How do you picture something you can’t see?
Suppose someone hands you a sealed box. You can’t open it, but you can shake it, listen to it, and weigh it. Could you work out what’s inside? That’s the situation scientists faced with atoms. Atoms are far too small to see, so scientists proposed a model模型 based on experiments, then tested it with new experiments. When a model couldn’t explain the results, they changed it.
A century of atomic models
Click each year to see what model was proposed, what evidence it rested on, and what it still couldn’t explain:
Solid sphere model
Key evidenceIn chemical reactions, elements always combine in fixed ratios by mass.
The modelMatter is made of atoms. Atoms are solid, indivisible spheres, and all atoms of an element have the same mass and properties.
What it couldn't explainAtoms turned out to contain even smaller particles: electrons.
Plum pudding model (proposed 1904)
Key evidenceWhile studying cathode rays, Thomson discovered the electron: a negatively charged particle far lighter than even the lightest atom, found in every kind of atom.
The modelAn atom is a sphere of positive charge with electrons scattered through it like raisins in a pudding. The positive and negative charges balance, so the atom is neutral.
What it couldn't explainIn the gold foil experiment, a few alpha particles were deflected sharply, and a very few even bounced back.
Nuclear model
Key evidenceThe gold foil experiment: almost all alpha particles passed straight through, a few were deflected sharply, and a very few bounced back.
The modelAt the center of the atom is a tiny, positively charged nucleus that holds nearly all the mass. Electrons move around it in the large space outside.
What it couldn't explainWhy don't the moving electrons spiral into the nucleus? Why does hydrogen give off only a few specific colors of light?
Bohr model (shell model)
Key evidenceHydrogen gives off light of only a few specific colors (a line spectrum).
The modelElectrons can only move in certain orbits with specific energies; the farther from the nucleus, the higher the energy. This is where electron shells come from.
What it couldn't explainIt works well for hydrogen but can't explain atoms with many electrons.
Electron cloud model (modern model)
Key evidenceElectrons behave both like particles and like waves, so their exact position and path can't both be known.
The modelElectrons don't follow fixed orbits. The model shows how likely an electron is to be found in each region around the nucleus: where the cloud is denser, the electron is more likely to be. In 1932, Chadwick discovered the neutron, completing the picture of a nucleus made of protons and neutrons.
The gold foil experiment
In 1909, Rutherford’s students Hans Geiger and Ernest Marsden fired positively charged alpha particles (helium nuclei, each about 7300 times the mass of an electron) at a very thin sheet of gold foil and watched where they went. In 1911, Rutherford used their results to propose the nuclear model.
Choose a model, fire a few batches of alpha particles, and compare what the two models predict:
- Went almost straight through 0
- Deflected noticeably 0
- Bounced back 0
Not to scale: the nuclei are drawn larger and the deflections are more common than in reality. In the real experiment, only about 1 alpha particle in 8000 was deflected by more than 90°.
The real results matched the Rutherford model, not the Thomson model: positive charge spread thinly through the whole atom could never bounce back a heavy, fast alpha particle. Rutherford later compared his surprise to firing an artillery shell at a sheet of tissue paper and having it bounce back.
Example 1: From observations to conclusions
The gold foil experiment gave three observations. What does each one tell us?
- Almost all alpha particles went straight through. Most of the atom is empty space.
- A few alpha particles were deflected at large angles. Alpha particles are positive, so they were repelled by something positive, and that positive charge must be concentrated in one place.
- A very few alpha particles bounced back. The positive part is tiny but very massive, holding nearly all of the atom’s mass: the nucleus.
Example 2: Couldn’t electrons have bounced the alpha particles back?
- Compare the masses. An alpha particle is about 7300 times as massive as an electron.
- An analogy. That’s like a bowling ball hitting a ping-pong ball: the bowling ball barely notices, and it certainly doesn’t bounce back.
- Conclusion. Only something far more massive than an alpha particle, and positively charged, could bounce it back: the nucleus of a gold atom.
- Discovered the electron
Thomson discovered the electron in 1897 while studying cathode rays.
- Described atoms as solid, indivisible spheres
That's Dalton's atomic theory.
- Proposed the nucleus based on the gold foil experiment
Rutherford proposed the nuclear model in 1911.
- Proposed that electrons move in specific orbits, or shells
Bohr proposed his shell model in 1913.
- Proposed the plum pudding model
Electrons scattered like raisins through a positively charged pudding.
- Used atoms to explain why elements combine in fixed mass ratios
Dalton's atomic theory explained the mass ratios seen in chemical reactions.
- Explained why hydrogen gives off only a few specific colors of light
Electrons jumping between orbits of different energy give off specific colors of light.
- Concluded that most of an atom is empty space
Since almost all alpha particles went straight through the foil, most of the atom must be empty.
Common mistakes
Mistake 1: “Each new model proved the old one completely wrong.” Each model explained the evidence available at the time. New evidence led to models being revised and refined, not thrown out entirely. Dalton’s core idea, that matter is made of atoms, still holds today.
Mistake 2: “The electron cloud is a cloud made of lots of electrons.” The electron cloud shows how likely an electron is to be found in each region. It isn’t a literal cloud of electrons: a hydrogen atom has only one electron, and it still has an electron cloud.
Mistake 3: “Some alpha particles bounced back because the nucleus is big.” It’s the opposite. So few bounced back because the nucleus is tiny, and an alpha particle rarely heads straight at one.
Quick check
In the gold foil experiment, a very few alpha particles bounced back. What does this show?
Only a tiny, massive, positively charged nucleus could bounce back a heavy, fast, positive alpha particle. A is what the straight-through particles show.
Which list puts the atomic models in the order they were proposed?
Dalton (1803) → Thomson (discovered the electron, 1897) → Rutherford (1911) → Bohr (1913).
Summary
- Dalton (1803): solid sphere model. Thomson (electron, 1897): plum pudding model. Rutherford (1911): nuclear model. Bohr (1913): shell model. Today: electron cloud model.
- Gold foil experiment: almost all alpha particles went straight through (the atom is mostly empty), while a few were deflected sharply and a very few bounced back (a tiny, massive, positive nucleus).
- Scientific models are built on evidence and revised when new evidence appears.
On to AP (for later)
Building and revising a model from evidence is AP Science Practice 6, Argumentation. Lesson 2.2.1 in Stage 2 covers the development of the atomic model in full, and lessons 2.2.2 and 2.2.3 explain the spectra and energy levels behind Bohr’s model. In AP topic 1.6, photoelectron spectroscopy (PES) gives direct experimental evidence of electron shells.