Developing models of atoms
Ideas about atoms have changed over time. Scientists develop new atomic An equation, diagram or analogy that helps explain a scientific idea. as they gather new experimental evidence.
John Dalton published his ideas about atoms in 1804. 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. Dalton used his atomic model to explain the properties of gases and the formulae of compounds.
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In 1897, J J Thomson carried out experiments and discovered the Subatomic particle, with a negative charge and a negligible mass relative to protons and neutrons.. The mass of an electron was much smaller than the mass of an atom, so electrons must be subatomic particles. 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.
In 1911 Ernest Rutherford used results from 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
Further experiments led to the idea that the nucleus contained small particles, called Subatomic particle with a positive charge and a relative mass of 1. The relative charge of a proton is +1.. Each proton has a small amount of positive charge.
In 1932 James Chadwick found evidence for the existence of particles in the nucleus with mass but no charge. These particles are called Uncharged subatomic particle, with a mass of 1 relative to a proton. The relative charge of a neutron is 0.. This led to another development of the atomic model, which is still used today.