Topic: Iosotopes and Avg. Atomic Mass

Topic: Iosotopes and Avg. Atomic Mass • Do Now:

Dalton 1766 • Atoms (tiny) • Atoms of same element are identical * • Indestructible * • Combine in whole # ratio (H2O) • In chemrxn. are combined, separated, rearranged • *turned out to not be entirely be true Remember #2…well Dalton was correct…

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

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

Consider U-234, U-235, & U-238 92 • 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? 92 234 – 92 = 142 235 – 92 = 143 238 – 92 = 146 92

Average Atomic Mass

Average Atomic Mass • The atomic masses reported in the periodic table representthe weighted average of the masses of the naturally occurring isotopes of that element

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

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

NOW YOU TRYAvg. 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

Avg. Atomic Mass of Si 0.9221 X27.97693  25.7975 amu 0.0470 X 28.97649  1.3619 amu + 0.0309 X 29.97376 0.9262 amu 28.0856 amu

Topic: Iosotopes and Avg. Atomic Mass