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ATOMIC THEORY
A. Particle Theory
1. Empedocles 450 B.C. states earth, air, fire, and water make up the world.
2. Democritus and Aristotle, around 420 B.C. suggested that matter could be broken down into
smaller and smaller components.
3. They coined the term “atoms”, which is Greek for “indivisible”.
4. Atoms were considered tiny particles that could not be seen.
B. Quantitative Studies
1. Law of Conservation of Mass- mass is neither created nor destroyed during chemical reactions
or physical changes. (Antoine Lavoisier 1789)
2. Law of Definite Proportions- a chemical compound contains the same elements in exactly the
same proportions by mass regardless of the size of the sample or source of the compound.
(Joseph Proust 1798)
3. Law of Multiple Proportions- if two or more different compounds are composed of the same
two elements, then the ratio of the masses is always a ratio of small whole numbers. (John
Dalton 1804)
C. Dalton’s Atomic Theory
1. In 1810, Dalton published the ideas of the atomic theory, explaining that atoms were
considered the smallest particles.
2. Information included structure and properties of atoms, properties of compounds, kinds of
compounds, and mass, energy, and volume relationships of atoms.
3. atom - the smallest unit of an element that can participate in a chemical reaction. Therefore, and
element is a substance composed of only one kind of atom.
D. Summary of Atomic Theory
1. All matter is made up of very small particles called atoms.
2. Atoms of the same element are chemically alike, atoms of different elements are chemically
different.
3. Atoms of an element have a definite average mass that is characteristic of the element.
4. Atoms of different naturally occurring elements have different average masses.
5. Atoms are not subdivided in chemical reactions.