1. Discovering the Atom

2. Early Ideas about Matter
2,000 years ago, Greek philosophers thought all matter was made up of the 4 elements- earth, water, fire, and air; but they could not test their idea because scientific tools and methods didn’t exist
Democritus would be the first philosopher to challenge the popular idea of matter
Atoms are small solid objects that cannot be divided, created, or destroyed
Atoms are constantly moving in empty space
Different types of matter are made of different types of atoms
The properties of the atoms determine the properties of matter
Aristotle challenged Democritus
Did not believe empty space existed
Favored the idea of fire, water, earth, and air
Because of his influence, his idea was accepted and Democritus’s were not studied for another 2,000 years

3. Dalton’s Atomic Model
In the late 1700s, English schoolteacher and scientist John Dalton revisited the idea of atoms
Dalton made observations and measurements of chemical reactions, combined data with other scientists and proposed the atomic theory that supported some of what Democritus presented
All matter is made of atoms that cannot be divided, created, or destroyed
During a chemical reaction, atoms of one element cannot be converted into atoms of another element
Atoms of one element are identical to each other but different from atoms of another element
Atoms combine in specific rations to make compounds

4. The Atom
Scientists have come to the agreement that matter is made of atoms with empty space between and within them
An atom is the smallest piece of an element that still represents that element
Have to use microscopes to see atoms, but scientists still cannot see inside the atoms

5. J.J. Thomson-Discovering Electrons
English scientist that did multiple experiments with cathode ray tubes
In performing these experiments, Thomson discovered a particle that had mass, but also had a negative charge
The negatively charged particle was called the electron; Thomson also proposed that atoms contained positively charged particles as well to balance out the electrons and give the atom a neutral charge

7. Rutherford-Discovering the Nucleus
Ernest Rutherford was a student of J.J. Thomson who began doing experiments with his students to Thomson’s atomic model
Through Rutherford’s gold foil experiment, it is concluded that most of the atom’s mass and positive charge is concentrated in a small area in the center of the atom called the nucleus
The positive charge comes from the atomic particle, the proton
All the while, the electron moves around the nucleus in empty space

9. Chadwick called it the neutron.
Discovering Neutrons
Rutherford’s colleague, James Chadwick would soon discover a neutral particle located in the nucleus of the atom with the proton
Chadwick called it the neutron.

10. Bohr’s Atomic model
Niels Bohr, a student of Rutherford’s, takes the Rutherford atomic model and picks it apart.
Bohr proposes that electrons move in circular orbits, or energy levels around the nucleus; electrons in an energy level have a specific amount of energy
Electrons closer to the nucleus have less energy than electrons farther away from the nucleus
Further research would show that energy levels are not arranged in circular orbits

12. The Modern Atomic Model
Electrons form an electron cloud, an area around an atomic nucleus where an electron is most likely to be located
The electron cloud is mostly empty space