OCR Specification focus:
‘State proton uud and neutron udd within the simple quark model.’
Protons and neutrons, collectively known as nucleons, are composed of more fundamental constituents called quarks, and understanding their quark content is essential for explaining nuclear structure, charge, and interactions.
The Quark Composition of Nucleons
The simple quark model provides a powerful framework for describing the internal structure of hadrons, including the proton and neutron. According to this model, all baryons—particles made of three quarks—derive their properties from the types and combinations of quarks they contain. The up (u) and down (d) quarks play the central role in forming the nucleons found in ordinary matter.
Baryons and Quarks
The proton and neutron each belong to the baryon family. Baryons are defined as composite particles made from three quarks bound together by the strong nuclear force.
Baryon: A hadron composed of three quarks bound by the strong nuclear force.
The strong nuclear force acts via the exchange of gluons, ensuring the quarks remain confined within the nucleon. These quarks carry fractional electric charges, and the total charge of the nucleon emerges from the sum of the charges of its constituent quarks.
Quark Charges
Before examining nucleon composition, it is necessary to recall the electric charges of the two quark flavours relevant to this subsubtopic:
Up quark (u): charge +⅔ e
Down quark (d): charge −⅓ e
These fractional charges combine in predictable ways to generate the overall charge of the nucleon. This relationship highlights the elegance of the quark model in explaining observable particle properties.
Proton Quark Content
The proton is the positively charged nucleon found in all atomic nuclei. Its quark structure directly accounts for its familiar charge of +1 e.
Composition of the Proton
The proton consists of:
Two up quarks (u)
One down quark (d)
Thus, its quark configuration is commonly written as uud.
Diagram of the internal quark structure of a proton, showing two up quarks and one down quark bound by the strong force. The coloured circles represent different colour charges, and the wavy lines represent gluons mediating the strong interaction. The colour charge detail exceeds OCR requirements but is consistent with the same model. Source.
The charges combine as follows:
Up quark: +⅔ e
Up quark: +⅔ e
Down quark: −⅓ e
The sum of these fractional charges gives:
+⅔ e +⅔ e −⅓ e = +1 e
This structure is crucial because it not only determines the proton’s charge but also influences its behaviour during particle interactions, particularly those involving the strong nuclear force.
Key Features of the Proton’s Quark Structure
The two up quarks dominate the proton’s charge contribution.
The down quark provides the balancing negative charge component.
All three quarks are held together tightly by the strong nuclear force.
The configuration uud is fixed for all stable protons.
The proton’s identity is defined by this exact quark combination; altering even one quark would transform it into a different particle entirely.
Neutron Quark Content
In contrast to the proton, the neutron is electrically neutral. However, it is structurally very similar to the proton, differing only in the arrangement of its constituent quarks.
Composition of the Neutron
The neutron consists of:
One up quark (u)
Two down quarks (d)
This gives the quark notation udd.
Diagram of the internal quark structure of a neutron, showing one up quark and two down quarks bound together. The coloured circles indicate different colour charges and the wavy lines represent gluons that mediate the strong nuclear force. The colour charge detail extends beyond the OCR requirement but remains consistent with the quark model. Source.
These quark charges combine as:
Up quark: +⅔ e
Down quark: −⅓ e
Down quark: −⅓ e
The charges sum to:
+⅔ e −⅓ e −⅓ e = 0
This exact balance of fractional charges explains the neutron’s lack of electric charge without requiring any additional assumptions.
Important Characteristics of the Neutron’s Quark Structure
The two down quarks contribute equal negative fractional charges.
The single up quark introduces a positive charge that cancels the two negative contributions.
Like the proton, the neutron’s quarks are bound by the strong nuclear force.
The udd configuration defines the neutron’s identity.
The neutron’s neutral nature, arising from its quark content, has significant consequences for its role in the nucleus and its behaviour under electromagnetic interactions.
Comparing Proton and Neutron Quark Structures
Despite their different charges, the proton and neutron are extremely similar in mass and overall structure. This close similarity arises from the interchange of one up quark and one down quark between them.
Summary of Structural Differences
Proton: uud
Neutron: udd
Implications of the Quark Combinations
The difference of only one quark flavour explains the small mass difference between the nucleons.
The shared presence of three light quarks means both particles are affected strongly by the strong nuclear force.
Charge differences lead to distinct roles within the atom:
Protons determine atomic number.
Neutrons contribute to mass number and nuclear stability.
Bullet-Point Comparison
Proton contains two up quarks, neutron only one.
Neutron contains two down quarks, proton only one.
Both consist of three quarks held by the strong nuclear force.
Fractional charges add differently to give either +1 e or 0.
These relationships form a central foundation for understanding nuclear physics at a more fundamental level and illustrate the simplicity and power of the quark model.
FAQ
The simple quark model focuses on up and down quarks because they are the lightest quark flavours and dominate the structure of stable matter. Their low masses allow them to combine into the lowest-energy configurations, forming long-lived baryons such as the proton and neutron.
Heavier quarks, such as strange or charm quarks, can appear in short-lived baryons but decay rapidly, so they do not form the nucleons found in everyday atoms.
Quarks are held together through a phenomenon called confinement, where the strong force increases with distance rather than weakening.
This force acts through gluons, which continually exchange “colour charge” between the quarks.
Because separating quarks requires more energy than creating new ones, individual quarks are never observed in isolation.
Fractional charge does not imply instability. What matters is how the strong force binds quarks so tightly that their individual charges combine to form a stable whole.
Additionally, quarks continuously interact via gluons, creating a dynamic system in which energy is constantly exchanged.
The stability of the proton in particular arises from its lowest-energy quark configuration, making decay impossible unless external interactions occur.
No. Quarks move rapidly inside nucleons and exchange gluons constantly. They behave more like a dynamic, fluctuating system than fixed structural components.
This motion means that the mass of a proton or neutron is dominated not by the quark masses themselves but by the energy of their interactions, according to mass–energy equivalence.
Although their quark compositions differ only by one quark flavour, this difference affects their stability outside the nucleus.
A free neutron (udd) undergoes beta decay, where one down quark changes into an up quark, turning the neutron into a proton.
A proton cannot undergo an equivalent decay because there is no lighter baryon with a suitable quark configuration for it to transform into while conserving charge and other quantum properties.
Practice Questions
Question 1 (2 marks)
State the quark composition of:
(a) a proton
(b) a neutron
Question 1 (2 marks)
(a) Proton quark content stated as uud
• 1 mark for stating uud
(b) Neutron quark content stated as udd
• 1 mark for stating udd
Question 2 (5 marks)
Using the simple quark model, explain how the proton and neutron differ in their quark composition and how this difference accounts for their electric charges. In your answer, refer to the charges of the up and down quarks and show how these combine to give the overall charge of each nucleon.
Question 2 (5 marks)
• States proton is made of uud (1 mark)
• States neutron is made of udd (1 mark)
• States up quark has charge +2/3 e (1 mark)
• States down quark has charge −1/3 e (1 mark)
• Correctly explains charge addition for both nucleons:
Proton: +2/3 + +2/3 + −1/3 = +1 e
Neutron: +2/3 + −1/3 + −1/3 = 0 (1 mark total for the explanation of how quark charges combine to form nucleon charge)
