Atomic Models — Dalton to Bohr
A Model That Keeps Changing
The history of atomic theory is a textbook case of how science actually works: each model served well until new experimental evidence forced scientists to revise or abandon it. No single scientist “discovered” the atom in its modern form — the picture we use today was built up over more than a century, with each breakthrough depending on the tools and questions of its time. Understanding this story helps you think like a scientist, not just remember facts.
The atomic model developed in stages: Dalton (solid sphere) → Thomson (plum pudding) → Rutherford (nuclear model) → Bohr (electron shells) → Chadwick (neutron discovered). Each change was driven by new experimental evidence, not by theory alone.
Dalton’s Solid Sphere (1803)
John Dalton proposed that all matter is made of tiny, indivisible particles called atoms. In his model, atoms were like tiny solid spheres — different for each element, but impossible to break apart. Dalton’s theory was based on careful measurements of the masses of reactants and products in chemical reactions (the law of definite proportions and the law of multiple proportions). At the time, it was the best explanation for why elements combine in fixed ratios.
Thomson’s Plum Pudding Model (1897)
In 1897, J.J. Thomson discovered the electron by experimenting with cathode rays — beams of particles that travelled from the negative terminal of a tube towards the positive terminal in a vacuum. He showed these particles had a negative charge and a mass far smaller than any atom. This proved that atoms were not indivisible — they contained smaller parts.
Thomson proposed the plum pudding model: the atom is a sphere of positive charge with negative electrons embedded in it, like plums in a pudding. The positive charge was spread evenly throughout the sphere. This model explained why atoms were electrically neutral overall.
Rutherford’s Nuclear Model (1911)
The plum pudding model was overturned by a famous experiment. Working in Manchester, Ernest Rutherford directed Hans Geiger and Ernest Marsden to fire alpha particles at a very thin sheet of gold foil (the Geiger–Marsden experiment, 1909; Rutherford’s interpretation published 1911). According to the plum pudding model, the alpha particles should have passed straight through with minimal deflection — positive charge was spread out, so it could not exert a strong force.
Most alpha particles did pass straight through the gold foil — suggesting that most of the atom is empty space. However, a small number were deflected at large angles, and a very few bounced almost straight back. Rutherford famously described this as “almost as incredible as if you fired a 15-inch shell at a piece of tissue paper and it came back and hit you.”
The only explanation consistent with these results was that almost all the mass of the atom, and all its positive charge, was concentrated in an extremely small, dense region at the centre: the nucleus. The electrons orbit this nucleus, and most of the atom is empty space between the nucleus and the electrons. This replaced the plum pudding model.
Bohr’s Shell Model (1913)
Rutherford’s model had a problem: electrons orbiting a nucleus should, according to classical physics, lose energy and spiral into the nucleus. The atom should be unstable. Niels Bohr solved this by proposing that electrons do not orbit freely — they can only exist in specific energy levels (called shells or orbits). Electrons can move between levels by absorbing or emitting energy, but within a level they do not radiate energy. This explained the stability of atoms and also accurately predicted the spectrum of light emitted by hydrogen atoms.
Bohr’s key addition was that electrons occupy fixed energy levels (shells). Electrons can jump to a higher shell by absorbing energy and drop to a lower shell by emitting energy (as light). This is why each element produces a unique spectrum of light.
Chadwick and the Neutron (1932)
One loose end remained: the mass of atoms was always greater than could be accounted for by protons alone. In 1932, James Chadwick bombarded beryllium with alpha particles and detected a previously unknown neutral particle with a mass similar to a proton. He had discovered the neutron. This completed the picture of the nucleus as containing both protons and neutrons, explaining why atoms of the same element can have different masses (isotopes).
Each model in this story was the best available explanation of the evidence at the time. No model was abandoned because someone decided it was wrong — each was replaced only when new experimental evidence (cathode rays, alpha scattering, emission spectra, neutron bombardment) could not be explained by the existing model. This is how science should work: models are provisional and open to revision. The fact that today’s atomic model is also more complex than Bohr’s (electrons are described as probability clouds, not fixed orbits) shows that the process continues.