Question: how did scients discover the atom

Ciarán O'Brien answered on 14 Nov 2014:

Mostly by watching how different types of matter behaved.

People reckoned that atoms existed since Ancient Greece, although they had very different ideas of what an atom would look like. It was mostly a philosophical concept back then and they had no way of looking into the question.

Around the late 1700s, chemists were starting to find evidence that atoms did indeed exist. French chemists Antoine Lavoisier and Joseph Proust worked out that the mass of the chemicals that go into a reaction is the same as the end product and that a chemical reaction will always use elements in the similar ratios (English chemist John Dalton worked out that 1g of tin would react with either 1 or 2g of oxygen depending on whether it created tin oxide or tin dioxide). It was around this time that scientists decided that atoms would be the most logical explanation for these patterns, it made sense that matter could only be broken down into so many elements, and atoms would be the smallest pieces of those elements.

Dalton used his discoveries to work out the weight of several elements and presented his first list of atomic weights in 1803, and that’s why atomic weight is measured in Daltons today. He made a couple of mistakes because nobody knew about the nature of atoms before, but those were quickly found and fixed by other scientists, like Avogadro.

It took nearly 100 more years before scientists could work out that atoms were made of smaller particles like protons and electrons, but the discoveries made between around 1790-1803 were the ones that showed atoms were real.

It was only in the last few years that scientists were able to actually see an atom under a microscope. IBM had managed to get fuzzy pictures of individual molecules as far back as the 80’s but atoms were just so small it was too tricky to focus a microscope on one of them. Here’s an article with some pictures of the first ever picture of a single hydrogen atom, which was only taken last year! http://physicsworld.com/cws/article/news/2013/may/23/quantum-microscope-peers-into-the-hydrogen-atom