Historical Development Of The Theory

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John Dalton

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Revived and greatly expanded the theory originally proposed by Democritus

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Dalton's Atomic Theory Key Points

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- All matter is composed by extremely small particles called atoms that were held together by forces of attraction
- Atoms are indivisible=> cany be created or destroyed
- Atoms of the same element=> identical, same mass
- Atoms of different elements=> different mass
- Chemical reactions occur when atoms are, joined, separated or rearranged=>atoms of an element cannot be changed into another
- Atoms of different elements could be combined in simple numerical ratios eg. 1:1

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Limitations to Dalton's Model

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- he could not account for new observations that chemical reactions could be used to create electric currents
- Other experiments were indicating that atoms were divisible into negative and positive parts

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J.J. Thompson

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Discovered that electrons made up cathode rays that could be deflected by magnets or electronically charged plates
- Discovered electrons, atoms were overall neutral
- Proposed that an atoms was a sphere of positive charges throughout which electrons were loosely embedded=> this is called the "Plum Pudding model"

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Ernest Rutherford

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Performed a classic experiment to test Thompson's Plum Pudding model
- Bombarded a thin gold foil with positively charged alpha particles
- He discovered that some of the particles bounced back => indicating repulsion by a highly positive charge
- Discovered that atoms were mostly empty space => they contained nuclei (nucleus)=> containing the mass and positive charge, surrounded by negatively charged electrons

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Limitations to Rutherford's model

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It could not explain why electrons did not emit energy and spiral into the nucleus

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Niels Bohr

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Developed a new atomic structure based on experiments involving spectra produced by hydrogen
- Bohr hypothesized that if atoms emit only discrete wavelengths => must have discrete energies
- Proposed that electrons moved around a nucleus in certain fixed paths called orbits => each orbit corresponded to a different energy level
- An electron could change its energy from going to an energy level to another but cannot remain between energy levels
- Movement from one energy level to another=> QUANTUM JUMP=> an electron could move from a lower energy level to a higher one by absorbing energy eg. from a flame
- When an electron moves back to its ground state it releases energy as a consequence, each element gives of different colours

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Erwin Schrodinger

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- Proposed that electrons should be regarded as wavelike properties=> electrons not confined in a definite path
- Electrons are found in a region of space around the nucleus called an orbital
- Visualized as a cloud of negative charge

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James Chadwick

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- Bombarded a sample of beryllium with alpha particles=> found a ray was given off by the beryllium
- The particles that made up the ray were not deflected by magnetic field, or electrical charge=> indicating they held no charge
- Suggested that the alpha particles displaced uncharged particles called neutrons from the nuclei of beryllium atoms