The Discovery of the Atom
Democratus and Aristotle (400 BC)
-Democratus hypothesized that matter was indivisible and presented the term ‘atmos’ meaning indivisible
-Aristotle suggested that matter was made from either Earth, water, wind or fire
John Dalton (1803)
- said that all matter is composed of a fundamental particle called atoms
-atoms can not be divided, created nor destroyed
-all atoms of the same element are identical in size and mass, but are different from the atoms of other elements
-compounds are made when atoms combine in whole number ratios ( the Law of Definite Proportions)
-Model : ‘The Billiard Ball’
J.J. Thomson (1897)
-all atoms contain both protons and electrons
-all protons are identical. All electrons are identical. Electrons are different from protons.
-An electron has a negative charge. A proton has a positive charge.
-Electrons and protons have the same amount of charge, however they are opposite.
-A proton has much more mass than an electron.
-Model : ‘ The Plum Pudding’ or ‘Raisin Bun’
Note : Thomson based his ideas on experiments with cathode ray tubes upon the research conducted by Michael Faraday. Faraday was the first to suggest matter was electrical in nature.
Ernest Rutherford (1911)
-by firing alpha particles at a very thin gold foil, Rutherford was able to see that the majority of particles sailed through the object. Several particles however were deflected on an angle or returned towards the source. This meant that there was something heavy and dense in the core of the atom. He termed this the nucleus.
-He was unable however to determine the correct mass of the nucleus
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Sir James Chadwick (1932)
-Chadwick was able to explain the atomic mass of the elements by incorporating a subatomic particle with no charge, but the mass of a proton
-He termed his finding the neutron
Niels Bohr (1915)
-Bohr had studied the character and properties of the electron and its association with speed and energy
-He suggested that the electron must exist in a shell with a certain distance and speed from the nucleus so that it would not spiral towards the centre
-this similarity to a solar system coined the term ‘planetary model’ for his interpretation of the atom
-Bohr based his ideas on experiments that looked at the line spectra of hydrogen. He could explain the move from lower levels to higher energy levels by looking at the visible lines of light that were emitted. This supported the idea that there were orbits that were filled with electrons. His ideas however lacked explanation beyond hydrogen, so they have been modified.
The quantum mechanics model – our current model of the atom
The idea that there are only discrete energy levels remains. The idea that electrons will fill in successive shells is also kept, however there is no way to tell exactly where the electrons are. Heisenberg’s (1927) uncertainty principle states that ‘it is not possible to know the position and motion of an electron at any time’. This has created the idea that there is a series of overlapping electron clouds that have a high probability of containing electrons.
The quantum mechanical model also proposes that there is a ‘principal quantum number’. This is based on the previous ideas of shells and increases from the inside outward. This number is called ‘n’.
The maximum number of electrons that can fit into each quantum level is equal to 2n2 . In a chart it appears as :
n / Maximum number of electrons per shell1 / 2 (1)2 = 2
2 / 2 (2)2 = 8
3 / 2 (3)2 =18
4 / 2 (4)2 =32