Fill in the blank. The___of an element is the total number of protons and neutrons in the____of the atom.

The atomic mass of an element is the total number of
protons and neutrons in the nucleus of the
atom.

Almost.

Now, here's the
rest of the story. The atomic mass of an atom, as given on the periodic table of the
elements, is an average. If you had a way to weigh individual atoms
one by one, you would find that most of the atoms of a substance had a weight equal to
the (rounded) atomic mass, but every now and then you'd have one that was different. The
different atoms are called isotopes. They occur naturally, and have an abnormal number
of neutrons in the nucleus.

For example, carbon has an
atomic mass of 12.0107. In reality, you will never find an atom of carbon that weights
12.0107 amu's. You'll find a lot of carbon atoms that weight exactly 12.000, and a very
few that weigh 14.000. For the common size atoms, which we call carbon-12, the weight
tells us that there are 6 protons and 6 neutrons in the nucleus. A carbon-14 still has 6
protons (the number of protons is really what determines the identity of the atom), but
it has 14-6= 8 neutrons.