1. Atomic Theories: How do we know what atoms are made of?

2. Democritus (400 BC) Ancient Greek philosopher Believed that everything in the universe was made of indivisible particles called “atoms” (or “atomos” in Greek) Said atoms were constantly moving Atoms only differed in shape, position, and arrangement

3. Democritus Cont’d

4. John Dalton (1803) Visualized atoms to look like a pool ball/bowling ball All elements are made of atoms. All atoms of the same element are exactly alike and have same mass. An atom of one element cannot be changed into an atom of a different element. Atoms cannot be created nor destroyed, only rearranged. Compounds are made from atoms of different elements.

5. John Dalton Cont’d

6. J.J. Thomson (1897) Visualized the atom to look like a chocolate chip muffin or plum pudding An atom consists of negative charges scattered throughout a ball of positive charges. Proved that an atom could be divided Said the atom is neutral.

7. JJ Thompson Cont’d

8. Ernest Rutherford (1908) Rutherford was a student of Thomson. Visualized the atom to look like a peach Positive charge (protons) is located in the center of the atom. Center is called the nucleus. Almost all of the atom’s mass is located in the nucleus. Atom is mostly empty space with the electrons moving around the nucleus.

9. Ernest Rutherford Cont’d

10. Niels Bohr (1913) Student of Thomson and Rutherford Visualized the atom to look like a sliced red onion & planets in the solar system Electrons orbit around the nucleus in a definite orbit Electrons could have only specific amounts of energy, leading them to move in certain orbits

11. Niels Bohr Cont’d

12. The Wave Model (1920s) Based on principles of wave mechanics Electrons do not move in a definite path It is impossible to determine the exact location of an electron The atom has a small, positively charged nucleus, surrounded by the electron cloud

13. The Wave Model Cont’d