| The vertical columns (major classes or divisions) into which elements are arranged in the periodic table of elements. There
are three common numbering systems for these groups:
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| The new IUPAC system, which numbers each column with Arabic numbers from 1 (one) through 18 (eighteen). To reduce confusion caused by the
other two systems, this is the system that is used in articles on this web site.
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| The old IUPAC system, which labeled columns with Roman numerals followed by either the letter 'A' or 'B'. Columns were numbed such that
columns one through seven were numbered 'IA' through 'VIIA', columns 8 through 10 were labeled 'VIIIA', columns 11 through
17 were numbered 'IB' through 'VIIB' and column 18 was numbered 'VIII'.
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| The CAS system, which also used Roman numerals followed by an 'A' or 'B'. This method, however, labeled columns 1 and 2 as
'IA' and 'IIA', columns 3 through 7 as 'IIIB' through 'VIB', column 8 through 10 as 'VIII', columns 11 and 12 as 'IB' and
'IIB' and columns 13 through 18 as 'IIIA' through 'VIIIA'.
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| Because of the confusion the old IUPAC and the CAS system created, the IUPAC adopted their new system.
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| Elements are arranged in these groups according to whose proprieties are similar. All elements in Group 1 for instance are alkali metals. They have only one electron in the outer shell (valence electron) and as a result are highly reactive. Elements in Group 17 are the halogens. They all have seven electrons in the outer orbital (two in level s and five in level p). They are also very reactive because they have seven electrons in the outer shell and will readily accept an electron in order to reach the ion configuration with the ideal number of eight electrons in the outer shell. Elements Group 18 have a complete outer shell with eight electrons. These noble gases are highly stable and do not react to form compounds under normal conditions.
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