2.1: introduction to atoms, elements, and ions

Democritus (460-370 BCE)—coined term atomus (“indivisible”)
- not backed by Aristotle, who pushed elemental (water, air, earth, fire) theory which was developed by Empedocles; politics has always been involved with science
Antoine Lavoisier—developed law of conservation of mass (1789): the total mass of substance present at the end of a chemical process is the same as the mass of substances present before the process took place
John Dalton—developed atomic theory of matter (1803): atoms are the fundamental building block of matter
- Dalton was the first modern scientist (used scientific method)
- developed four postulates:
- all matter is composed of indivisible particles, atoms (Democritus)
- chemical reactions combine or recombine atoms, they do not destroy them (Lavoisier)
- all atoms of an element are identical atoms of different elements are different
- when elements react to form compounds, they react in defined, whole-number ratios

of protons dictate which element is which

modeled using the “billiard ball” model
law of multiple proportions: if two or more compounds can be made from elements A and B, the masses of B that combine with a given mass of A are in the ratio of small whole numbers
when two or more compounds exist from the same elements, they cannot have the same relative number of atoms
- radioactivity: the spontaneous emission of high levels of energy
discovered by Marie Curie (1867-1934)
eventually used to discover that atoms are not the smallest particles (protons, neutrons, etc. exist—subatomic particles)