1. The Development of Atomic Theory

2. Early Philosophers Enter...the ATOM
Heraclitus: maintained that everything is in a state of flux. (everything changes)
Parminedes: everything is what it is (only permanent things are considered “real”)
Enter...the ATOM
Democritus: around the 5th Century B.C. proposed that matter is composed of tiny, indestructable units called “atoms.”
Atoms themselves do not change
Move around and combine with other atoms
Properties of an atom are determined by the shape of the atom

3. Popular Does Not Equal Correct!
Because Aristotle completely dismissed the idea of atoms, many philosophers and scientists went along with his idea.
The study of atoms was reintroduced in Europe during the Renaissance
If philosophers like Democritus had not recorded their ideas in some understandable matter, the idea of atoms may not have been “rediscovered”
Did You Know?
The word atom comes from the Greek word “atomos” meaning invisible.

4. John Dalton Early 1800’s Each element has it’s own type of atom
The masses of different types of atoms are different
Started a system of chemical symbols
Arranged them on a chart
Theorized that elements combine in definite ratios according to their weight
Compounds are made of molecules
Atoms determine the composition of matter

5. J.J. Thomson's Theory
ELECTRONS are negatively charged particles that make up an atom
Since atoms are neutral and negative charges exist, there must be some positive charge in an atom as well
1904 – Thomson proposed a structure of an atom that consisted of a positive sphere with negative charges distributed throughout the charge
This model became known as the “plum-pudding model”
Chocolate chip cookie dough anyone?!

6. Ernest Rutherford Niels Bohr
In 1911 he described a small, heavy nucleus with electrons in orbit around the nucleus
1919 – discovered a proton and determined that it exists in the nucleus of an atom
Protons are positively charged
Niels Bohr
1913 – determined that electrons move in orbital shells
The electrons in the outer orbital shells determine the properties and interactions of the atom

7. Modern Atomic Theory
As a result of the work of the scientists listed here, as well as hundreds of others, we now use the “Electron Cloud” model of the atom.
An atom consists of:
A nucleus
Protons and neutrons
Positively charged
An electron cloud
Negatively charged