Earth, Air, Wind and Fire

Until fairly recently most people thought that everything was made of Earth, Air, Wind and Fire. But due to the hard work of some dilligent scientists we have discovered atoms. So let's take a quick look at this gang of heroes!

By the end of this page you should be able to:
  • To be able to name the scientists whose work lead to the discovery of the atom
  • To be able to explain how the discovery of the electron lead to the plum pudding model
  • To be able to explain how the work of Rutherford and Marsden lead to the nuclear model

John Dalton - Discovered the Atom

John Dalton was one of those all-rounders, he made contributions to many areas of science. His biggest achievement was the development of atomic theory, he came up with five "key ideas":

  • Elements are made of extremely small particles called atoms.
  • Atoms of a given element are identical in size, mass, and other properties; atoms of different elements differ in size, mass, and other properties.
  • Atoms cannot be subdivided, created, or destroyed.
  • Atoms of different elements combine in simple whole-number ratios to form chemical compounds.
  • In chemical reactions, atoms are combined, separated, or rearranged.

That's a lot of stuff..... the key thing to remember is that he proved the existence of atoms. While he was doing this he also discovered atomic mass, developed the shorthand symbols we use for formulae, wrote the first chemical formulae and helped to flesh out the water cycle. Busy guy.

John Dalton

JJ Thompson - Discovered The Electron and Liked Pudding

Plum Pudding Model

In 1897 J.J. Thomson announced an amazing discovery - atoms were made of smaller particles. Through his experiments he had discovered electrons, he determined that they i) have a negative charge and ii) they're far smaller than an atom .

He came up with a model/theory to explain his observation. Atoms consists of a cloud of positive charge with negatively charged electrons embedded. This idea is called the Plum Pudding model.

Rutherford/Geiger/Marsden - Discovered the Nucleus - Not Fans of Plums

These three chaps were pretty much playing around in the lab from 1909-1911 when they broke Thomson's Plum Pudding model by discovering that the positive charge in an atom was concentrated in the atom's centre. They named this ball of positive charge the nucleus.

They then refined Thomson's model to create the nuclear model of the atom - this is shown in the picture to the right. This shows a key principle in science - a model is developed that fits the available data, when new data is discovered that contradicts the current model it is adapted or discarded for a better one. Rutherford took all the credit even though his students did the work......

Rutherford/Nuclear Model

Bohr - Not as Bohring as he Sounds

Bohr model of an atom

Bohr was epic. He's the Thor of science. He took Rutherford's model, in 1913, and adapted it to explain flame emission spectra (there's a page on this in the Trilogy section if you want to know what it is). He determined that electrons move around the nucleus at fixed distances.

This model is shown in the diagram to the left - compare it to the diagram above. Notice how the electrons are now held at fixed distances from the nucleus, we've met this idea before - they're called electron shells.

Atoms seemed a bit too heavy for their own good....

Lots of scientists tried to make mathematical models to explain the behaviour of the nucleus (this is an A-level physics concept, it's to do with something called spin and how mass affects it) and no-one could come up with something that worked. Atoms seemed to be about twice as heavy as they should be and many scientists were starting to think that there may be another particle involved. Proof finally came when James Chadwick managed to prove neutrons existed and determine their mass in the early 1930s. It also soon became clear that they had been difficult to discover as they're neutral, so they don't interact with protons and electrons.

Scientific Theories

Each time a particle was discovered our model of the atom was refined to best fit the available experimental evidence. Best not to get too attached to an idea in science - a new one will be along soon!

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