The unit of measure for atomic
mass is the atomic mass unit (amu), sometimes called the atomic weight. The
atomic mass of an atom is about the sum total of its neutrons and protons, because
each proton and neutron weighs about 1 amu. Electrons have much less mass than
protons and neutrons, so they aren’t counted when measuring atomic mass.The periodic table tells us the atomic mass of
each element. Find the atomic mass on the lower chart of the periodic table. For
example, a sodium atom has eleven protons (its atomic number is 11) and twelve
neutrons. To get the atomic mass for sodium, the protons (11) and the neutrons
(12) are added together to reach the atomic mass (23). Notice that on this
periodic table, the atomic mass is written as a decimal number, not a whole
number. Does this mean there are partial protons or neutrons? No! It means there
are slightly different forms (known as isotopes) of the element, and the number
is the average of the atomic masses of the different forms. For our purposes, we
can round off to the nearest whole number.
Atomic Mass
What is the atomic mass of Al rounded to the nearest whole number?
27
Your Turn . . .
How many neutrons are in the element Ar?
22 neutrons
