Download Calculation of Average Atomic Masses

Gen Chem 123
9/09/03
Dr. J
Names: ____________________ __________________ __________________________
Mass Units
Earlier we showed that if the mass of a H atoms was assigned a value of 1, then the mass of an oxygen
atoms had a value of 16.
Most of the mass of an atom is contributed by the nucleus (protons and neutrons)
The modern unit of mass is the atomic mass unit (amu or the new unit u), is based on carbon 12 (6
protons, 6 neutrons), the most abundant naturally-occurring isotope of C. The atomic mass unit is defined
as:
1 amu = 1/12 the mass of a carbon-12 atom (which included electrons whose mass is negligible
compared to the proton and neutron).
1 amu = 1.9426 x 10-23 g/12 = 1.6605 x 10-24 g
The table below gives the masses of the electron, proton, and neutron in g and amu.
Particle
Electron
Proton
Neutron
g
9.1094 x 10-28
1.6726 x 10-24
1.6749 x 10-24
amu
0.0005
1.0073
1.0087
When calculating the atomic mass of an isotope, we will assume that the mass of the electrons are
negligible, and the each proton and neutron contributes 1 amu to the mass of the isotope. These
calculations will prove to be close but not exact since the sum of the mass of the individual protons and
neutrons is usually greater than the actual mass of the nucleus containing that number of protons and
neutrons. The mass lost on formation of the nucleus from the individual nucleons has been converted to
energy, based on Einstein’s famous formula, E = mc2.
1. Calculate the average atomic mass of Mg from the data below:
2. Assume that the average atomic mass of Cl is 35.5. Chlorine (atomic number 17) has
17 protons and 17 electrons. Chlorine exists naturally as two different isotopes, differing
only in the number of neutrons in the nucleus. Come up with two different pairs of
isotopes and their relative natural abundancies such that the weighted average of the
pairs is 35.5.
a. Set 1:
b. Set 2: