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A Quantum Theory for Atomic Structure

Early Models

The 'Plum Pudding' model of the atom. Negatively charged electrons (the plums) are embedded in a sphere of uniform positive charge (the pudding).

The first model of the atom was developed by J.J. Thomson and Lord Kelvin in 1904. They called it the 'plum pudding' model because the negative electrons (the plums) were embedded in a sphere of uniform positive charge (the pudding). This model was soon replaced. In 1911, Ernest Rutherford suggested that his students Geiger and Marsden fire alpha particles (produced by some naturally radioactive materials) at a thin gold foil. They observed that some of the alpha particles were scattered through large angles, indicating that the positive charge in the atom was concentrated in a small volume – the nucleus. Rutherford subsequently proposed a model of atom based on a central nucleus of positive charge circled by negative electrons – like planets orbiting the sun. Rutherford was awarded the Nobel Prize in Chemistry in 1908 for his work on understanding radioactive decay.