Early Atomic Models: From Ideas to Evidence

How do you study something you can't see?

A single atom is far too small to see with any ordinary microscope. A row of several million atoms side by side would stretch only about one millimetre. So how did scientists figure out what atoms are like?

They used indirect evidence: they did experiments, looked at the results, and built models that explained them. When new evidence didn't fit, they changed the model. That is how science works, and the story of the atom is one of the best examples.

Ideas without experiments

Around 400 BCE, the Greek philosopher Democritus suggested that if you kept cutting matter into smaller and smaller pieces, you would reach a tiny piece that could not be cut. He called it atomos, meaning "uncuttable". Thinkers in other places, such as the philosopher Kanada in ancient India, also described matter as made of tiny particles.

Another famous Greek thinker, Aristotle, disagreed. He taught that everything was made of four elements: earth, air, fire and water. Aristotle was so respected that his view was widely accepted in Europe for about 2000 years. Neither side did experiments to test their ideas, so there was no evidence to settle the question.

This is an example of science being shaped by its social context (A2.3): who was respected, and which tools existed, affected which ideas people accepted.

Evidence arrives: careful measuring

In the late 1700s, chemists began to measure mass very carefully.

• Antoine Lavoisier (France) did reactions in sealed containers and found that the total mass did not change. This is the law of conservation of mass.
• Joseph Proust (France) found that a compound always has the same elements in the same proportions by mass. For example, pure water is always about 1 g of hydrogen for every 8 g of oxygen, no matter where the water came from. This is the law of definite proportions.

These laws described what happens. The next step was to explain why.

Dalton's atomic theory (1803)

The English teacher and scientist John Dalton explained these laws with an atomic theory:

1. All matter is made of tiny particles called atoms, which cannot be created, destroyed or divided.
2. All atoms of the same element are identical in mass and properties. Atoms of different elements are different.
3. Compounds form when atoms of different elements combine in fixed, whole-number ratios.
4. In a chemical reaction, atoms are rearranged, not created or destroyed.

Point 4 explains conservation of mass: the same atoms are there before and after, so the mass is the same. Point 3 explains definite proportions: water always has the same ratio of atoms, so it always has the same ratio of masses.

Dalton pictured atoms as solid spheres, like tiny billiard balls with nothing inside.

Thomson finds a particle inside the atom (1897)

By the late 1800s, scientists could pump most of the air out of a glass tube and pass electricity through it. A glowing beam, called a cathode ray, travelled from the negative electrode (the cathode).

The British physicist J. J. Thomson tested the beam:

• When he placed electrically charged plates beside it, the beam bent toward the positive plate. Opposite charges attract, so the beam must be made of negatively charged particles.
• Magnets bent the beam too, and from how much it bent he worked out that the particles were far lighter than a hydrogen atom, the lightest atom. (We now know an electron has about 1/1836 the mass of a hydrogen atom.)
• The beam was the same no matter which metal the cathode was made of. So these particles must be inside all atoms.

These particles are now called electrons. Dalton had said atoms cannot be divided, but Thomson had found something smaller than an atom. The model had to change.

The "plum pudding" model (1904)

Atoms are electrically neutral (no overall charge). So if atoms contain negative electrons, they must also contain an equal amount of positive charge. Thomson suggested an atom is a sphere of positive charge with negative electrons scattered through it, like plums in a pudding. A Canadian might say: like blueberries in a muffin.