What did John Dalton discover about the atom?

John Dalton discovered that elements are only able to combine atomically when they are in fixed ratios. This discovery led to the basis for Dalton's Atomic Theory.

Dalton's Atomic Theory consists of five different laws:

1. The first law states that all elements on Earth are composed of small particles called atoms.
2. The second part of Dalton's Atomic Theory states that all atoms of an element are the same as that element. For example, gold is considered gold even when it is at the smallest atomic level.
3. The third law dictates the atomic weights of atoms, and it is directly related to how atoms are distinguished by the different atomic weight they carry.
4. In the fourth law, Dalton expresses how elements are able to combine. Atoms from elements are able to connect with atoms from other elements; chemical compounds are formed when this occurs.
5. The fifth law states that atoms cannot be created or destroyed in a chemical compound.

Some changes have been made to Dalton's laws over the years as new discoveries have been made, however overall the basis of his theory still stands. Examples of modifications include that the fifth law only applies to chemical compounds, due to the fact that nuclear fission and nuclear fusion can be used for other atomic reactions. Additionally, the first law is somewhat less accurate than it was in Dalton's time, as it has been found that atoms can be made into smaller particles.