Atomic Mass
Atomic Number
Atoms are arranged on the periodic table in order of atomic
number, or the number of protons.
e
P
N
N P
e
Atomic Number
Isotopes
 Isotopes are atoms of the same element that have the same
number of protons but a DIFFERENT number of neutrons.
 Each isotope is a specific nuclide
Note that the # of
protons stays the same,
so the nucleus’ charge
stays the same
Note that isotopes do
not vary chemically in
any significant way
Mass Number
 Mass number = # protons + # neutrons
 Nuclear Symbol is the proton # and the mass number next to the
Element symbol. We’re going to write them with the mass # on
the top left and the atomic # on the bottom left
 Ex: Helium-3 has 2 protons and 1 neutrons
Mass# =
3
2𝐻𝑒
Helium-4 has 2 protons and 2 neutrons
Mass# =
4
2𝐻𝑒
Element and Isotope Practice
Isotope
# of
Protons
# of
Neutrons
6
6
6
8
Hydrogen-1
Hydrogen-3
Copper-63
Copper-65
# of
Electrons
Mass #
Nuclear
Symbol
Atomic Mass Units
 These are relative mass units: we assign the mass of 1 proton to
be 1/12th of a Carbon-12 atom
 Why do you think we do this?
 Scientists decided they liked carbon a lot
 1 carbon atom has roughly 12 times more mass than hydrogen (1
proton)
 Because the mass of a proton and neutron are very miniscule and
hard to think of in terms of grams
Average Atomic Mass
 If we know there are different Isotopes of 1 element, how can we
arrange the elements on the periodic table by mass correctly?
 Remember when we didn’t mass our filter paper in the Separation
Lab? How did we compensate for the fact that our filter paper would
be similar to another sheet of filter paper, but not identical? How did
we get a good value?
 A weighted average!
How to do a Average Atomic Mass
 Let’s say we have an element named Derpium from the alternate
universe of the Land of Ooo.
 Derpium exists in 2 isotopes, one with an atomic mass of 16 amu
and the other with an atomic mass of 19 amu.
 Derpium-19 is less stable and thus less common in the Land of
Ooo, so it’s only found 25% of the time.
 Derpium-16 is more stable and thus found 75% of the time.
How to do a Average Atomic Mass
 If the head wizard of the Land of Ooo had a chunk of Derpium
Ore and didn’t feel like explaining to the King that 25% of The
Derpium had less mass than the rest of the 75% of the Derpium,
he could create a weighted average
 25% of the ore is 19 amu:
19 * 0.25 =
 75% of the ore is 16 amu:
16 * 0.75 =
 Average Atomic Mass of Derpium =
Practice Average Atomic Mass
 A. Copper-63 occurs 69.15% of the time
 B. Copper-65 occurs 30.85% of the time
 Step 1: Turn the percentage of A into a decimal:
 Step 2: Multiply the amu of A by this decimal:
 Step 3: Turn the percentage of B into a decimal:
 Step 4: Multiply the amu of B by this decimal:
 Step 5: Add the result of Step 2 and Step 4 together