Modern Periodic Table: Periods and Groups

The periodic table has undergone extensive changes in the time since it was originally developed by Mendeleev and Moseley.  Many new elements have been discovered, while others have been artificially synthesized.  Each fits properly into a group of elements with similar properties.  The periodic table is an arrangement of the elements in order of their atomic numbers so that elements with similar properties appear in the same vertical column or group.

The figure below shows the most commonly used form of the periodic table.  Each square shows the chemical symbol of the element along with its name.  Notice that several of the symbols seem to be unrelated to the name of the element: Fe for iron, Pb for lead, etc.  Most of these are the elements that have been known since ancient times and have symbols based on their Latin names.  The atomic number of each element is written above the symbol.

A period is a horizontal row of the periodic table.  There are seven periods in the periodic table, with each one beginning at the far left.  A new period begins when a new principal energy level begins filling with electrons.  Period 1 has only two elements (hydrogen and helium), while periods 2 and 3 have 8 elements.  Periods 4 and 5 have 18 elements.  Periods 6 and 7 have 32 elements because the two bottom rows that are separated from the rest of the table belong to those periods.  They are pulled out in order to make the table itself fit more easily onto a single page.

A group is a vertical column of the periodic table, based on the organization of the outer shell electrons.  There are a total of 18 groups.  There are two different numbering systems that are commonly used to designate groups and you should be familiar with both.  The traditional system used in the United States involves the use of the letters A and B.  The first two groups are 1A and 2A, while the last six groups are 3A through 8A.  The middle groups use B in their titles.  Unfortunately, there was a slightly different system in place in Europe.  To eliminate confusion the International Union of Pure and Applied Chemistry (IUPAC) decided that the official system for numbering groups would be a simple 1 through 18 from left to right.  Many periodic tables show both systems simultaneously.

Glossary

• group:  Elements with similar chemical properties appear at regular intervals, within the vertical columns.
• period:  A period is a horizontal row of the periodic table.

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