Atomic Structure (AQA)

Atoms

Atoms have a nucleus containing protons and neutrons, with electrons moving in shells.

Atomic Number

The number of protons in an atom, determining the element.

Mass Number

The total number of protons and neutrons in an atom's nucleus.

John Dalton (1803)

Proposed that atoms are tiny, solid spheres that cannot be divided.

J. J. Thomson (1897)

Discovered the electron and proposed the plum pudding model.

Plum Pudding Model

An atomic model with a positive charge containing scattered electrons.

Ernest Rutherford (1909)

Disproved the plum pudding model with the gold foil experiment.

Rutherford’s Nuclear Model

Suggested a small, dense, positively charged nucleus in an atom.

Niels Bohr (1913)

Proposed that electrons orbit the nucleus in fixed shells.

Discovery of Protons

Rutherford confirmed the existence of positively charged protons.

James Chadwick (1932)

Discovered neutrons, explaining atomic mass differences.

Modern Atomic Model

Atoms have a nucleus and electron clouds, following quantum mechanics.

Size of an Atom

Approximately 0.1 nm, with a nucleus around 1 × 10⁻¹⁴ m.

Subatomic Particles

Atoms contain protons, neutrons, and electrons with specific properties.

Proton

Subatomic particle with a +1 charge and relative mass of 1.

Electron

Subatomic particle with a -1 charge and very small relative mass.

Neutral Atom

An atom with equal numbers of protons and electrons.

Isotopes

Atoms with the same number of protons but different neutrons.

Chemical Properties of Isotopes

Identical because isotopes have the same number of electrons.

Relative Atomic Mass (Ar)

Weighted average of an element's isotopes based on abundance.

Chlorine Isotopes

Chlorine-35 (75%) and Chlorine-37 (25%), giving Ar = 35.5.

Gold Foil Experiment

Rutherford’s test that showed atoms have a dense nucleus.

Electron Shells

Electrons orbit the nucleus in specific energy levels.

Quantum Model of Atoms

Electrons exist in probability clouds rather than fixed orbits.