1. Review!!! A neutral atom has an Atomic Number of 82 and a Mass Number of 207. What is this element? How many protons? How many electrons? How many neutrons?

2. Review! Discuss with your tablemates the experiments and conclusions of each scientist. Who did the gold foil experiment? Who discovered the electron? Who discovered the neutron? Who used the cathode ray tube? What are Dalton’s Postulates? What is the Bohr Model of the atom?

4. What is an isotope?  Atoms of the same element can have different numbers of neutrons; the different possible versions of each element are called isotopes.  For example, the most common isotope of hydrogen has no neutrons at all; there's also a hydrogen isotope called deuterium, with one neutron, and another, tritium, with two neutrons. HydrogenDeuteriumTritium

5. Symbols for Isotopes Two ways to write isotope symbols: 1.35 Mass Number Cl Element Symbol 17 atomic number 2. chlorine - 35 name mass number or symbol

6. Naturally occurring carbon consists of three isotopes, C-12, C-13, and C-14. State the number of protons, neutrons, and electrons in each of these carbon atoms: 12 13 14 CC C 6 6 6 #P _______ _______ _______ #N _______ _______ _______ #E _______ _______ _______

7. Example #1 Nitrogen-15 How many p? How many e? How many n?

8. Example #2 14 How many p? How many e? How many n? C

9. How many isotopes can one element have? Can an atom have just any number of neutrons? There ARE "preferred" combinations of neutrons and protons –Light elements tend to have about as many neutrons as protons –Heavy elements apparently need more neutrons than protons in order to stick together. –Atoms with a few too many neutrons, or not quite enough, can sometimes exist for a while, but they're unstable.

10. How many isotopes can one element have? Can an atom have just any number of neutrons? Do atoms just fall apart if they don't have the right number of neutrons? –YES! Unstable atoms are radioactive: their nuclei change or decay by spitting out radiation, in the form of particles or electromagnetic waves.

11. Quick Question! How do teachers (and gradebooks) calculate your grade in your class? Remember that you have Daily Grades, Quiz Grades, and Test grades (each worth different amounts)? Your total average in your class is a WEIGHTED AVERAGE of all of your grade categories!

12. Average Atomic Mass Listed on the periodic table Gives the mass of “average” atom of each element. Average atom based on all the isotopes and their percent abundance Atomic mass is NOT a whole number

13. Steps for Determining Average Atomic Weights Turn the percent into a decimal. Multiply the decimal by the mass. Repeat for each isotope Add the masses up.

14. Example: Magnesium Isotopes Mass Percent of Isotope Abundance Mg-24 = 24.0 amu 78.70% = _____________ Mg-25 = 25.0 amu 10.13% = _____________ Mg-26 = 26.0 amu11.17% = ____________ Atomic mass (average mass) Mg= ___________ amu