The Nuclear Model of AtomCopy

O Level / IGCSE Physics Cheat Sheet – Nuclear Physics: Section 5.1 The Nuclear Model of the Atom

5.1.1 The Atom

Structure of the Atom:

• An atom consists of a small, central nucleus (positive charge) and electrons (negative charge) orbiting around it.
• Electrons are arranged in energy levels or shells.
• The nucleus is dense and contains nearly all the mass of the atom.

Alpha Particle Scattering Experiment (Rutherford’s Gold Foil Experiment):

• Alpha particles (⁴₂He nuclei) directed at a thin gold foil.
• Observations and Conclusions:
  1. Most particles passed straight through → atoms are mostly empty space.
  2. Some particles deflected at large angles → atom has a dense, small, positively charged nucleus.
  3. Very few particles rebounded → nucleus contains most of the atom’s mass.

Evidence Summary:
a) Very small nucleus → very few alpha particles rebound.
b) Nucleus contains most of atom’s mass → deflects high-energy alpha particles.
c) Nucleus is positively charged → repels positively charged alpha particles.

5.1.2 The Nucleus

Nucleus Composition:

• Made up of protons (positively charged) and neutrons (no charge).
• Proton: charge = +1e, mass ≈ 1 u.
• Neutron: charge = 0, mass ≈ 1 u.
• Nucleons = protons + neutrons.

Ion Formation:

• Positive ion (cation): atom loses electrons → more protons than electrons.
• Negative ion (anion): atom gains electrons → more electrons than protons.

Atomic and Mass Numbers:

• Proton number (Z) = number of protons in nucleus (also equal to number of electrons in a neutral atom).
• Nucleon number (A) = total number of protons + neutrons.
• Neutrons = A − Z

Nuclide Notation:

• Written as:

A
Z X

Where:

• X = chemical symbol
• Z = proton number
• A = nucleon number

Example:
14
6 C (Carbon-14)

Isotopes:

• Atoms of the same element with the same number of protons but different numbers of neutrons.
• Same chemical properties, different mass numbers.

Examples:

• Hydrogen-1 (¹₁H), Hydrogen-2 (²₁H, deuterium), Hydrogen-3 (³₁H, tritium)
• Carbon-12 (¹²₆C), Carbon-14 (¹⁴₆C)