There are 18 groups in the periodic table according to the modern IUPAC numbering system.
Here's why:
The periodic table is organized into vertical columns called groups (or families). These groups contain elements with similar chemical properties due to having the same number of valence electrons. Historically, different numbering systems were used to identify these groups. However, the International Union of Pure and Applied Chemistry (IUPAC) standardized the numbering from 1 to 18 in 1988.
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IUPAC Numbering: This system, which is now the standard, numbers the groups sequentially from left to right, from 1 to 18.
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Older Numbering Systems: Older systems sometimes used Roman numerals (I to VIII) along with letters (A and B) to differentiate between main group elements and transition metals. This could lead to confusion.
While there are 7 periods (horizontal rows) in the periodic table, the question specifically asks about the number of groups.
Therefore, the correct answer is 18.
