1. Brief History of Atomic Theory

2. 1 st atomic models In 400 BC, the model looked like a solid indivisible ball In 400 BC, the model looked like a solid indivisible ball

3. Atomos Democritus’ theory: Matter could not be divided into smaller and smaller pieces forever Democritus’ theory: Matter could not be divided into smaller and smaller pieces forever He named the smallest piece of matter “atomos,” an indivisible particle He named the smallest piece of matter “atomos,” an indivisible particle

4. This theory was ignored and forgotten for more than 2000 years! This theory was ignored and forgotten for more than 2000 years!

5. Why? Aristotle & Plato Aristotle & Plato thought earth, fire, air and water were the elements. Alchemy used this model for about 2000 years.

6. Dalton’s Model In the early 1800s, the English chemist In the early 1800s, the English chemist John Dalton performed experiments that confirmed the idea of atoms. John Dalton performed experiments that confirmed the idea of atoms.

7. Dalton’s Atomic Theory All elements are made of indivisible atoms All elements are made of indivisible atoms Atoms of the same element are identical, but differ from atoms of other elements Atoms of the same element are identical, but differ from atoms of other elements Different elements’ atoms can combine to form compounds. Different elements’ atoms can combine to form compounds. Reactions occur when atoms are separated, joined, or rearranged. Reactions occur when atoms are separated, joined, or rearranged. WHICH OF THESE ARE NO LONGER CONSIDERED TO BE TRUE ?

8. Thomson’s Plum Pudding Model In 1897, J.J. Thomson discovered that atoms are made of even smaller particles. In 1897, J.J. Thomson discovered that atoms are made of even smaller particles.

9. Thomson Model Thomson used cathode ray tube Thomson used cathode ray tube The ray consisted of negatively charged “electrons”. The ray consisted of negatively charged “electrons”.

10. Thomson Model He proposed a model of the atom called the “Plum Pudding” model.* He proposed a model of the atom called the “Plum Pudding” model.* Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Atoms were made from a positively charged substance with negatively charged electrons scattered about, like raisins in a pudding. Aka: “raisin pudding model” Aka: “raisin pudding model”

11. Millikan (1909) OIL DROP experiment OIL DROP experiment Measures charge on electron Measures charge on electron + -

12. Rutherford’s gold foil experiment (1911) Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Rutherford’s gold foil experiment (1911) Involved firing a stream of tiny positively charged particles at a thin sheet of gold foil (2000 atoms thick) Result: most of space occupied by atom is empty space! Except for small, dense, positively charged NUCLEUS. Result: most of space occupied by atom is empty space! Except for small, dense, positively charged NUCLEUS. Positive particles named PROTONS Positive particles named PROTONS all protons are in the nucleus. all protons are in the nucleus.

14. Rutherford Nuclear Model all of an atom’s positively charged particles are contained in the nucleus. The negatively charged particles were scattered outside the nucleus. all of an atom’s positively charged particles are contained in the nucleus. The negatively charged particles were scattered outside the nucleus.

15. Bohr Model In 1913, the Danish scientist Niels Bohr proposed that each electron is in a specific energy level. In 1913, the Danish scientist Niels Bohr proposed that each electron is in a specific energy level.

16. Bohr Model According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun.

17. Wave Model Louis DeBroglie (1924) e’s have wavelike properties

18. Wave Model Erwin Schrodinger (1927) Quantum mechanics model e’s location probability

19. Chadwick (1932) Discovers particle with same mass as proton but no charge …the Discovers particle with same mass as proton but no charge …the NEUTRON NEUTRON

21. IndivisibleElectronNucleusOrbit Electron Cloud Greek X Dalton X Thomson X Rutherford X X Bohr X X X Wave X X X