Models of the atom

Why we need a model

An atom is about a ten-millionth of a millimetre across. Even the best microscopes, which can now show individual atoms as fuzzy bumps, cannot show what is inside them. So scientists build models: simplified pictures that explain the evidence and make predictions that can be tested. When an experiment gives a result the model cannot explain, the model has to change. The story of the atom is one of the clearest examples in science of that happening again and again.

From an idea to a theory: Democritus and Dalton

About 2400 years ago the Greek thinker Democritus argued that if you kept cutting something in half you would eventually reach a piece that could not be cut. He called it atomos, "uncuttable". It was a clever idea, but it was not science yet: there was no way to test it.

The evidence came from careful weighing. In the 1780s Antoine Lavoisier showed that mass is not lost or gained in a chemical reaction. Chemists then found that a compound always contains its elements in the same proportion by mass: the carbon dioxide from burning charcoal and the carbon dioxide you breathe out both contain 2.66 g of oxygen for every 1 g of carbon. In 1803 John Dalton, a teacher in Manchester, explained these patterns with an atomic theory:

• All matter is made of tiny particles called atoms, which cannot be created, destroyed or divided.
• All atoms of one element are identical, with the same mass. Atoms of different elements are different.
• Atoms join in simple whole-number ratios to form compounds, such as 2 hydrogen to 1 oxygen in water (H₂O).
• In a chemical reaction atoms are rearranged, not created or destroyed.

Dalton pictured atoms as tiny solid balls. Most of his theory still stands. Two parts were later shown to be wrong: atoms can be divided, and atoms of the same element are not all identical in mass (you'll meet isotopes in 1C).

Thomson finds the electron

In the late 1800s physicists were experimenting with cathode ray tubes: sealed glass tubes with most of the air pumped out and a metal plate at each end. With a high voltage across them, a mysterious beam travelled from the negative plate (the cathode) and made the glass glow where it hit.

In 1897 J. J. Thomson passed the beam between two charged plates. It bent towards the positive plate, so it had to be made of negatively charged particles. By measuring how far it bent, he showed the particles were more than a thousand times lighter than a hydrogen atom (we now know the figure is about 1800), the lightest atom known. They were the same whatever metal the cathode was made of. Thomson had found a particle smaller than an atom that is part of every atom: the electron.

That broke Dalton's "indivisible" atom. Since atoms are neutral overall, they must also contain positive charge to balance the electrons. In 1904 Thomson suggested the plum pudding model: a ball of spread-out positive charge with electrons dotted through it like fruit in a pudding.

Rutherford's gold foil experiment

Ernest Rutherford, a New Zealander working in Manchester, used alpha particles as tiny bullets. Alpha particles are fast, heavy and positively charged, and are given off by some radioactive elements. In 1909 his students Hans Geiger and Ernest Marsden fired them at a very thin gold foil and counted the flashes they made on a screen around it, one by one, in a darkened room.

If the plum pudding model were right, the thin spread-out charge in each atom could only nudge the alpha particles, so they should all go nearly straight through. Here is what they actually saw:

• Almost all the alpha particles went straight through. So the atom must be mostly empty space.
• A few were deflected through large angles. So there must be something with a strong positive charge that repels them.
• About 1 in 8000 bounced back. Only something that is tiny, very dense and strongly positive could turn a fast alpha particle right around.

In 1911 Rutherford proposed the nuclear model: nearly all the atom's mass and all its positive charge sit in a tiny central nucleus, with the electrons moving in the empty space around it. The nucleus is about 100 000 times smaller across than the atom. If an atom were blown up to the size of Sydney Cricket Ground, the nucleus would be a grain of sand on the centre wicket.

Bohr's shells and the quantum model

Rutherford's model had a problem. An electron circling a nucleus should steadily lose energy and spiral inwards, so atoms should collapse in a fraction of a second. They clearly don't. In 1913 the Danish physicist Niels Bohr suggested that electrons can only exist in certain fixed energy levels, or shells, at set distances from the nucleus. An electron can jump between shells but cannot sit in between. This explained why hot elements give out light of only certain colours (see 1D), and it is the model you will use to draw atoms in this course.

In 1932 James Chadwick found the third particle, the neutron, which has no charge and sits in the nucleus with the protons. Since the 1920s physicists have used the quantum model, in which electrons behave partly like waves. We can't know exactly where an electron is, only where it is likely to be, so electrons are pictured as a fuzzy cloud. The shell model is a simpler version of this that still makes good predictions for chemistry.