John Dalton's Atomic Theory

You may take it for granted that matter is made up of atoms, but what we consider common knowledge was unknown until relatively recently in human history. Most science historians credit John Dalton, a British physicist, chemist, and meteorologist, with the development of modern atomic theory.

Early Theories 

While the ancient Greeks believed atoms made matter, they disagreed on what atoms were. Democritus recorded that Leucippus believed atoms to be small, indestructible bodies that could combine to change properties of matter. Aristotle believed elements each had their own special "essence," but he did not think the properties extended down to tiny, invisible particles. No one really questioned Aristotle's theory, since tools did not exist to examine matter in detail.

Along Comes Dalton

So, it wasn't until the 19th century that scientists conducted experiments on the nature of matter. Dalton's experiments focused on gases -- their properties, what happened when they were combined, and the similarities and differences between different types of gases. What he learned led him to propose several laws, which are known collectively as Dalton's Atomic Theory or Dalton's Laws:

• Atoms are small, chemically indestructible particles of matter. Elements consist of atoms.
• Atoms of an element share common properties.
• Atoms of different elements have different properties and different atomic weights.
• Atoms that interact with each other obey the Law of Conservation of Mass. Essentially, this law states the number and kinds of atoms that react are equal to the number and kinds of atoms in the products of a chemical reaction.
• Atoms that combine with each other obey the Law of Multiple Proportions. In other words, when elements combine, the ratio in which the atoms combine can be expressed as a ratio of whole numbers.

Dalton is also known for proposing gas laws (Dalton's Law of Partial Pressures) and explaining color blindness. Not all of his scientific experiments could be called successful. For example, some believe the stroke he suffered might have resulted from research using himself as a subject, in which he poked himself in the ear with a sharp stick to “investigate the humours that move inside of my cranium.”

Sources

• Grossman, M. I. (2014). "John Dalton and the London atomists: William and Bryan Higgins, William Austin, and new Daltonian doubts about the origin of the atomic theory." Notes and Records. 68 (4): 339–356. doi:10.1098/rsnr.2014.0025
• Levere, Trevor (2001). Transforming Matter: A History of Chemistry from Alchemy to the Buckyball. Baltimore, Maryland: The Johns Hopkins University Press. pp. 84–86. ISBN 978-0-8018-6610-4.
• Rocke, Alan J. (2005). "In Search of El Dorado: John Dalton and the Origins of the Atomic Theory." Social Research. 72 (1): 125–158. JSTOR 40972005