1. Standard Atomic Weights on Periodic Table (red numbers) are found through the terrestrial (Sol III) relative abundances of the masses (in amu) of the isotopes of that element. Atomic mass unit (1 amu) is defined as 1/12 the mass of the neutral Carbon-12 atom (includes the 6 protons, 6 neutrons, 6 electrons)

2. Average Atomic Mass If there are 3 naturally occurring isotopes of
Neon including
Ne-20 Ne-21 Ne-22.
then the average mass of Neon atoms should be ___ amu.
Why does the PT lists it as amu?
Calculated as a weighted average of atoms in a sample of naturally occurring isotopes.

3. 63Cu is 69.17% abundant 65Cu is 30.83% abundant Average Atomic Mass
A weighted average atomic mass is calculated from both the
masses of each isotope and the
relative abundance (%) of each isotope.
63Cu is 69.17% abundant
65Cu is 30.83% abundant
To which will the
average mass be closer,
63 or 65?

4. Calculating Average Atomic Mass
copper-63
has a mass of amu
and natural abundance of 69.17%
copper-65
has a mass of amu
and has a natural abundance of 30.83%
Calculating Average Atomic Mass
Avg. Mass = (Mass1 x %frac) + (Mass2 x %frac) …
= (62.93)(0.6917) + (64.93)(0.3083) =
= amu

5. Neon-20 Neon -21 Neon-22
Which isotope is in greater relative abundance in natural samples of neon?
because…
the weighted average atomic mass of neon from the PT is amu.
90% - 20, <1% - 21, 9% - 22

6. =28.8 + 3.40 = 32.2 Avg. = (M1)(%) + (M2)(%)
Quick Quiz!
1. If an unusually selected sample of sulfur contained 90.0% sulfur-32 (31.97 amu) and % sulfur-34 (33.97 amu), what would be its average atomic mass?
32.2 amu
32.9 amu
33.5 amu
34.2 amu
Avg. = (M1)(%) + (M2)(%)
= (31.97)(0.900) + (33.97)(0.100) =
= 32.2

7. Changes on WorkSheet Question #2 has 3 isotopes Neon - 20 ( 19
Changes on WorkSheet Question #2 has 3 isotopes Neon - 20 ( amu at 90.48%) Neon – 21 ( amu at 0.27%) Note 2 Sig Fig on Abundance… Neon – 22 ( amu at 9.25%) Question #3 Silicon – 28 ( amu at %) Silicon – 29 ( amu at 4.683%) Silicon – 30 ( amu at 3.087%)