1. Development of our understanding of the atom

2. Early Models Democritus was first to suggest atom in 4 th century B.C. Believed matter was made of tiny individual particles called atomos meaning indivisible. Believed that atoms could not be created, destroyed, or further divided. Aristotle was one of the most respected Greek philosophers. Disagreed with Democritus because it was against his views. Matter is made of earth, fire, air and water. Did not believe that that atoms moved through empty space.

3. John Dalton Dalton’s model was the “Billiard Ball” Atomic Theory Matter is composed of atoms Atoms are indivisible Atoms of a given element have same size, mass and chem props and are different from those of another element Different atoms combine in whole number ratios to form compounds and are separated, combined and rearranged in chem reactions Conservation of Mass-is the result of the separation, combination, or rearrangement of atoms-atoms are not created, destroyed, or divided in the process.

4. Dalton’s Theory had to be revised because he … Dalton was incorrect about atoms being indivisible. Atoms can be divisible into several subatomic particles. Dalton was also incorrect about all atoms of a given element having identical properties. Atoms of the same element can have slightly different masses.

5. Early Models Thomson used the Cathode Ray – determined atoms were made of particles Negative charges in atom “Plum Pudding” model Millikan used oil drop experiment Found mass and charge of an electron

6. Important discoveries Rutherford experimented with Gold Foil Found nucleus Discovered its positive charge and tiny size Electrons moving around the nucleus Came up with “Nuclear Model”

7. Moving Forward… Bohr said electrons move in orbits Found in energy levels Explains bright-line spectrum Called “Planetary Model” But Bohr knew that it was flawed

8. What we believe now Orbital proposed by Schrödinger Describes most likely where electrons were Quantum mechanics new theory “Electron Cloud” model