Why is it called the periodic table?

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The periodic table is called periodic because the physical and chemical properties of the elements repeat at regular intervals, or periods, when the elements are listed by increasing atomic number. After lithium, sodium, potassium and the other alkali metals, the same soft, highly reactive behavior returns at intervals of 8, 8, 18, 18 and 32 elements, so each row ends in a noble gas and the next row starts again with an alkali metal. This repetition is the periodic law, which Mendeleev stated in 1869 using atomic weight and Henry Moseley restated in 1913 using atomic number. The pattern comes from the way electrons fill shells and subshells, so atoms with the same outer valence electron arrangement line up in the same vertical column, or group. The word periodic comes from the Greek periodos, meaning a going around or cycle. The 'table' part is simply the chart layout of rows (periods) and columns (groups). Periodicity shows up in many properties. Atomic radius, ionization energy, electronegativity and metallic character all rise and fall in a repeating pattern when plotted against atomic number: ionization energy peaks at each noble gas and drops sharply at the next alkali metal. The pattern was noticed before the cause was known; once quantum mechanics explained electron shells and subshells in the 1920s, chemists could see that periodicity comes from filling s, p, d and f orbitals in order. Not every property repeats exactly, since the period lengths change from 8 to 18 and then 32, so the periods are not all the same size. Mendeleev called his statement the periodic law, and in modern wording it says that the properties of the elements are a periodic function of their atomic number. Without this repeating pattern, the elements would simply be a long list. This recurring pattern is called periodicity, and it is what the name periodic table means.