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Edexcel GCSE Chemistry Development of the atom

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).

The Dalton model

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.

The plum pudding model

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.

Particle scattering experiment

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.

The nuclear model

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When was the dalton model suggested?

Edexcel GCSE Chemistry Development of the atom When was the dalton model suggested?
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2

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Did Dalton say that atoms can rearrange to make different substances? If not, explain how his statement was different.

Edexcel GCSE Chemistry Development of the atom Did Dalton say that atoms can rearrange to make different substances? If not, explain how his statement was different.
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3

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Draw the plum pudding model of the atom.

Edexcel GCSE Chemistry Development of the atom Draw the plum pudding model of the atom.
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4

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Why upon the discovery of electrons they were defined as subatomic particles rather than just another element?

Edexcel GCSE Chemistry Development of the atom Why upon the discovery of electrons they were defined as subatomic particles rather than just another element?
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5

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How much of the plum pudding model of the atom is empty space?

Edexcel GCSE Chemistry Development of the atom How much of the plum pudding model of the atom is empty space?
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6

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What unexpected was observed during the particle scattering experiment?

Edexcel GCSE Chemistry Development of the atom What unexpected was observed during the particle scattering experiment?
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7

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Why was the vacuum important in the particle scattering experiment?

Edexcel GCSE Chemistry Development of the atom Why was the vacuum important in the particle scattering experiment?
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8

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A snapshot of an Ξ± particle passing through an atom during a particle scattering experiment has been taken and is shown in the image. Would the results registered by the detector from this atom agree with the plum pudding model of the atom? Why?

Edexcel GCSE Chemistry Development of the atom A snapshot of an Ξ± particle passing through an atom during a particle scattering experiment has been taken and is shown in the image. Would the results registered by the detector from this atom agree with the plum pudding model of the atom? Why?
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9

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Which scientist provided evidence to support the nuclear model, specifically the composition of the nucleus?

Edexcel GCSE Chemistry Development of the atom Which scientist provided evidence to support the nuclear model, specifically the composition of the nucleus?
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10

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Where is the positive charge in an atom?

Edexcel GCSE Chemistry Development of the atom Where is the positive charge in an atom?
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