1. Topic: Iosotopes and Avg. Atomic Mass  Do Now:

2. Dalton 1766 Remember #2…well Dalton was correct… 1)Atoms (tiny) 2)Atoms of same element are identical * 3)Indestructible * 4)Combine in whole # ratio (H 2 O) 5)In chem rxn. are combined, separated, rearranged *turned out to not be entirely be true

3. These three hydrogens are found in nature  All H’s have 1 proton  H-1: mass # = 1 1-1 = 0 neutrons 1-1 = 0 neutrons  H-2: mass # = 2 2-1 = 1neutron 2-1 = 1neutron  H-3: mass # = 3 3-1 = 2 neutrons 3-1 = 2 neutrons

4. Isotopes  atoms of same element with different # neutrons (mass # is different) different # neutrons (mass # is different) # protons stays same! (atomic # is same) # protons stays same! (atomic # is same)

5. Consider U-234, U-235, & U-238  What’s the atomic number of U?  How many protons in U?  How many neutrons in U-234?  How many neutrons in U-235?  How many neutrons in U-238?  How many electrons in U? 92 92 234 – 92 = 142 235 – 92 = 143 238 – 92 = 146 92

6. Average Atomic Mass

7.   The atomic masses reported in the periodic table represent the weighted average of the masses of the naturally occurring isotopes of that element

8. Unit for Avg. Atomic Mass =  Amu (or regents says just u) – atomic mass unit Define as 1/12 the mass of a Carbon-12 atom Define as 1/12 the mass of a Carbon-12 atom

9. Avg. Atomic Mass of Chlorine (Cl has 2 isotopes) 24.230%Cl-37 75.770%Cl-35 % Abundance Mass(percent) + mass(percent) +…= avg. atomic mass 35(.75770) + 37(.24230) = 35.453 amu

10. NOW YOU TRY Avg. Atomic Mass of Si 92.21% of Si has a mass of 27.97693 amu 4.70% of Si has a mass of 28.97649 amu 3.09% of Si has a mass of 29.97376 amu

11. Avg. Atomic Mass of Si 0.9221 X 27.97693  25.7975 amu 0.9221 X 27.97693  25.7975 amu 0.0470 X 28.97649  1.3619 amu 0.0470 X 28.97649  1.3619 amu + 0.0309 X 29.97376  0.9262 amu + 0.0309 X 29.97376  0.9262 amu 28.0856 amu 28.0856 amu