GCSE - Models of the Atom

In Ancient Greece, it was first believe that everything was composed of four 'elements'; earth, wind, fire and water. However, a rare few a were prepared to think differently. Enter the Greek philosopher Demokritos, who believed that everything was in fact made up from small pieces indivisible matter called atoms, taking their name from the Greek word 'atomos', which means 'uncuttable'.

For a long time, nobody was able to provide any reasonable suggestion of what atoms may be made from, until the English Physicist J J Thompson. In 1897 he proposed the plum pudding model of the atom, which was sensible for its time.

The plum pudding model appeared plausible as a completely solid atom would explain why they couldn't be squashed. It also included positive and negative parts, which could cancel each other out, making an atom neutral overall, as they were known to be.

However, in 1905 Ernest Rutherford performed an experiment that would force scientists to think differently about atoms. In his experiment, he fired alpha particles (positively charged) at a thin leaf of gold foil. According to the plum pudding model, the alpha particles should have passed straight through the atoms with little or no deflection, but that was not the case.

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While most of the alpha particles did pass through, some were deflected by more than 4°, indicating a concentration of positive charge in an atom, later named the nucleus. This experiment was repeated over 100,000 times, with the same outcome each time and the plum pudding model simply did not account for the experimental results. For this reason, Rutherford proposed the nuclear model of the atom. 

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The nuclear model was later revised by Niels Bohr. who gave us the Bohr model of the atom that we still use today. 

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And so concludes our history of atomic structure. The key points to remember in an exam situation are:

• The plum pudding model is made of randomly arranged negative electrons embedded inside a positive pudding. 
• The gold leaf provided data that the plum pudding model could not explain which prompted the development of an alternative model.
• The data from the gold leaf experiment was thought to be reliable as over 100,000 readings were taken.