ATOMIC WEIGHT
Looking at the periodic table, we see that elements have weights expressed like:
16.00
8
79.90
35
O
Br
We know that the 8 in oxygen stands for the number of protons and the 16.00 is
it’s mass, but we also know that oxygen doesn’t weight 16. So why aren’t the
masses reported to reflect the mass of the protons, neutrons and electrons…why
are these numbers so large and whole?
Isn’t the following how they should be expressed?
8 protons = 8 (1.673 10-24g) = 1.338 10-23g
35 protons = 35 (1.673 10-24g) =
8 neutrons = 8 (1.67510-24g) = 1.340 10-23g
45 neutrons = 45 (1.67510-24g) =
8 electrons = 8 (9.10910-28g) = 9.101 10-28g
35 electrons = 35 (9.10910-28g)=
2.679 10-23g
Total
2.679 1023
8
O
Total
7.7081023
35
7.70810-23g
Br
The reason chemists have chosen a standard to simplify how atoms are written,
this makes working in the lab easier. All Atomic Masses are most easily measured
relative to carbon-12.
Carbon-12 is automatically given a mass of 12 Atomic Mass Unit (AMU).
1amu 
1
mass
12
12
C
amu = dimensionless, no units (as a side note: amu is being phased out and will be
replaced with the Dalton in honor of John Dalton…whom as you remember
expressed the first Atomic Theory)
Atomic Weights are calculated using an instrument called a Mass Spectrometer,
which can ONLY measure masses relative to 12C. For instance a sample of 16O
and 12C are put in the Mass Spectrometer…the weight of the 16O is different the
12
C and the 16O comes out of the machine more slowly and is measured to be
1.3329 times slower then carbon. 17O comes out 1.1466 times more slow then 12C
and 18O, the slowest comes out 1.4999 times less slow then 12C.
So for Oxygen:
16
1.3329
Relative Ratio  12 =
Mass
15.995
17
1.1466
16.999
18
1.4999
17.999
Relative Ratio:
A
X/12C
O
O
O
Then the mass is calculated using the Relative Ratio and is multiplied by 12 to
give the mass of that Isotope.
The same method is used for EVERY Isotope on the Periodic Table (don’t forget
to throw in the % Abundance in your calculation…..then you have it).