Difficulty: Foundational | Prerequisites: none. This is a good starting point if you are coming in cold.
The history of atomic theory is the story of how our understanding of matter evolved from "stuff is made of tiny things" to precise quantum mechanical models. Each scientist built on (or overturned) the work before them. This topic also covers SI units and measurement systems, nuclear decay types, and the reference tables you will need for calculations. If you know nothing else, start here: the people and their experiments are tested on nearly every general chemistry midterm.
Ten scientists shaped the atomic model from Democritus's "atomos" through Dalton's billiard balls, Thomson's plum pudding, Rutherford's nucleus, Bohr's planetary orbits and de Broglie's electron cloud. Each model was disproved or refined by experimental evidence. This section also collects the SI units, nuclear decay equations and measurement conversions you need as reference material.
Democritus
First to propose that all matter is composed of tiny, indivisible particles. He called them "atomos" (meaning indestructible). In simple terms, he said "keep cutting something in half and eventually you get a piece that cannot be cut further."
John Dalton
Formulated the first modern atomic theory. Four key points: elements are made of atoms; atoms cannot be divided, created or destroyed in chemical reactions; all atoms of a given element are identical in mass and properties; compounds contain elements in fixed proportions. He also discovered the mole concept. (Later revised: isotopes showed that not all atoms of an element are identical in mass.)
J.J. Thomson
Discovered the electron using the cathode ray tube (CRT) experiment. He measured how much the rays were deflected by magnets and how much heat they generated, allowing him to estimate their mass. Proposed the plum pudding model: a sphere of positive charge with negatively charged electrons embedded in it.
Ernest Rutherford
Discovered the nucleus using the gold foil experiment. He fired alpha particles at thin gold foil and found that most passed through, but some deflected at large angles or bounced straight back. Conclusion: the positive charge of an atom is concentrated in a tiny, dense nucleus, and most of the atom is empty space.
Niels Bohr
Proposed the planetary (quantised orbits) model. Electrons orbit the nucleus in fixed energy levels, each holding a specific number of electrons. This is the most commonly drawn atomic model and was the first to successfully predict the hydrogen spectrum.
Louis de Broglie
Proposed that electrons (and all moving particles) have wave-like characteristics. His equation (λ = h/mv) predicts the wavelength of any moving particle. This led to the electron cloud model, replacing Bohr's fixed orbits.
John Newlands
Proposed the law of octaves: elements repeated their properties in a periodic pattern every eight elements. This was an early attempt at organising elements before the periodic table.
Dmitri Mendeleev
Organised the first periodic table by arranging elements in order of increasing atomic mass. Some elements ended up in groups with different properties, so the system was not fully accurate.
Henry Moseley
Discovered that atoms of each element contain a unique number of protons. Rearranged the periodic table in order of atomic number instead of atomic mass, fixing Mendeleev's errors.
Robert Millikan
Discovered the charge of an electron using the oil-drop experiment.
The progression runs: Democritus (indivisible particles) → Dalton (solid spheres, chemical laws) → Thomson (electrons embedded in positive charge) → Rutherford (dense positive nucleus, mostly empty space) → Bohr (electrons in quantised orbits) → de Broglie / Schrodinger (electron cloud, wave-particle duality).
Each step was driven by experimental evidence. Thomson's cathode ray experiment revealed electrons. Rutherford's gold foil experiment revealed the nucleus. Bohr's model explained hydrogen's emission spectrum. De Broglie's wave hypothesis and the Schrodinger equation replaced fixed orbits with probability clouds.
Alpha emission (α decay): parent → ⁴₂He + daughter. The nucleus loses 2 protons and 2 neutrons.
Beta emission (β decay): parent → ⁰₋₁e + daughter. A neutron converts to a proton, emitting an electron.
Positron emission: parent → ⁰₊₁e + daughter. A proton converts to a neutron.
Gamma emission (γ): parent → ⁰₀γ + daughter. Energy is released with no change in mass or atomic number.
Electron capture: parent + ⁰₋₁e → daughter. An inner electron is absorbed by the nucleus.
Alpha bombardment: parent + ⁴₂He → daughter.
Fission / neutron bombardment: parent + ¹₀n → fragments + neutrons.
Induced (artificial) transmutation: ⁴₂He + target → product + ¹₁p.
Penetrating power (most to least): gamma > beta > alpha.
Base units:
Quantity | Unit |
|---|---|
Time | Second (s) |
Length | Metre (m) |
Mass | Kilogram (kg) |
Temperature | Kelvin (K) |
Amount of substance | Mole (mol) |
Derived units:
Quantity | Unit |
|---|---|
Volume | m³ |
Speed / Velocity | m/s |
Density | kg/m³ |
Energy | Joule (J) |
SI prefixes:
Factor | Prefix | Example |
|---|---|---|
10⁹ | giga- | gigametre |
10⁶ | mega- | megagram |
10³ | kilo- | kilometre |
10¹ | deca- | decalitre |
10⁻¹ | deci- | decilitre |
10⁻² | centi- | centimetre |
10⁻³ | milli- | milligram |
10⁻⁶ | micro- | microgram |
10⁻⁹ | nano- | nanometre |
10⁻¹⁰ | angstrom | |
10⁻¹² | pico- | picometre |
Volume conversions: 1000 mL = 1 L = 1000 cm³ = 1 dm³.
Students often credit Dalton with discovering the atom. Democritus proposed the idea of atoms roughly 2,000 years before Dalton. Dalton's contribution was the first testable atomic theory with chemical laws behind it.
Thomson did not discover the proton. He discovered the electron. The proton was identified later, and Rutherford's experiment revealed the nucleus (where protons reside).
Bohr's model is useful for hydrogen but does not work well for multi-electron atoms. Students sometimes assume it is universally correct.
Alpha particles are the least penetrating form of radiation, not the most. Gamma particles are the most penetrating.
⚠️ Know which scientist goes with which discovery and which experiment. This is a direct-recall section on most exams.
⚠️ Be able to describe Thomson's CRT experiment, Rutherford's gold foil experiment and Millikan's oil-drop experiment in 2 to 3 sentences each.
⚠️ Know the order of atomic models and what evidence disproved each one.
⚠️ Nuclear decay types (alpha, beta, positron, gamma, electron capture) are commonly tested. Know the particle symbols and mass/charge changes.
⚠️ Penetrating power order (gamma > beta > alpha) is a classic multiple-choice question.
⚠️ SI prefix conversions appear in calculation problems. Know at least giga through pico.
True or false: Rutherford discovered the electron. (False, Thomson discovered the electron. Rutherford discovered the nucleus.)
Fill in the blank: Mendeleev arranged elements by ____, while Moseley arranged them by ____. (atomic mass, atomic number)
True or false: Alpha particles have the highest penetrating power of the three radiation types. (False, gamma particles do.)
Fill in the blank: The SI base unit for temperature is the ____. (Kelvin)
True or false: In beta decay, a proton converts to a neutron. (False, a neutron converts to a proton.)
Q: Describe Rutherford's gold foil experiment and state what it proved.
A: Rutherford fired alpha particles at a thin sheet of gold foil coated on the far side with zinc sulfide (to detect hits). Most particles passed straight through, but some deflected at large angles and a few bounced almost straight back. This proved that the atom's positive charge is concentrated in a very small, dense nucleus, and that most of the atom is empty space.
Q: What did Millikan's oil-drop experiment determine?
A: The charge of a single electron (approximately 1.6 × 10⁻¹⁹ coulombs).
Q: How does Bohr's model differ from the current quantum mechanical model?
A: Bohr's model places electrons in fixed circular orbits at set distances from the nucleus. The quantum mechanical model (from Schrodinger and de Broglie) describes electrons as probability clouds (orbitals) with no fixed path.
Q: Write the nuclear equation for alpha decay of uranium-238.
A: ²³⁸₉₂U → ⁴₂He + ²³⁴₉₀Th. The parent loses 2 protons and 2 neutrons.
Q: Convert 5.0 kilometres to metres.
A: 5.0 × 10³ = 5,000 metres. (Kilo- = 10³.)
The atomic models covered here set up the quantum mechanics you need for electron configuration (Atomic Structure notes). Rutherford's discovery of the nucleus leads directly to understanding nuclear decay and radioactivity. SI units and measurement conversions are used in every calculation throughout the course, from stoichiometry to gas laws.
Democritus, atomos, Dalton, atomic theory, J.J. Thomson, cathode ray tube, CRT, plum pudding model, electron, Rutherford, gold foil experiment, nucleus, Bohr, planetary model, quantised orbits, de Broglie, electron cloud, wave-particle duality, Newlands, law of octaves, Mendeleev, periodic table, Moseley, atomic number, Millikan, oil-drop experiment, alpha decay, beta decay, gamma radiation, positron emission, electron capture, fission, transmutation, penetrating power, SI units, base units, derived units, SI prefixes, mole, Kelvin, kilogram, metre, second