Arrangement of the Periodic Table

In this section we will discuss some of the basic terminology related to the arrangement of elements on the periodic table. First, let's review how to read an individual cell of the table.

In most periodic tables at least two numbers are shown in the box for each element, as shown above. There will be the atomic number and also the atomic mass. In this periodic table the atomic number is shown in the upper left-hand corner and the atomic weight is shown below the symbol for the element. The name is also given.

Periods

Notice how the elements are arranged on the periodic table in increasing order of atomic number (Z). There is, first of all, hydrogen (Z=1) then to its right is helium (Z=2). There the line ends, so the next line starts with lithium (Z=3) then to its right is beryllium, boron, and so forth on across with increasing atomic number.

There is a special name given to the horizontal rows on the periodic table. They are called periods. The first row, the one containing hydrogen and helium, is called the first period. The second row is called the second period. And so on. There are seven periods in all

You may wonder why helium, with atomic number 2, is put way over on the right side instead of next to hydrogen. The reason is that the chemical properties of helium are similar to those of the other elements in the far right column. They are called the inert gases because they are all very unreactive gases. Remember that the periodic table is arranged so that elements with similar chemical properties are lined up in vertical columns. It would not be correct to place the inert gas, He, in the second column, the one just to the right of H, because He's properties aren't anything like those of the other elements in the second column (Be, Mg, etc.) They are all reactive metals, not inert gases. In many ways, H does not belong in the first column either, but it is usually put there anyway for reasons that we will get into later.

Groups

The vertical columns on the periodic table have special names, too. They are called groups or families. Remember that all elements in a group or family have similar chemical properties (though H is an exception sometimes).

There are two different systems for numbering groups. One method is to use arabic numerals (1, 2, 3 ...) and number starting with 1 at the left end going up to 18 at the right end. The other method distinguishes between different parts of the table; the first two groups, plus the last six, are numbered IA through VIIIA (or 8A, if you don't like roman numerals). As you can see above, things get a little crazy for the groups labeled with a B. We start with IIIB (or 3B) above Sc ... an odd place to start, but okay. We increase through VIIB above Mn, and then things go off the rails. We have a block of three groups all labeled VIIIB, and then for no obvious reason we start over with IB and IIB to wrap up.

The good news is that you will not have to memorize this mess. You will always have access to a periodic table when doing work for this class. So, while you should be able to look at it and answer questions like "what element is in the fifth period, Group VIB?" (molybdenum), you will not be asked things like "what is the group number of the column starting with copper?"

Metals and Nonmetals

In addition, there are some other designations on the periodic table that you need to know. Most of the elements on the periodic table shown below are blue in color. Those elements are metals. The elements that are shown in green are the nonmetals. The orange ones are called metalloids or semi-metals. While their properties are often somewhat between the two, you can consider them as nonmetals for purposes of this class.