Topic: Iosotopes and Avg. Atomic Mass

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Dalton1766

Remember #2…well Dalton was correct…

1) Atoms (tiny)2) Atoms of same element

are identical *3) Indestructible *4) Combine in whole # ratio

(H2O)5) In chem rxn. are

combined, separated, rearranged

*turned out to not be entirely be true

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

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

92234 – 92 = 142

235 – 92 = 143

238 – 92 = 146

92

Average Atomic Mass

Average Atomic Mass

The atomic masses reported in the periodic table represent the 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)

24.230%Cl-37

75.770%Cl-35

% Abundance

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 X 27.97693 25.7975 amu

0.0470 X 28.97649 1.3619 amu

+ 0.0309 X 29.97376 0.9262 amu

28.0856 amu