Which period do the elements where first ionisation energy takes place?

The first ionization energy of an element typically occurs in the period in which the element is located on the periodic table. For example, the first ionization energy of elements in the first period (e.g. hydrogen and helium) occurs in the first period, while the first ionization energy of elements in the second period (e.g. lithium and beryllium) occurs in the second period, and so on.

Third period