How many periods are in the periodic table?

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Accepted Answer

There are 7 periods in the periodic table , the horizontal rows (also called periods) numbered 1 to 7 from top to bottom. They hold 2, 8, 8, 18, 18, 32 and 32 elements, which add up to 118, and each period number matches the number of electron shells occupied in the ground state of its atoms, with a few exceptions such as palladium. Period 1 holds only hydrogen and helium, periods 2 and 3 are the short periods of eight elements, periods 4 and 5 add the transition metals and run to 18, and periods 6 and 7 add the lanthanides and actinides to reach 32 elements each, although those two series are usually drawn separately below the table. Period 7 was completed in 2016 when nihonium, moscovium, tennessine and oganesson were named. Seven periods are enough because the heaviest known element, oganesson, has its outermost electrons in the seventh shell. In the common 18-column drawing the sixth and seventh rows look like they hold only 18 elements, because 14 of each are shown separately below the table as the f-block: the lanthanides (lanthanum to ytterbium) and the actinides (actinium to nobelium). To identify the period of an element, count the rows from the top, or find its highest occupied electron shell in its electron configuration; for example, gold ([Xe] 4f 14 5d 10 6s 1 ) is in period 6 because its outermost shell is the sixth. Across each period the elements change from reactive metals on the left to nonmetals on the right and end in a noble gas, so the number of periods also counts how many times that pattern has repeated. Period 7 became complete only in 2016, when the last four elements were named. An eighth period would begin with element 119 if scientists succeed in making it, and calculations suggest it could hold around 50 elements, though predictions vary.