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Humans have been trying to determine what constitutes an atom since the time of the ancient Greeks.

Six key time points in the evolution of atomic theory. 1. Atomism by Democritus in 5th century BC. The image shows many blue circles in a group. All matter is composed of small invisible particles called atoms. 2. Solid sphere model by John Dalton in 1803. The image shows one large blue circle. Atoms are invisible, and compounds are combinations of different types of atoms. 3. Plum pudding model by J J Thompson in 1897. The image shows a blue circle with a large white plus sign inside and 7 small negative labeled circles inside the blue circle. The atom is a sphere, but the positive and negative charges are embedded within it. Nuclear model by Ernest Rutherford in 1911. The image shows a small positive circle being orbited by multiple independent orbits each with two small negative charges. Electrons orbiting in a set, predictable paths around fixed, positively charged nucleus. 5. Planetary model by Niels Bohr in 1913. The image shows a small positive circle orbited by 3 increasingly large circles. Inner circle with 2 negative charged particles, next layer with 6, and last with one. Electrons are arranged in concentric specific circular orbits around the nucleus. 6. Quantum model by Erwin Schrodinger in 1926. The image shows a small positive charge surrounded by a large white opaque circle. The locations of the electrons could only describe as being part of a cloud around the nucleus.

Figure 1The evolution of Atomic Theory

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