How many groups are in the periodic table?

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Accepted Answer

There are 18 groups in the periodic table , numbered 1 to 18 from left to right under the current IUPAC system, so the number of groups equals the number of vertical columns in the standard layout of the modern periodic table. Group 1 starts with hydrogen and the alkali metals and group 18 ends with the noble gases. Groups 1 and 2 form the s-block, groups 3 to 12 the d-block (transition metals) and groups 13 to 18 the p-block (helium, in group 18, is an s-block element), while the f-block lanthanides and actinides sit in two rows below and are not given group numbers in most tables. Older North American and European tables counted eight A groups and eight B groups with roman numerals, which is why some textbooks list the main groups as IA to VIIIA plus the B groups, and some older charts show 16 labeled groups. Because the groups run from 1 to 18, you can name any column by its number: group 1 (hydrogen and the alkali metals), group 2 (alkaline earth metals), groups 3 to 12 (transition metals), group 13 (boron group), group 14 (carbon group), group 15 (pnictogens), group 16 (chalcogens), group 17 (halogens) and group 18 (noble gases). Group 3 is the subject of an ongoing debate: some tables list scandium, yttrium, lanthanum and actinium there, while others list scandium, yttrium, lutetium and lawrencium, which changes only how the two f-block rows are attached, not the total of 18. The count of 18 follows from filling electron subshells: two s columns, ten d columns and six p columns add up to 18. Fewer, more crowded tables such as Mendeleev's early version with eight groups appeared before the noble gases and the transition metals were fully understood. In the wide 32-column layout the periodic table has 32 columns, but it still has 18 groups.