What is a period in the periodic table?

What is a period in the periodic table? A period in the periodic table is a horizontal row of elements (a period is also simply called a row), listed by increasing atomic number from left to right.

There are seven periods, and in nearly every case the period number equals the number of electron shells (energy levels) in an atom of the element, so lithium in period 2 has two shells and cesium in period 6 has six. Period lengths are 2, 8, 8, 18, 18, 32 and 32 elements, set by how many electrons each shell can hold. From alchemy to plutonium and uranium, the periodic table records how the history of the elements unfolded.

Moving across a period from the alkali metals on the left to the noble gases on the right, atomic number rises, atomic radius generally shrinks, electronegativity and ionization energy generally rise, and elements shift from metals through metalloids to nonmetals. Periods differ from groups, which are the vertical columns. The same series of answers also covers “where are the metals located on a periodic table”.

A period number also tells you which electron shell is being filled. Period 1 fills only the 1s subshell, which holds two electrons, so it has just hydrogen and helium. If you are studying, you may also want the answer to “how to read the periodic table”.

Periods 2 and 3 fill s and p subshells, giving eight elements each, from lithium to neon and from sodium to argon. Periods 4 and 5 add the d subshells of the transition metals, giving 18 elements each, and periods 6 and 7 add the f subshells of the lanthanides and actinides, giving 32 each. For a different question, see “how many metals are in the periodic table of elements”.

Each period except the first begins with an alkali metal and ends with a noble gas, so the same pattern repeats row after row, which is the reason for the name periodic table. To locate a period, count rows from the top: sodium is in period 3, iron in period 4, silver in period 5, gold in period 6 and uranium in period 7. The lanthanides and actinides belong to periods 6 and 7 even though they are drawn in two rows below the main table to keep it compact.