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 you're 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 choose 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.

Periods and Groups

A row on the periodic table is called a period. There are seven periods on the periodic table. You might look and think "wait, I counted nine", and that would be technically wrong because those bottom two rows with elements 58-71, and 90-103 are actually from rows (periods!) 6 and 7 from the main table above them. Later, you will find out that those row numbers will match perfectly with the principle quantum number \(n\) from atomic theory. The periodic table has all sorts of cool information just based on its layout.

A column on the periodic table is known as a group or family. The groups are actually numbered up at the top of the table. They go from 1 to 18 which is the more internationally known numbering system and the official one according to IUPAC. But there is often another set of numbers which are split into the "A" and "B" groups. This is the older American system of numbering and there are 1-8 for each group of A and B. The "A" elements are also known as the representative elements (1A-8A) and correspond to groups 1, 2, 13-18 on the IUPAC numbering. Those 10 groups in the middle of the table starting with scandium are the B-groups (IUPAC 3-12) and are known as the d-transition metals.

Some of the groups have a very specific family name. The important ones you need to know are listed below.

  • Group 1 : the Alkali Metals (not counting H)
  • Group 2 : the Alkaline Earth Metals
  • Group 17 : the Halogens
  • Group 18 : the Noble Gases (aka the Inert Gases)

Those last two rows (although part of rows 6 and 7 as mentioned earlier) under the main table are the \(f-\)transition metals and are know as the lanthanides and the actinides. You can figure out that their names come from the two elements that immediately preceed them - lanthanum and actinium. And yes, both lanthanum and actinium are a part of those two groups which is why I have split colors for those elements.

Below is a lovely figure I made that illustrates many of these groups mentioned above.

Oh yeah, there is one more thing... If you take the group of all the lanthanides and add the two elements scandium and yttrium, you'll have yet another classification or group known as the rare earth elements. There are other names I didn't mention, but I'm going to let that be it for us for now. 😉

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