Democritus was the Greek philosopher (in 4th century BC) that first discussed the idea of matter being made of tiny things called atoms.
John Dalton changed how scientists though about matter based on observations of how chemicals behave (chemical properties). He looked at the ideas created by Joseph Priestly and Antoine Lavosier and,in 1803, created the beginning of modern day atomic theories ...
His theory was based on 5 assumptions:
- All matter consists of tiny particles called atoms. Dalton and others imagined the atoms that composed all matter as tiny, solid spheres in various stages of motion.
- Atoms are indestructible and unchangeable. Atoms of an element cannot be created, destroyed, divided into smaller pieces, or transformed into atoms of another element. Dalton based this hypothesis on the law of conservation of mass as stated by Antoine Lavoisier and others around 1785. (This has been since refuted, what have we learned about what an atom is made of?)
- Elements are characterized by the weight of their atoms. Dalton suggested that all atoms of the same element are identical. (This was later refuted, what have we learned about the mass of atoms?)
- In chemical reactions, atoms combine in small, whole-number ratios. Experiments that Dalton and others performed indicated that chemical reactions proceed according to atom to atom ratios which were precise and well-defined.
- For example, we know that every water molecule has a specific number of hydrogen atoms and a specific number of oxygen atoms.
- When elements react, their atoms may combine in more than one whole-number ratio. Dalton used this assumption to explain why the ratios of two elements in various compounds, such as oxygen and nitrogen in nitrogen oxides, differed by multiples of each other.
Dalton's theory was accepted because it explained:
- the laws of conservation of mass,
- definite proportions,
- multiple proportions,
- and other observations.