What are Atomic Weights?
You already know that the atomic number is based upon the number of protons in an atom. You also know how that is what
makes an atom different from all others. Another way to describing atoms is by using the number of protons and neutrons. The
number of protons and neutrons in an atom is called the Atomic Mass or Weight.
Atomic weights or masses are not given in grams or pounds. It is given in Atomic Mass Units, (AMU’s). With Atomic Mass Units,
each particle is given a weight of one (1). In other words, each proton is given a value of one (1), each neutron is given a value of
one (1).
Atomic masses are useful when determining the number of neutrons in an atom. All you need to do is subtract the number of protons (atomic number) from the atomic mass, and that tells you the number of neutrons.
Atomic weight compares the ‘weight’ of one type of atom with the ‘weight’ of another kind of atom. Look at the items below:
Atomic Number
Atomic Mass or Weight
Sulfur (S) has an atomic weight of 32.07. Iron (Fe) has an atomic weight of 55.845. This means that iron is heavier or more massive
than sulfur.
One often asked question is, “Why is the atomic mass of an atom a decimal and not a whole number?” For example, Sulfur has an
atomic mass of 32.07 and not 32. This is because a small percentage of sulfur atoms have 17 neutrons and not 16. These are called
isotopes of sulfur. Isotopes are atoms of the same element with different amounts of neutrons. When you average all of the isotopes of sulfur, you get an atomic mass or weight of 32.07. Therefore most atomic masses are averages.
Some atomic masses are in parentheses. This means that no isotopes of this element exist, or the isotopes are unstable and exist
for only a short time.
Electron
Nucleus—Protons and Neutrons.
Each protons has a value of one.
Electron
Each neutron has a value of one.
Electron
What is the atomic
weight of this atom?
What is the atomic
weight of this atom?
_________ AMU’s
_________ AMU’s
Complete this chart.
Name of Element
Number of Protons
Number of Neutrons
Cobalt
27
32
Zinc
30
35
Iodine
53
74
Tungsten
74
110
Lead
82
125
Atomic mass or weight
Use your periodic table to complete this chart.
Element name
Listed Atomic Mass
Rounded Atomic Mass
Silver
Hydrogen
Calcium
Barium
Questions:
1) What is an AMU?
2) What two particles are counted to determine atomic mass or weight? __________________________________________
3) Why are some atomic masses is parentheses? ______________________________________________________________