Ancient to Modern Atomic Models: Key Concepts and Laws

Democritus

Proposed that matter consists of tiny, indivisible particles called atoms that differ in size, shape, and properties.

Aristotle

Rejected the atomic theory and advocated the four classical elements (earth, air, fire, water).

Dalton's Atomic Theory

Formulated four postulates regarding atoms in the early 19th century.

Dalton's First Postulate

All matter is composed of extremely small particles called atoms.

Dalton's Second Postulate

Atoms of the same element are identical, whereas atoms of different elements differ.

Dalton's Third Postulate

Compounds form when atoms of different elements combine in simple whole‑number ratios.

Dalton's Fourth Postulate

Atoms are indivisible; they cannot be created or destroyed, only rearranged in chemical reactions.

Flaw in Dalton's Theory: Identical atoms

Modern isotopic theory shows that isotopes are atoms of the same element with different numbers of neutrons, giving different masses.

Flaw in Dalton's Theory: Indivisibility

Experiments revealed atoms consist of protons, neutrons, and electrons.

Law of Conservation of Mass

Matter cannot be created or destroyed in a chemical reaction; total mass remains constant.

Law of Definite Proportions

A given compound always contains the same mass percentages of its constituent elements.

Law of Multiple Proportions

When elements combine, they do so in ratios of small whole numbers.

J.J. Thomson

Discovered the electron (negatively charged particle) using cathode-ray experiments.

Plum-Pudding Model

Atoms consist of a uniform positively charged matrix (the 'pudding') with evenly distributed electrons (the 'plums').

Rutherford's Gold-Foil Experiment

Demonstrated that atoms contain a tiny, dense, positively charged nucleus.

Setup of Rutherford's Experiment

α‑particles (He²⁺) fired at a thin gold foil.

Observation of Rutherford's Experiment

Most α‑particles passed straight through, but a small fraction were deflected at large angles or bounced back.

Conclusions of Rutherford's Experiment

Atoms have a massive, dense nucleus occupying a very small volume; most of the atom is empty space.

Bohr Model

Proposed that electrons occupy fixed energy levels (orbits) at set distances from the nucleus.

Key idea of Bohr Model

Electrons can jump between levels, emitting or absorbing quantized energy.

Limitation of Bohr Model

Electrons do not truly orbit like planets; later quantum mechanics refined this view.

Quantum Mechanical Model

Describes electrons as existing in orbitals—regions of space with a high probability of finding an electron.

Features of Quantum Mechanical Model

Electrons are delocalized within orbitals; they are most likely found close to the nucleus but can be found farther away with lower probability.