Early ideas about atoms
Ideas about atoms have changed over time. Scientists developed new atomic An equation, diagram or analogy that helps explain a scientific idea. as they gathered new experimental evidence.
John Dalton published his ideas about atoms in 1803. He thought that all matter was made of tiny particles called The smallest part of an element that can exist., which he imagined as tiny spheres that could not be divided.
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Nearly 100 years later, J J Thomson carried out experiments and discovered the Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. This led him to suggest the An early model of the atom in which an atom is a ball of positive charge with negative electrons embedded in it. of the atom. In this model, the atom is a ball of positive charge with negative electrons embedded in it - like currants in a Christmas pudding.
In 1909 Ernest Rutherford designed an experiment to test the plum pudding model. In the experiment, positively charged Subatomic particle comprising two protons and two neutrons (the same as a helium nucleus). were fired at thin gold foil. Most alpha particles went straight through the foil. But a few were scattered in different directions.
This evidence led Rutherford to suggest a new model for the atom, called the The scientific idea that an atom has electrons surrounding a nucleus that contains protons and neutrons.. In the nuclear model:
- the mass of an atom is concentrated at its centre, the The central part of an atom. It contains protons and neutrons, and has most of the mass of the atom. The plural of nucleus is nuclei.
- the nucleus is positively charged