SQA Higher Chemistry Development of the atom
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In 1803 John Dalton published his ideas on the atom, describing how all matter is made up of small particles called atoms. His theory has disproved previously used misconception that some elements are made up of different atoms, such as gold being made using other metals. The main three ideas of his theory are:
1. All matter is made of tiny particles called atoms which cannot be created, destroyed or divided.
2. All atoms of one element are identical, all atoms of different elements are different.
3. Different atoms come together to form different substances (compounds).
In 1897 Joseph John Thomson discovered smaller than atoms negatively charged particles that were produced by a cathode. Since they had a significantly larger charge to mass ratio, that meant they were first discovered subatomic particles, electrons. Subsequently a model of the atom was proposed where it looked like a plum pudding. In this model the atom is a positive 'dough' with negative electrons stuck in it like plums.
In 1905 Ernest Rutherford tested the plum pudding experiment by firing a directed beam of alpha particles, that have a charge of +2, at a thin gold leaf suspended in a vacuum. Gold was used in this experiment since it is a metal that can be rolled very thinly without cracking. Vacuum was important to ensure any deflection of particles was from the atoms in the gold leaf, not air, for example.
If the plum pudding model was correct, all alpha particles would pass through the gold undeflected. It was observed that most particles passed through, however a small proportion were deflected by angles, and an even smaller proportion were relfected straight back off the foil.
Rutherford concluded the atom must be mostly empty space, as most alpha particles passed straight through. The small number of particles that deflected at angles suggests there is a concentration of positive charge, as the positive alpha particles were repeled by like charges. As only a very small amount of alpha reflected straight back suggests the presence of the nucleus, a tiny volume of positive charge in the atom. This experiment led to the proposal of the nuclear model.
Ernest Rutherford used the results from the particle scattering experiment to propose the nuclear model describing a central positively charged nucleus with orbiting electrons. Niels Bohr described that electrons were occupying energy shells in this way surrounding the nucleus. In 1932 James Chadwick discovered the neutron which suggested that the nucleus consists of protons and neutrons. These discoveries led to the theory of the nuclear model, which is the model of the atom we know today, nucleus containing protons and neutrons surrounded by electrons in energy shells.
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