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FormulasNow, since we know that atoms can combine with one another we have to have some way of representing this. The way that we show combinations of atoms on paper is to write down combinations of symbols for those atoms. These combinations of symbols are called formulas. Much like we can make words by combining letters, we can make formulas by combining symbols. Dalton's formula for water would be HO. We write down HO together, with no space in between them, and that would represent one atom of hydrogen combined with one atom of oxygen. (Again, that is not the correct formula for water.) Avogadro's formula for water would be H2O. H2O simply means that there are two atoms of hydrogen bonded to one oxygen atom, a ratio of two hydrogen atoms for every one oxygen atom. The subscript follows the symbol of the element to which it refers. The way that you would represent sodium chloride is to write down the symbol Na followed by the symbol Cl with no space between them. That represents sodium chloride--a combination of sodium and chlorine in a 1:1 ratio by atoms. The formula for hydrogen would be written as H2. The 2 subscript immediately following the H means that there are two hydrogen atoms combined with one another. The formula for molecular oxygen is O2. Again, the subscript 2 shows that there are 2 of those oxygen atoms bonded together. Note that the formulas for these elements is different from the symbols for these elements because the formulas represent the molecules of the elements and the symbols represent the atoms of these elements. Here is the formula for some kind of a sugar molecule. C6H12O6 represents 6 carbon atoms, 12 hydrogen atoms, and 6 oxygen atoms. In the formula Fe(NO3)3 you have to contend with parentheses. The 3 at the end of the formula--the one that is outside the parentheses--means that we have 3 of everything inside the parentheses. So there are 3 NO3's which would be a total of 3 nitrogen atoms and nine oxygen atoms, along with the 1 iron atom. Notice that the last 3 doesn't apply to the iron. It only applies to what's inside the parentheses. The 3 inside the parentheses applies only to the oxygen. You can also write down the formula of the compound if you are given the information about how many atoms are in it. For example, if I told you that a particular compound had 1 sulfur atom for every 3 oxygen atoms, you should be able to write down the formula SO3. Or if I told you that in a particular molecule there were 2 nitrogen atoms and 5 oxygen atoms, you should be able to write down the formula N2O5. Practice Interpreting FormulasTo give you a little more practice on this, why don't you try your hand at figuring out how many calcium atoms, how many phosphorus atoms, and how many oxygen atoms are represented by the formula Ca2(PO4)3 (in example 11d). Take a moment to figure that out, the answer follows. AnswerYou should have come up with 2 calcium atoms, 3 phosphorus atoms, and 12 oxygen atoms. Hopefully, you didn't have any problem figuring that out.
E-mail instructor: Sue Eggling Clackamas Community College |
