How many periods are in the periodic table?
Quick Answer
There are 7 periods, or horizontal rows, in the periodic table. Each new period generally begins when electrons start filling a new shell.
The Full Story
The periodic table has seven periods, each corresponding broadly to a new principal electron shell. As atomic number increases across a period, many properties change in a repeating pattern because of electron arrangement. Period 1 contains just hydrogen and helium, while period 7 includes the heaviest naturally occurring and synthetic elements. The table’s repeating structure is why the word “periodic” is used at all. This pattern was recognized long before the structure of the atom was fully understood.
Key Facts
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