17-Jul-2017 10:32

In the early 1800s, John Dalton used the concept of atoms to explain why elements always react in ratios of small whole numbers (the law of multiple proportions).For instance, there are two types of tin oxide: one is 88.1% tin and 11.9% oxygen and the other is 78.7% tin and 21.3% oxygen (tin(II) oxide and tin dioxide respectively).This common pattern in chemistry suggested to Dalton that elements react in whole number multiples of discrete units—in other words, atoms.


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The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.

Atoms are very small; typical sizes are around 100 picometers (a ten-billionth of a meter, in the short scale).

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