11.1 Atoms, nuclei and radiation

Atoms of all elements are made up of 3 subatomic particles → protons, neutrons and electrons.
Proton number (Z) → The number of protons in a nucleus.
Nucleon number (A) → The total number of protons and neutrons in a nucleus.
- A nucleon is a particle found in the nucleus → either a proton or a neutron.
Isotopes → Forms of the same element with the same number of protons but a different number of neutrons.
Nuclide → A specific type of nucleus defined by its proton number (Z) and nucleon number (A).
Nuclides are represented as follows: $^A_Z \text{X}$ $_{Z}^{A} X$
- Where X is the symbol for the element.
In all nuclear processes (e.g., radioactive decay), the nucleon number and charge are conserved.

🔥 Unified Atomic Mass Unit (u)

The SI unit used to measure the mass of atoms and their constituent particles is the unified atomic mass unit (u), not the kilogram (kg).

Subatomic particle	Mass (u)
Proton	1
Neutron	1
Electron	~1/2000

Radioactivity

In some atomic nuclei, the combination of protons and neutrons is unstable.
To become more stable, these nuclei emit particles and/or electromagnetic (e.m.) waves. This process is known as radioactivity.
Common emissions from radioactive nuclei include:
- Alpha particles (α)
- Beta particles (β)
- Gamma rays (γ)

Alpha (α) radiation

Composition: 2 protons + 2 neutrons (a Helium nucleus)
Mass: 4u
Charge: +2e
Denoted as: $^4_2 \alpha$
Example of α-decay:

Beta (β) radiation

There are two types of beta radiation:
- Beta-minus (β⁻): The emission of an electron.
- Beta-plus (β⁺): The emission of a positron.

🔥 Antiparticles

An antiparticle has the same mass but the opposite charge as its corresponding particle. A positron (β⁺) is the antiparticle of an electron (β⁻).

Example of β⁻ decay:
- An antineutrino ( $\bar{\nu}$ ) is also emitted. It has zero mass and zero charge.
Nuclear change during β⁻ decay: A neutron transforms into a proton.
Example of β⁺ decay:
- A neutrino ( $\nu$ ) is also emitted. It has zero mass and zero charge.
Nuclear change during β⁺ decay: A proton transforms into a neutron.

Gamma (γ) radiation

Composition: Gamma rays (high-energy electromagnetic radiation).
Mass: 0
Charge: 0 (neutral)
Example of γ-decay: An excited nucleus releases energy as a gamma ray to become more stable.

Radiation Type	Composition	Mass (u)	Charge (e)
α (Alpha)	Helium nucleus (2p + 2n)	4	+2
β⁻ (Beta-minus)	Electron	~1/2000	-1
β⁺ (Beta-plus)	Positron (anti-electron)	~1/2000	+1
γ (Gamma)	Electromagnetic wave	0	0

Kinetic energy of emitted particles

α-particles emitted from a specific nuclide all have the same kinetic energy.
β-particles emitted from a specific nuclide have a continuous range of kinetic energies. This is because the total energy released is shared between the beta particle and the neutrino/antineutrino.

Alpha particle scattering experiment

Setup:
- A beam of alpha particles (positively charged) was directed at a thin gold foil.
- A circular detector screen measured the deflection angle of the particles.
Observations & Conclusions:
- Most α-particles passed straight through: The atom is mostly empty space.
- A few α-particles were deflected at small angles: The atom contains a concentrated positive charge.
- Very few α-particles were deflected at large angles (>90°): The positive charge is concentrated in a tiny, dense core called the nucleus.

11.2 Fundamental particles

Fundamental particles are particles that cannot be broken down into smaller components.
They are categorized into two main groups: Quarks and Leptons.

Flavour of quark	Charge (e)
Up (u)	+2/3
Top (t)	+2/3
Charm (c)	+2/3
Down (d)	-1/3
Bottom (b)	-1/3
Strange (s)	-1/3

Antiquarks (e.g., anti-up, $\bar{u}$ ) have the same mass but opposite charge to their corresponding quark.
Electrons and neutrinos are types of leptons.
Subatomic particles are classified as follows:

Hadrons	Leptons
Affected by the strong nuclear force	Not affected by the strong force
Composed of quarks (not fundamental)	Are fundamental particles
e.g., Protons and Neutrons	e.g., Electrons and Neutrinos

Hadrons are further divided into two types:

Baryons	Mesons
Made of 3 quarks	Made of 1 quark and 1 antiquark
e.g., Protons and Neutrons	e.g., Pions and Kaons

Quark Composition of a Proton (p):
- Composition: two up quarks, one down quark (uud)
- Charge: $(+2/3) + (+2/3) + (-1/3) = +1e$
Quark Composition of a Neutron (n):
- Composition: one up quark, two down quarks (udd)
- Charge: $(+2/3) + (-1/3) + (-1/3) = 0$

Quark changes in β⁻ decay

During beta-minus decay, a down quark within a neutron changes into an up quark, transforming the neutron into a proton.