11. Particle physics AS

Exam analysis

Most tested topics

11.1 Atoms, nuclei and radiation

Most alpha particles passed through thin foil, but a small fraction were deflected through large angles; this implies most atomic volume is empty with mass and positive charge concentrated in a tiny nucleus.

Use the scattering observations as evidence and distinguish the nuclear model from the older diffuse-charge model.

A near head-on deflection is rare because the nucleus occupies a tiny cross-section, yet it reveals strong repulsion from concentrated positive charge.

The experiment did not show electrons orbiting like planets; it established nuclear concentration, not a complete modern atom model.

A simple nuclear model places positively charged protons and neutral neutrons in a tiny nucleus, with negatively charged electrons occupying the surrounding region.

Mass is concentrated mainly in nucleons while atomic size is set by the electron region. This is a model, not a literal set of classical orbits.

Changing the number of electrons can make an ion without changing the nucleus or the element identity.

Electrons are not inside the nucleus in this model, and a neutral atom does not mean it contains no charged particles.

Proton number Z is the number of protons; nucleon number A is total protons plus neutrons, so neutron number is A−Z.

Z determines chemical identity. A can vary between isotopes while Z remains fixed.

A nuclide with A=23 and Z=11 contains 11 protons and 12 neutrons.

Nucleon number is not the number of electrons, and changing A alone does not create a new element.

Isotopes have the same proton number but different numbers of neutrons, so they share chemical identity but differ in mass and nuclear properties.

Compare Z first, then A. Isotopic abundance affects relative atomic mass without changing the element symbol.

Carbon-12 and carbon-14 both have Z=6; carbon-14 has two more neutrons and is radioactive.

Isotopes are not different elements, and isotope differences are not caused by different electron counts in neutral atoms.

$⁽ᴬ⁾₍Z₎X: A = nucleon number, Z = proton number, X = element symbol$

Quantity	Count
protons	Z
neutrons	A − Z
electrons in a neutral atom	Z
electrons in an ion of charge +ne	Z − n
electrons in an ion of charge −ne	Z + n

For ²³⁹₉₄Pu⁺, protons = 94, neutrons = 239 − 94 = 145 and electrons = 94 − 1 = 93. The + sign means one electron has been removed; it does not change the nucleus.

To write a nuclide symbol, use the periodic-table identity to select X and Z, then place total protons plus neutrons as A at upper left and Z at lower left.

Do not swap A and Z. A is not neutron number, and an ionic charge changes electron count only; it does not change A, Z or the isotope.

$ΣA_before = ΣA_after and Σ(q/e)_before = Σ(q/e)_after$

Emitted object	nucleon number A	charge number q/e
α particle	4	+2
β⁻ electron	0	−1
β⁺ positron	0	+1
γ photon or (anti)neutrino	0	0

Write every reactant and product, then total the upper numbers for nucleon number and the signed lower charge numbers independently. A missing particle or wrong daughter is exposed when either ledger fails.

Alpha decay conserves A through A = (A − 4) + 4 and charge through Z = (Z − 2) + 2. In beta-minus decay, the daughter's nuclear proton number rises by one, but total charge still balances: Z = (Z + 1) + (−1).

Proton number and neutron number are not each separately conserved in beta decay: one nucleon changes type. The conserved quantities here are total nucleon number and total electric charge across all products.

Radiation	Composition	Rest mass	Charge
α	helium nucleus: 2 protons + 2 neutrons	about 4 u	+2e
β⁻	electron	9.11 × 10⁻³¹ kg ≈ 5.49 × 10⁻⁴ u	−e
β⁺	positron	same as electron	+e
γ	electromagnetic photon	0	0

Identify radiation from all three requested properties. Positive charge alone is insufficient: both α and β⁺ are positive, but α has charge magnitude 2e and a mass thousands of times larger.

$α = ⁴₂He, β⁻ = ⁰₋₁e, β⁺ = ⁰₊₁e, γ = ⁰₀γ$

Gamma radiation is electromagnetic radiation, not a neutral nucleon. A beta particle has the electron/positron rest mass; its high speed does not make its rest mass zero.

The antiparticle corresponding to a particle has the same rest mass and electric charge of equal magnitude but opposite sign.

Particle	Antiparticle	Mass relation	Charge relation
electron e⁻	positron e⁺	equal	−e and +e
proton p	antiproton p̄	equal	+e and −e

If a top quark has charge +(2/3)e, its antiquark has the same mass and charge −(2/3)e. Reverse the sign; do not change the magnitude or mass.

A positron is specifically the electron's antiparticle. It is not a proton: both carry +e, but their masses and particle identities are different.

Decay	Nucleon change	Charged beta particle	Neutral lepton
β⁻	neutron → proton	electron e⁻	electron antineutrino ν̄ₑ
β⁺	proton → neutron	positron e⁺	electron neutrino νₑ

$n → p + e⁻ + ν̄ₑ p → n + e⁺ + νₑ$

Match the beta sign first: β⁻ is the electron and therefore accompanies the antineutrino; β⁺ is the positron and accompanies the neutrino. Both (anti)neutrinos have zero charge and zero nucleon number.

The (anti)neutrino is an additional emitted particle, not another name for the beta particle. Omitting it makes a beta-decay equation incomplete for this syllabus.

Alpha decay between quantised nuclear levels gives particles with characteristic discrete energies; beta decay shares energy with a neutrino, producing a continuous range.

Interpret line spectra and broad beta spectra as evidence about the number of bodies sharing decay energy.

A detector can show sharp alpha peaks but a broad beta distribution ending at a maximum energy.

A continuous beta spectrum does not mean beta particles are emitted with random violation of energy conservation; total energy is conserved event by event.

$α: ᴬ_ZX → ᴬ⁻⁴_Z₋₂Y + ⁴₂He β⁻: ᴬ_ZX → ᴬ_Z₊₁Y + ⁰₋₁e + ⁰₀ν̄ₑ β⁺: ᴬ_ZX → ᴬ_Z₋₁Y + ⁰₊₁e + ⁰₀νₑ$

Identify the decay type, calculate the daughter A and Z, use Z to identify its element symbol, add every emitted particle, then check that upper numbers and signed lower numbers balance separately.

Decay	Complete example	Check
α	²³⁸₉₂U → ²³⁴₉₀Th + ⁴₂He	238 = 234 + 4; 92 = 90 + 2
β⁻	³₁H → ³₂He + ⁰₋₁e + ⁰₀ν̄ₑ	3 = 3; 1 = 2 − 1
β⁺	¹²₇N → ¹²₆C + ⁰₊₁e + ⁰₀νₑ	12 = 12; 7 = 6 + 1

A is unchanged in beta decay. The daughter Z rises for β⁻ and falls for β⁺, and the correct electron (anti)neutrino must be included even though it contributes 0 to both number checks.

One unified atomic mass unit, 1 u, is one twelfth of the mass of a neutral carbon-12 atom. Use 1 u = 1.66 × 10⁻²⁷ kg.

$mass in kg = mass in u × 1.66 × 10⁻²⁷ mass in u = mass in kg ÷ (1.66 × 10⁻²⁷)$

A particle of mass 12.0 u has mass 12.0 × 1.66 × 10⁻²⁷ = 1.99 × 10⁻²⁶ kg.

A nucleus of mass 7.30 × 10⁻²⁶ kg has mass (7.30 × 10⁻²⁶)/(1.66 × 10⁻²⁷) = 44.0 u.

Nucleon number A is an integer count, not an exact nuclear mass in u. Binding energy causes the actual mass of a nucleus to differ from the simple sum of free nucleon masses.

11.2 Fundamental particles

A fundamental particle is not known to contain smaller constituent particles: it cannot be subdivided into a simpler internal structure. Quarks are fundamental particles.

Group for charge learning	Quark flavours
up-type	up (u), charm (c), top (t)
down-type	down (d), strange (s), bottom (b)

The six flavour names are up, down, strange, charm, top and bottom. Each also has a corresponding antiquark flavour, so there are six antiquark flavours.

Meson, baryon, proton and neutron are not quark flavours; they are composite hadrons. For this syllabus, learn quark flavour and charge only—no other quark properties are required.

Flavours	Quark charge	Corresponding antiquark charge
u, c, t	+(2/3)e	−(2/3)e
d, s, b	−(1/3)e	+(1/3)e

Replace every constituent by its signed charge and add. An antiquark keeps the flavour family but reverses the charge sign.

Proton uud: +(2/3)e +(2/3)e −(1/3)e = +e. Neutron udd: +(2/3)e −(1/3)e −(1/3)e = 0. Antiproton ūūd̄: −(2/3)e −(2/3)e +(1/3)e = −e.

Do not reverse flavour when forming an antiquark: anti-up is still the antiparticle of up, but its charge is −(2/3)e. Fractional constituent charges can sum to an integer hadron charge.

Nucleon	Quark composition	Charge sum
proton	uud	2/3 + 2/3 − 1/3 = +1
neutron	udd	2/3 − 1/3 − 1/3 = 0

Protons and neutrons are not fundamental because each is a composite baryon containing three quarks.

$For Z protons and N neutrons: number of u = 2Z + N, number of d = Z + 2N$

Tritium has Z = 1 and N = 2, so it contains 2(1)+2 = 4 up quarks and 1+2(2) = 5 down quarks. An alpha particle has two protons and two neutrons, giving 6 up and 6 down quarks.

A neutron is neutral because its fractional constituent charges sum to zero, not because it contains no charged quarks.

Hadron class	Constituent pattern	Examples
baryon	three quarks	proton uud, neutron udd
antibaryon	three antiquarks	antiproton ūūd̄
meson	one quark + one antiquark	pion

A hadron is a composite particle made from quarks. For the syllabus classification, hadrons are baryons or mesons; flavour combinations need not use identical quarks.

A neutral meson can pair a quark with its corresponding antiquark, such as uū: +(2/3)e −(2/3)e = 0. Charge validity must agree with the constituent signs.

Electrons, positrons, neutrinos and antineutrinos are leptons, not hadrons. A meson is not two baryons and does not contain two ordinary quarks.

In β− decay a down quark changes to an up quark, turning a neutron udd into a proton uud and emitting an electron and antineutrino; β+ reverses the quark change with a positron and neutrino.

Track the quark charge change and the emitted lepton charges together. The weak interaction mediates the flavour change.

A neutron’s d→u raises hadron charge by +e, balanced by the emitted electron’s −e in β− decay.

The beta electron is not an orbital electron released from the atom; it is created in the weak decay process.

Leptons are fundamental particles: they are not made from quarks. In this syllabus, recognise electrons and neutrinos and their antiparticles as leptons.

Lepton	Antiparticle	Electric charge
electron e⁻	positron e⁺	−e; antiparticle +e
electron neutrino νₑ	electron antineutrino ν̄ₑ	0; antiparticle 0

Fundamental leptons	Composite hadrons (not leptons)
electron, positron, neutrino, antineutrino	proton, neutron, baryon, meson

Beta-minus emission contains two leptons: an electron and an electron antineutrino. Beta-plus emission contains a positron and an electron neutrino.

A positron is a positively charged lepton, so it is false that every charged lepton has charge −e. A neutrino is not a neutral hadron or a photon.

11. Particle physics AS

Exam analysis

Most tested topics

11.1 Atoms, nuclei and radiation

Alpha scattering showed that atoms contain a tiny dense positive nucleus

The nuclear atom contains protons and neutrons in a small nucleus with electrons outside it

Proton number identifies the element, while nucleon number counts protons plus neutrons

Isotopes are atoms of the same element with different neutron numbers

Read nuclide notation and count particles in atoms and ions

Balance nucleon number and electric charge separately

Compare alpha, beta-minus, beta-plus and gamma radiation

An antiparticle has equal mass and opposite charge

Beta-minus emits an antineutrino; beta-plus emits a neutrino

Alpha particles have discrete energies, whereas beta particles have a continuous energy spectrum

Write complete alpha, beta-minus and beta-plus decay equations

Convert particle and nuclear masses using the unified atomic mass unit

11.2 Fundamental particles

Quarks are fundamental particles with six flavours

Use quark and antiquark charges to find composite charge

Protons are uud and neutrons are udd

Hadrons are baryons or mesons according to constituent pattern

Beta decay changes quark flavour while conserving total charge and nucleon number

Electrons and neutrinos are fundamental leptons