1. Modern Physics The Nucleus

2. Modern Physics Three main particles in an atom

3. Modern Physics Three main particles in an atom Protons Neutrons Electrons

4. Modern Physics Three main particles in an atom Mass Protons1.007276 amu Neutrons1.008665 amu Electrons.001097 amu

5. Modern Physics Lets look at a Helium atom Helium 4, symbolized as 4 2 He 2 – protons 2 – neutrons 2 – electrons The “4” is called the “mass number” and is the sum of protons and neutrons The “2” is called the “atomic number” and is the number of protons

6. Modern Physics Protons have a positive charge + Neutrons have NO charge Electrons have a negative charge -

7. Modern Physics Protons are in the nucleus Neutrons are in the nucleus Electrons are in clouds at different energy levels away from the nucleus

8. Modern Physics

9. Wait, some elements come in different varieties, like Carbon-12 and Carbon 14

10. Modern Physics Wait, some elements come in different varieties, like Carbon-12 and Carbon 14 These are called ISOTOPES Different in the number of neutrons We identify elements by the number of protons IF an atom has 6 protons, it is called Carbon

11. Modern Physics Carbon comes in two forms Carbon -12 – 6 protons and 6 neutron Carbon -14 – 6 protons and 8 neutrons Carbon -12 is the most common Carbon -14 is less common and is called an ISOTOPE of Carbon

12. Modern Physics Lets go back to the main particles

13. Modern Physics Protons have a positive charge + Neutrons have NO charge Electrons have a negative charge – Remember like charges repel SO If the nucleus is made up of Positively charged protons and neutral neutrons – why does it stay together?? Shouldn’t it fly apart?

14. Modern Physics Three main particles in an atom Actual MassTotal Mass 2 Protons1.007276 amu2.014552 2 Neutrons1.008665 amu2.017330 2 Electrons.0005486 amu.001097 total combined mass should be = 4.032979

15. Modern Physics Three main particles in an atom Actual Mass Total Mass 2 Protons1.007276 amu2.014552 2 Neutrons1.008665 amu2.017330 2 Electrons.0005486 amu.001097 total combined mass should be = 4.032979 HOWEVER THE ACTUAL MEASURED MASS IS 4.00260.03038 amu is MISSING

16. Modern Physics Three main particles in an atom Actual MassTotal Mass 2 Protons1.007276 amu2.014552amu 2 Neutrons1.008665 amu2.017330amu 2 Electrons.0005486 amu.001097amu total combined mass should be = 4.032979amu HOWEVER THE ACTUAL MEASURED MASS IS 4.00260.03038 amu is MISSING THIS DIFFERENCE IS CALLED “THE MASS DEFECT”

17. Modern Physics Where did the mass go???

18. Modern Physics Where did the mass go??? E=mc 2 Mass was changed into the BINDING energy to hold together the positively charged protons This is called the “Nuclear FORCE”

19. Modern Physics The Nuclear Force must be strong enough to hold together the positively charged protons For smaller atoms, an equal number of protons and neutrons generate a sufficient Nuclear Force to hold the atoms together For larger atoms, more neutrons are needed to generate a sufficient Nuclear Force

20. Modern Physics Belt of Stability

21. Modern Physics Atoms are stable if they are inside the Belt of Stability Atoms are unstable if they are outside the Belt of Stability An unstable atom is said to be RADIOACTIVE

22. Modern Physics Are all ISOTOPES radioactive??

23. Modern Physics Are all ISOTOPES radioactive?? NO, only the ones outside of the belt of stability Lets take a look at Carbon -12 and Carbon 14

24. Modern Physics Belt of Stability

25. Modern Physics Carbon – 12 is stable with 6 protons and 6 neutrons Carbon – 14 is unstable with 6 protons and 8 neutrons

26. Modern Physics Carbon – 12 is stable with 6 protons and 6 neutrons Carbon – 14 is unstable with 6 protons and 8 neutrons Carbon -14 is RADIOACTIVE

27. Modern Physics Carbon – 12 is stable with 6 protons and 6 neutrons Carbon – 14 is unstable with 6 protons and 8 neutrons Carbon -14 is RADIOACTIVE Carbon – 14 occurs naturally in our environment – a certain percentage of our bodies contain carbon -14

28. Modern Physics Carbon -14 dating

29. Modern Physics Carbon -14 dating All living things have a mixture of Carbon -12 and Carbon -14 atoms incorporated into our living cells Carbon – 14 is always slowly decaying into Carbon -12 The ratio of Carbon-12 to Carbon -14 in the cells of living things is representative of the environment

30. Modern Physics When an organism is living, the carbon in our bodies is constantly exchanging with carbon in our environment Therefore the ratio of Carbon-12 and Carbon- 14 in our cells reflects that of the environment

31. Modern Physics However, when an organism dies, the exchange with the environment stops The carbon-14 in the dead organism continues to decay and is not replaced with an exchange with the environment This changes the ratio of Carbon-12 to Carbon -14 in the dead organism’s cells

32. Modern Physics We know how fast Carbon -14 decays, therefore by measuring how much is missing we can determine how long the exchange of carbon with the environment has been stopped i.e. when the organism died This allows us to use Carbon-14 dating to tell how old things are

33. Modern Physics Mesa Verde cliff dwellings have been dated using Carbon -14 dating

34. Modern Physics By taking samples of the cedar wood beams and measuring the ratios of Carbon -12 to Carbon 14 atoms it was determined that the tree died (was cut down) between 900 and 1300 AD (over 1000 years ago) Therefore we believe this is the time the cliff dwellings were built

35. Modern Physics Carbon dating can only be used on previously living things Cannot be used to measure the age of rocks Other similar dating systems exist that use radioactive isotopes and their decay rates

36. Modern Physics Belt of Stability