum numbers must be conserved:
- relative charge
- baryon number
- electron lepton number
- muon lepton number
Strangeness must also be considered:
- It is conserved in strong interactions.
- It may change by 0, +1 or −1 in weak interactions.
A particle interaction is only possible if all relevant conservation rules are satisfied simultaneously. If any conserved quantity is not balanced, the interaction cannot occur.
In particle interactions, several quantum numbers must be conserved for the reaction to be possible.
The key conservation rules are:
- baryon number
- electron lepton number
- muon lepton number
- charge
Strangeness must also be considered in strong interactions and weak decays involving strange particles.
The type of interaction can often be identified using particle content and conservation rules.
For example, leptons do not participate in the strong interaction, which only acts between quarks (within hadrons), whereas the weak interaction can involve both leptons and hadrons and is responsible for processes in which the particle type changes. A useful distinction is:
- Strong interaction: Strangeness is conserved.
- Weak interaction: Strangeness can change (due to quark flavour change).
Question walkthrough
Identifying the Antineutrino in Neutron Decay
Formulates the equation for neutron decay into a proton and electron, then identifies the undetectable massless particle as an electron antineutrino.
Question walkthrough
Classifying an antimuon capture reaction
Checks the particle transformation n+μ⁺→p+antimuon neutrino to show it must be a weak interaction, since converting a neutron into a proton requires a change in quark flavour.
Question walkthrough
Classifying neutron-to-proton decay as weak
Checks conservation laws for n→p+μ⁻+ν̄_μ to show the decay is possible only as a weak interaction, since the underlying quark flavour change (d→u) occurs only via W boson exchange.
Question walkthrough
Classifying kaon decay as weak
Checks whether K⁺→μ⁺+ν_μ conserves charge, lepton number, and strangeness to show the decay is possible only as a weak interaction, since strangeness is not conserved.
Question walkthrough
Identifying an unknown particle from collision
Uses conservation of charge, baryon number, lepton number, and strangeness to identify an unknown particle X produced when two pions collide to form a neutral kaon and X.
Hadrons are subatomic particles composed of smaller particles called quarks. These hadrons can be categorised into two main types:
- Baryons
- Made up of three quarks
- Examples of these include protons and neutrons
- Mesons
- A quark–antiquark pair.
- Examples of these include pions and kaons.
In an anti-hadron, each quark is swapped for its antiquark compared to the corresponding hadron.
The simple quark model of hadrons includes three types of quarks (along with their charges):
- Up
- Down
- Strange
Their corresponding antiquarks are:
- Anti-up
- Anti-down
- Anti-strange
All baryons and mesons possess integer charges (whole number values like etc.).
A mixture of quarks and antiquarks is not possible in baryons: combining quarks with antiquarks in a group of three would lead to a non-integer charge.
Mesons are a quark–antiquark pair, but some combinations would lead to a non-integer charge, so they are not allowed.
For example, there is no meson consisting of a pair since this would lead to a charge of
Baryons are hadrons composed of three quarks.
Baryon number is always conserved in particle interactions:
- All baryons have a baryon number of
- Antibaryons have a baryon number of
Examples of baryons include protons and neutrons:
- Proton: A baryon with a baryon number of composed of two up quarks and one down quark
- Neutron: A baryon with a baryon number of composed of one up quark and two down quarks
All quarks have a baryon number of while all antiquarks have a baryon number of
An up quark, has a charge of and a down quark, has a charge of Their antiquarks have opposite charges.
Antiprotons and antineutrons are antibaryons. Each quark in its corresponding baryon is replaced by an antiquark.
Protons and neutrons are not fundamental particles; they are composite particles made up of quarks.
Both protons and neutrons are baryons, which are composed of three quarks:
- Protons are composed of two up quarks and one down quark Its quark structure is
- Neutrons are composed of two down quarks and one up quark Its quark structure is
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Question walkthrough
Identifying a Particle from Its Properties
Identifies a proton and its uud quark composition from clues about baryon number, quark count, charge, and its role as an atomic constituent.
Question walkthrough
Identifying a Baryon from Its Quark Content
Finds the overall charge of a particle made of one up and two down quarks, then identifies it as a baryon called the neutron.
Pions are mesons made up from a combination of up, down, anti-up and anti-down quarks.
Some examples of pions are shown below.
Charged pions are made up of one up and one anti-down quark (positive) or one down and one anti-up quark (negative).
Neutral pions consist of an up anti-up pair or a down anti-down pair, where the relative charges cancel out.
The pion acts as the exchange particle of the strong nuclear force.
Kaons are mesons containing either a strange or an anti-strange quark.
Strange quarks have a charge of
Some examples of kaons are shown below.
Charged kaons include either an up or anti-up quark, with their overall relative charge reflecting the charge of that quark.
Neutral kaons are those which include a down or anti-down quark.
Kaons will quickly decay into pions
The charge of a particle is the sum of the charges of its quarks.
Up quark
Down quark
To find the charge of a proton, we identify the quark composition:
Total charge calculation:
The proton has a charge of
Similarly, to find the charge of a neutron, we know the quark composition is:
Total charge calculation:
The neutron is neutral and has no net charge.
Be careful not to confuse the quark compositions for protons and neutrons.
Question walkthrough
Counting Up and Down Quarks in Nitrogen-14
Finds the total number of up and down quarks in a nitrogen-14 nucleus by counting its protons and neutrons and their quark composition.
Question walkthrough
Total Charge of a Meson from Its Quarks
Calculates the total charge of a meson made of an up quark and an anti-strange quark by summing the individual quark charges.
In beta-minus decay ( decay), a neutron within the nucleus of an atom decays into a proton.
During this process, the neutron emits a high-energy electron (beta-minus particle, and an anti-electron neutrino
The equation for beta-minus decay is:
In beta-minus decay ( decay), a parent nucleus transforms into a different element, known as the daughter nucleus.
- The proton number increases by one: a neutron has turned into a proton, so the element has gained an extra proton.
- The nucleon number (mass number) remains the same since a neutron decays to a proton, so the total number of protons and neutrons stays constant.
For a parent nucleus with proton number and mass number the daughter nucleus is given by:
The decay equation can be checked by confirming that mass and charge are conserved:
- On the left-hand side of the arrow, the total mass is and the charge is
- On the right-hand side, we have a mass of (as beta-minus particles and anti-electron neutrinos have no mass) and a charge of
Thus, no charge or mass has been created or destroyed in this decay.
In beta-plus decay ( decay), a proton within the nucleus is converted into a neutron.
Memory tool: beta-plus decay means a proton changes.
During this process, the proton emits a high-energy positron (a beta-plus particle, ) and an electron neutrino,
The general equation for beta-plus decay is:
During beta-plus decay ( decay):
- The proton number decreases by one: since a proton has been converted into a neutron, the atom has lost a proton.
- The nucleon number (mass number) remains the same: no nucleons are lost or gained, so the total number of protons and neutrons stays constant.
This may be written in a general form for an unknown parent nucleus with the proton number and mass number
Checking mass and charge conservation:
- On the left-hand side of the arrow, we have a mass of and a charge of (each proton has a charge of ).
- On the right-hand side, we have a mass of (as beta-plus particles and electron neutrinos have no mass) and a charge of
- Thus, no charge or mass has been created or destroyed in this decay.
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Nuclear Equation for Magnesium-23 Beta-Plus Decay
Writes the nuclear equation for the beta-plus decay of magnesium-23 and identifies the resulting daughter element as sodium-23.
In beta-minus decay ( decay), a neutron inside the nucleus is transformed into a proton, emitting an electron and an anti-electron neutrino, This can be further explained at the subatomic level.
At the subatomic level, particles like protons and neutrons are composed of quarks:
- A neutron consists of two down quarks and one up quark
- A proton consists of two up quarks and one down quark
The beta-minus decay process can be understood in terms of quark transformation:
- During the decay, one of the down quarks in the neutron is converted into an up quark
- This conversion changes the neutron into a proton while emitting an electron and an anti-electron neutrino.
The equation for beta-minus decay is:
At the quark level, this can be written as:
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Quark Changes in Carbon-14 Beta Decay
Describes the change in quark composition when a neutron in carbon-14 decays into a proton, and writes the full nuclear equation for the decay.
In beta-plus decay ( decay), a proton inside a nucleus is converted into a neutron, accompanied by the emission of a positron ) and an electron neutrino This can be further explained at the subatomic level.
Beta-plus decay involves a transformation in the quark composition of the proton.
- A proton consists of two up quarks and one down quark
- A neutron consists of one up quark and two down quarks
During beta-plus decay, one of the up quarks in the proton converts into a down quark, causing the proton to change into a neutron.
The quark-level equation for this decay can be written as:
The overall equation for beta-plus decay is:
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Nuclear Equation for Fluorine-18 Beta-Plus Decay
Writes the nuclear equation for the beta-plus decay of fluorine-18 into oxygen-18, and explains the up-to-down quark transformation involved.
Quark transformations occur during various types of particle interactions, such as beta-minus and beta-plus decay.
Each type of decay involves the conversion of one type of quark into another.
Charge, energy, and momentum must be conserved during these interactions.
We can determine the number of unknown particles in a decay equation by balancing charge on both sides of the equation.
The steps to balance a quark transformation equation are:
1. Identify the initial quark composition: determine the initial quarks involved in the transformation, noting their respective charges.
2. Identify the final quark composition: determine the final quark composition after the transformation, again noting their respective charges.
3. Set up the equation: The equation should represent the initial quark composition transforming into the final composition. For example:
4. Calculate total charge:
5. Check for charge conservation: Ensure that the total initial charge equals the total final charge:
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Charge Balance in Beta-Minus Decay
Shows that charge is conserved in the beta-minus decay of a neutron into a proton, electron, and antineutrino, using quark composition.
The decay of particles can be explained using the quark model.
The weak nuclear force is one of the four fundamental forces of nature, alongside gravity, electromagnetism, and the strong nuclear force.
It is responsible for radioactive decay processes that involve the transformation of quarks. It allows quarks to change their type or ‘flavour’ from one to another.
The decay of hadrons often involves the transformation of one quark into another and the emission of leptons (electrons, positrons, and neutrinos).
- In beta-minus decay ( decay), a neutron decays into a proton This occurs because one of the down quarks in the neutron transforms into an up quark via the weak nuclear force.
- In beta-plus decay ( decay), a proton decays into a neutron This occurs because one of the up quarks in the proton transforms into a down quark
Decays involving the strange quark also occur through the weak nuclear force. The strange quark can transform into an up quark or a down quark.
For example, a kaon has a quark composition It can decay into a pion and a neutrino–antineutrino pair via the weak nuclear force.
The following conservation laws apply to particle decays:
- Charge conservation: the total charge before and after a decay must be the same.
- Lepton number conservation: the total lepton number is conserved (within a family of leptons).
- Baryon number conservation: the total baryon number remains unchanged during a decay.
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Charge Conservation in Kaon Decay
Explains the weak decay of a K+ kaon to a pi+ pion using the quark model, and verifies charge conservation from their quark compositions.