One of the values of the mole is that every mole of any element has the same number of atoms in it. We have not yet discussed what that number is in this course (we will deal with that later when we consider the size of atoms), but we do know that it is the same amount for each element.
If you were to weigh out an equal number of oxygen and hydrogen atoms, the amount of oxygen that you have would weigh 16 times the weight of the amount of hydrogen you have because each oxygen atom weighs 16 times more than each hydrogen atom. Conversely, if you weigh out 16 times more oxygen (by weight) than hydrogen, you will have equal amounts (by atoms) of each. As long as the weight of the oxygen measured was 16 times heavier than that of the hydrogen used, we would have the same number of atoms. Pause for a moment to make sure that makes sense to you. If not, talk with an instructor about this relationship. (You can use e-mail if you are not in the lab.)
We can focus on one particular combination of numbers and units to define the quantity that we call a mole. We use the number given by the relative atomic weight for each element and we use the mass unit gram. Thus, if we weigh out 1 gram of hydrogen, we have 1 mole of hydrogen atoms. If we weigh out 16 g of oxygen, we have 1 mole of oxygen atoms.
This gives us a new perspective on the units that can be used with atomic weights. Atomic weights can be used as unitless quantities that give the relative weights of atoms. Atomic mass units were defined so that the atomic weights would represent the weights of individual atoms. Moles were defined so that atomic weights in grams would represent that particular quantity of an element.
In summary, a mole is the quantity of an element that you have when you weigh out in grams the amount of that element specified by the atomic weight. The special property of a mole is that one mole of an element contains the same number of atoms as one mole of any other element, even though at this moment we don't know what number of atoms that happens to be.
Incidentally, the weights of elements could be measured in grams, kilograms, ounces, pounds or tons. To emphasize that we selected grams, the term gram-mole is used by some chemists. Similarly, "kilogram-moles" or "pound-moles" could be defined if you had a reason to do so.
As we continue working with elements, atoms, and moles, you will find that we use the symbol of an element in a variety of ways. You will find that we not only use the symbol of an element as the abbreviation of the name of the element and to represent individual atoms of the element, we also use the symbol to represent one mole of an element. You must pay attention to the context to keep track of what the symbol is being used to represent.
E-mail instructor: Sue Eggling
Clackamas Community College