Property:Periodic table row

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A period is a horizontal row in the periodic table of the elements.

Although groups are the most common way of classifying elements, there are some regions of the periodic table where the horizontal trends and similarities in properties are more significant than vertical group trends. This can be true in the d-block (or "transition metals"), and especially for the f-block, where the lanthanides and actinides form two substantial horizontal series of elements. The period number also shows how many electron shells there are in an element.