Chemistry – Atomic structure (C1.2)
Changing ideas about atoms Learning objectives: • describe how the atomic model has changed over time • explain why the atomic model has changed over time • understand that a theory is provisional until the next piece of evidence is available.
KEY WORDS electron shell Ernest Rutherford Geiger and Marsden experiment J. J. Thompson James Chadwick John Dalton Niels Bohr
The idea of atoms has changed hugely over the years. At the moment, scientists believe atoms are very small, have a very small mass and are made of protons, electrons and neutrons. Our current theories were developed by imagination, evidence and advances in technology, with each new idea being built on the ideas of earlier scientists.
Developing the atomic theory Explanations about atoms began about 400 BC, when the Greek philosopher Democritus described materials as being made of small particles. He called these particles ‘atoms’. However, he had no evidence. It was just an idea. Little more was suggested for more than 2000 years, but in 1803 the British scientist John Dalton used his observations to describe the atom in more detail. His model described an atom as a ‘billiard ball’.
Figure 1.5 Dalton’s idea of atoms: they were like tiny billiard balls
Dalton’s model was then changed as new evidence was found. In 1897, 94 years later, J. J. Thomson discovered the electron. Thomson developed the way that the atom was thought of by using a ‘plum pudding’ model to describe atoms. Negative electrons were thought to be embedded in a ball of positive charge, rather like the fruit (the electrons) are part of a pudding (the ball of positive charge).
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1
Suggest why Dalton’s atomic model did not include positive and negative charge.
2
Explain why the discovery of the electron changed the Dalton model of the atom.
A current simple model of an atom with protons and neutrons in the nucleus surrounded by electrons in shells around it. This model was developed over time.
KEY INFORMATION At each stage, the explanations of atomic theory were provisional until more convincing evidence was found to make the model better.
OCR Gateway GCSE Chemistry: Student Book
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