Hello, Amber,
The "atomic mass" of each element can be found at the bottom of its symbol on the periodic table. For Ar, the average atomic mass is 39.948 AMU (atomic mass unit). I'll round that to 40, for this explanation. Argon is element 18 on the table, which means it has 18 protons. Each proton is assigned 1 AMU as its mass. Each Ar also has 18 electrons, for a neutral atom (the default, unless noted with a superscript after its symbol that shows any net charge). Electrons are considered to have 0 AMU. They do have a slight mass, but too little for these types of questions. Their mass becomes important in physics and in nuclear energy, however.
If Ar's mass is 40 AMU, we can subtract out the protons at 1 AMU each to gives us the number of neutrons, also 1 AMU each. This means that Ar has 22 AMU left after subtracting the mass of the 18 protons. The 22 AMU comes from the neutrons, also at 1 AMU each. So we have 22 neutrons.
Why? you might ask, does the table show a mass that is slightly off from its calculated value of 40, if you count neutrons and protons at 1 AMU each. That's because the table represents an average of Ar atoms found on Earth and the number of neutrons can vary even within the same element type. Atoms with an additional neutron are still neutral, but their mass will increase by 1 AMU. So there could be an argon 41 atom having 18 protons, 18 electrons, and 23 neutrons. Ar 39 has 21 neutrons, and so on. These variations on Ar are called "isotopes," and these can occur on all elements. Atomic energy is built on the unstable nature of some of the neutron-heavy elements such as Uranium and Plutonium isotopes.
So the atomic masses on the Periodic Table reflect a weighted average of those isotopes.
An atomic mass unit (amu; dalton) is a unit of mass equal to exactly 1/12 the mass of one 12C atom, or 1.660538921 x 10−27 kg. This is the approximate mass of one proton or one neutron and is the basis of atomic weights.
Getting back to your question, the answer would be
39.948amu x 1.660538921 x 10−27 kg/amu
Too many numbers for my taste, so I'll leave the final answer to you. If I round off in my head, I get 40x1.5 = 60,
so approximately 60 x 10−27 kg or 6.0 x 10−26 kg
Too little to satisfy my hunger, but enough to make it interesting for physicists trying to release that mass as energy, according to E = mc2, where c is the speed of light at 3 x 108m/s. Square that and you have a number large enough (9 x 1016 ) to make something interesting with only a mole of Ar atoms (6.03 x 1023).
6.0 x 10−26 kg/atom x 9 x 1016 m2/s2 x 6.03 x 1023 atoms/mole = 324 x 1013 kg m2/s2 per mole (40 grams) of argon. [1 kg m2/s2 is 1 Joule.]
40 grams of argon contains 3.24 x 1015 Joules of energy. That is equivalent to 774 kilotons of TNT!
Sorry for the digression, but it helps explain why someone might be interested in the actual mass of an atom. Beware!
If this didn't help - sorry. I hope it was at least interesting.
Bob
