1. A Brief History of Atomic Theory This Section will focus on Scientists who have had an impact on the study of the atom.

2. Democritus Democritus proposed that matter cannot be broken down indefinitely. At some point you end up with a piece that can’t be divided. atom The smallest piece of matter he called an atom, from the Greek word atomos, which means “indivisible”. 470-380 B.C.

3. Democritus’ Model “Atomos”

4. John Dalton John Dalton was a British chemist. He was the first modern scientist to propose the existence of atoms. He described an atom as an invisible indestructible, solid sphere, like a billiard ball. 1766 - 1844

5. Dalton’s Model The “Indivisible Sphere”

6. Sir Joseph “J.J.” Thomson J.J. Thomson was a British physicist who was the first scientist to propose the plum pudding model. He was also the first to propose the theory of the negatively charged electron. 1856 - 1940

7. Sir Joseph “J.J.” Thomson 1856 - 1940

8. Thomson “plum pudding” model + -

9. Ernest Rutherford Ernest Rutherford experiments proved that atoms are mostly empty space. Discovered the nucleus, which contains positively charged particles. Was the first to suggest that electrons circle the dense nucleus. 1871- 1937

10. Ernest Rutherford 1871- 1937

11. Rutherford’s Model Electron Negative charge Nucleus It has a + charge

12. Niels Bohr Niels Bohr stated that electrons move in different orbits, or energy levels, around the nucleus like planets orbit the sun. Each energy level is located a specific distance from the nucleus and contains a certain number of electrons. 1885 - 1962

13. Bohr Model Energy levels

14. Current Model This model is based upon Bohr’s model, except that electrons orbit the nucleus in random patterns. The region where these particles are found is referred to as the electron cloud.

15. Determining PEN Numbers

16. Demitri Mendeleev Demitri Mendeleev was a Russian scientist. He was the first person to make the periodic table. The periodic table is a tool to help organize the elements.

17. Mendeleev’s Table

18. Modern Periodic Table

19. 6 C Carbon 12.01115 How to Use the Table Atomic Mass = P + E + N Number of Protons & Electrons Atomic Number Chemical Symbol Element Name

20. How to find number of Protons & Electrons 6 C CARBON 12.01115 The number of Protons in atom is also the Atomic number, so therefore the Atomic number also represents the amount of Protons in the nucleus of that Atom. In a neutral Atom Protons = Electrons Atomic Number/ number of Protons/Electrons

21. How to Find the Amount of Neutrons in an Atom To find the amount of neutrons in an atom you have to subtract the atomic mass from the atomic number. 16 S sulfur 32.064 Atomic Mass 32 Atomic Number 16 Number of Neutrons

22. Calculating P, E, N 6 C Carbon 12.01115 P = E = N =

23. Calculating P, E, N 6 C Carbon 12.01115 P = 6 E = 6 N = 6

24. Calculating P, E, N 20 Ca Calcium 40.08 P = E = N =

25. Calculating P, E, N 20 Ca Calcium 40.08 P = 20 E = 20 N = 20

26. Calculating P, E, N 26 Fe Iron 55.847 P = E = N =

27. Calculating P, E, N 26 Fe Iron 55.847 P = 26 E = 26 N = 30

28. Complete Worksheet