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.

The nuclide notation ^A_ZX places nucleon number A at the upper left, proton number Z at the lower left and element symbol X beside them.

Read particle counts directly: protons=Z, neutrons=A−Z, and neutral electrons=Z. For ions, adjust electron count by charge.

^14_6C has 6 protons, 8 neutrons and 6 electrons when neutral.

Do not swap A and Z; the upper number is total nucleons, not the number of neutrons.

In a nuclear equation, total nucleon number A and total proton number Z are conserved across the reaction or decay.

Balance A and Z separately; emitted particles carry their own nucleon and charge numbers.

In alpha decay, A falls by 4 and Z by 2 because the emitted alpha particle is ^4_2He.

Conservation of nucleon number does not mean the same nucleus remains; identity and energy can change while totals balance.

Alpha radiation is helium nuclei, beta-minus is electrons, beta-plus is positrons, and gamma is high-energy electromagnetic radiation.

Compare mass, charge, ionising power and penetration only after identifying the particle or photon; beta signs depend on β− or β+.

An alpha particle has charge +2e and large mass; a beta-minus particle has charge −e and tiny mass; a gamma photon has zero rest mass and charge.

Gamma is not a stream of neutral nucleons, and beta-plus is not an electron with the usual negative charge.

A particle’s antiparticle has equal rest mass and opposite electric charge; for example, a positron is the antiparticle of an electron.

Keep particle identity separate from radiation type and track conservation laws when a particle–antiparticle pair is created or annihilated.

Electron–positron annihilation can produce photons whose total energy reflects the particles’ rest energy and motion.

An antiparticle is not simply a particle travelling backwards in space, and equal mass does not mean equal charge.

In β− decay a neutron becomes a proton, electron and electron antineutrino; in β+ decay a proton becomes a neutron, positron and electron neutrino.

The neutrino carries away variable energy and momentum, explaining the continuous beta spectrum while preserving conservation laws.

The electron in β− decay does not receive one fixed energy because the available decay energy is shared with the antineutrino and recoil.

The neutrino is not optional bookkeeping and is not the same as the emitted beta particle.

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.

An alpha decay lowers A by 4 and Z by 2; beta-minus raises Z by 1; beta-plus lowers Z by 1, with A unchanged. Gamma leaves A and Z unchanged.

Balance the daughter nuclide first, then include the emitted particle and check both A and Z totals.

^238_92U→^234_90Th+^4_2He is balanced for alpha decay; β− changes a neutron-rich nucleus’s Z upward by one.

Beta decay changes element identity through Z, while gamma decay changes only nuclear energy state, not the element.

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

Use u for comparing nuclear and particle masses, then convert to kilograms when equations require SI units.

A nuclide with mass number about 4 has mass roughly 4 u, before accounting for binding-energy mass defect.

u is a unit of mass, not atomic number or energy; mass number is an integer count, not an exact mass in u.