A Gram of Hydrogen:Let's say that I told you to get a gram of hydrogen and bring it to me. How would you do that? Well, you could go to the atom store, grab some hydrogen, and weigh it to a gram on the nearest scale, but that would be much too easy. What if I were to tell you that in a gram of hydrogen, there are always 6.022 x 1023 atoms? So, now all you have to do is count out that many atoms and hand em' over to me. You better start now; it'll probably take you a while.
Moles and the Periodic Table:In the section above we explained how many atoms are in a gram of hydrogen. That number is a special number called "Avogadro's Number". If you have 6.022 x 1023 atoms of any substance, we say that you have a "mole" of that substance. If I have 6.022 x 1023 water molecules, I have a mole of water. If I have 6.022 x 1023 cats, I have a mole of cats. (That would be a lot of cats. How would I feed them, I wonder? I suppose I'd need a few moles of that canned cat food stuff.) So, a mole is a unit of measurement that tells how many objects (atoms, molecules, or... cats) you have.
Remember in the first part of Atomic Nomenclature when we explained the periodic table and how to calculate atomic masses? Well, if you have a mole of hydrogen atoms, you'd have a 1 gram of hydrogen, right? This is what we explained in the first section of this page. So, let's think about this for a second. Hydrogen has 1 nucleon, a proton. Helium, however, usually has 4 nucleons. It has 2 protons and 2 neutrons. That means that each helium atom is about 4 times the mass of a hydrogen atom, right? It has 4 nucleons, while hydrogen has 1. That means that if I have 2 hydrogen atoms and 2 helium atoms, the mass of my 2 helium atoms will be 4 times that of the mass of my 2 hydrogen atoms. So, if I have a mole of hydrogen atoms and a mole of helium atoms, the mass of my collection of helium atoms will also be 4 times that of the mass of my hydrogen atoms. So, if a mole of hydrogen atoms weighs 1 gram, then a mole of helium will weigh 4 grams. Let's stop and think again. What is the atomic mass of the main isotope of helium? It's 4. So, the mass of a mole of helium (known as helium's molar mass) is the same as it's atomic mass. In fact, that's what the atomic mass really does measure. The atomic masses of the atoms really measure the mass of a mole of that atom. For example, the atomic mass of the most common isotope of carbon is 12. Thus, if I had a mole of carbon-12, I would have 12 grams of carbon.
Average Atomic Mass:You've got 20 of helium atoms sitting in a box. I tell you to find the average mass. Most of the atoms' masses are 4(4He), but 2 are 3(3He). How would you find the average atomic mass? Well, you would add up all the mass-4 atoms (18 of them), and then add in the mass-3 atoms (2 of them). 18x4 + 2x3 = 78. This is the sum of all their masses. To find the average mass, you need to divide by the total number of atoms. 78/20 = 3.9. Therefore, the average mass of the helium atoms in your box is 3.9. This is pretty simple, right? What if I didn't tell you the number of atoms in the box, and I only told you that, say, 10% are 3He, and the rest are 4He. You could still calculate the average in much the same way:
10% x 3 + 90%x4 = .1 x 3 + .9 x 4 = 3.9
We must then divide by 100%, which is 1, so the answer is still 3.9. Thus, the average mass is 3.9. This is exactly how scientists calculate the average atomic mass of an element. They don't know how many atoms of each isotope are in the universe, but they do know the percent of each type of atom. From this, they can calculate the average atomic mass for each element. Also, only non-radioactive isotopes are used in calculating atomic mass.
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