The Periodic Table

use it often

Elements and Symbols

The periodic table can often be presented with an abundance of data about each and every element listed. In it's simplest form (shown below), each entry only has three pieces of information that you will need to know. These three pieces of data are the elemental symbol, the atomic number (typically given the symbol, Z, and the atomic weight. *Note: If you click on the table, you'll launch it into its own window/page on your browser.

Periodic Table of the Elements

Make sure that you know what each of these parts is and what it represents. The diagram below illustrates the parts and their definitions.

A diagram showing where atomic weight and mass are on an element item in the periodic table.


Hey you! LOOK again at any periodic table - including the one above. Notice how the atomic weights have no units after them. Maybe your thinking... "Well, I know the weights are in grams because that is how I learned it in high school". Oof. Sure, you're not wrong. BUT it would be much much better for you to realize that those could be ANY unit of weight/mass you chose and the whole table would still be correct. Relative masses means that they are all corrected relative to each other. You could think in pounds, or kilograms, or ounces, or even tons, or heaven forbid... short tons, long tons, drams, grains, or stones. Not to mention the myriad of masses represented by all the metric prefixes to prepend to "gram". You can work chemistry mass problems in any mass you want and it will still work because the masses are relative to each other. All chemical ratios work just as well with masses as they do with our oh so familiar moles.

So why DO we seem to concentrate on the "gram" as our go to guy on the periodic table for atomic weights and ultimately for molar masses and molecular weights? Well the key here is the way we historically defined the mole. Because of that old definition, we were able to say that all those atomic weights are in grams per mole of substance or abbreviated g/mol. This helps tremendously when having to convert from moles to mass as we often do in chemistry. Counting by number is the molar amount, while measuring by mass is the... well, mass amount (duh). Those atomic weights are the number of grams you will need of that element in order to have exactly 1 mole of that element. It's a nice system.

In general, to work all types of stoichiometry problems, we say to convert all masses to grams first. Then, convert those grams in to moles and work the problem in moles only. Then (if need be) convert your answer in moles into grams. And finally, convert those grams into any other unit needed that might be asked for. Quite the rigorous path for "all" problems.

You could easily shorten that path. How? Well IF the problem is stated in say pounds, and then wants the answer in pounds... there is really no reason to convert to grams first and then back out to pounds later. Just work the problem in pounds - it will work. You'll only have to go to grams IF the number of moles is asked for. Knowing how numbers work and how ratios work is KEY to understanding and working chemistry stoichiometry problems. The periodic table is your ultimate conversion chart for converting any substance into another substance and doing so with exact proper amounts (masses and moles).

Here's a nice Periodic Table and more pdf for you to use for this class.

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