2.  You cannot see them, yet they make up everything…

3. THE ATOM  An atom is the smallest particle of an element that retains its identity in a chemical reaction.

4. Theories Over Time  The modern model of the atom has developed slowly over time as many philosophers and scientists contributed to the theory.

5. Early Models of the Atom  Let’s learn about the dead guys!

6. Democritus  Democritus was a Greek philosopher who lived from 460 – 370 B.C.  Democritus was the first to suggest the existence of atoms  Democritus believed that there where particles called “atomos”, which he believed made up everything around us.  “Atomos” in Greek means indivisible

7. Anton Lavoisier (late 1700s)  Developed the Law of Conservation of Mass  Matter cannot be created nor destroyed

8. Joseph Proust (late 1700s)  Developed the Law of Constant Composition  The particles of a pure substance are identical to each other.

9. Dalton’s Atomic Theory (1803)  John’s Dalton was a school teacher.  Although many philosophers and scientists before him developed ideas of the atom, John Dalton was the first to develop an Atomic Theory because Dalton was the first to use experimental methods.

10. Dalton’s Atomic Theory  All elements are composed of indivisible particles called atoms  Atoms of the same element are identical  Atoms of different elements can chemically combine in simple whole-number ratios to form compounds  Chemical reactions occur when atoms rearrange. Atoms of one element are never changed into atoms of another element.

11. Dalton’s Atomic Theory  Dalton’s atomic theory can be linked back to the ideas of Democritus, Lavoisier, and Proust.

12. Subatomic Particles  Much of Dalton’s theory remains true, however we do know that atoms are divisible.  Subatomic particles refers to the particles smaller than the atom.  Protons  Neutrons  Electrons

13. Electricity and the Atom  Ben Franklin (1700s)  First to determine that there were positive and negative charges.  Michael Faraday (1839)  Suggested that the structure of atoms was related to electricity.

14. Electrons  Electrons carry a negative charge  JJ Thompson was the first to discover the electron using a cathode ray tube.  JJ Thompson’s atomic model was called the plum pudding model

15. Cathode Ray Tube

16. Discovery of the Nucleus  Rutherford’s Gold Foil Experiment  Rutherford shot a beam of alpha particles (positively charged particles) at a very thin sheet of gold foil.  Rutherford expected the alpha particles to pass through the gold foil with a slight deflection from the positive charges spread throughout the gold foil  Most of the particles went through the gold foil but Rutherford was surprised to find a small amount deflecting at large angles.

17. Gold Foil Experiment

18. Rutherford’s Atomic Model  Rutherford determined that…  The atom is mostly empty space  The positive charge and almost all the mass are concentrated in a small region called the nucleus. http://www.youtube.com/watch?v=5pZj0u_XMbc&feature=related

19. The Nucleus  The nucleus is composed of protons and neutrons  Protons have a positive charge  Neutrons have no charge (neutral)  Therefore… the nucleus has a positive charge overall  Almost all of an atom’s mass is located within the nucleus

20. Properties of Subatomic Particles

21.  What makes atoms different if they are all made up of protons, electrons, and neutrons?  The number of protons, electrons and neutrons  Most importantly, the number of protons gives the element it’s identity

22. Atomic Number  Elements are different because they contain a different number of protons  The atomic number = number of protons

23. Mass Number The mass number = number of protons and neutrons  Mass of electrons is negligible

24. Calculating number of Protons, Neutrons, and Electrons  Atomic number and Mass number can be found on the periodic table  Number of Protons = Atomic Number  Number of electrons = Number of Protons (in a neutral atom)  Number of Neutrons = Mass # - Atomic #

25. Practice Problems  Carbon  Aluminum  Lithium  Hydrogen  Silicon p = 6 n = 6 e = 6 p = 13 n = 14 e = 13 P = 3 n = 4 e = 3 P = 1 n = 0 e = 1 P = 14 n = 14 e = 14

26. Isotopes  An isotope is an atom with the same number of protons, but a different number of neutrons  Therefore…. Isotopes have different mass numbers

27. Example

28. Atomic Mass  The atomic mass of an element is the weighted average of the elements naturally occurring isotopes.  This is why the atomic masses on your periodic table are not whole numbers