What is 1st ionization energy?

With a few notable exceptions, the first ionization energy generally rises as the atomic number increases over the course of a period. In the second period, the II A is more than the III A due to the stability of the filled S subshell, while the I A ionization is less than the II A.

The VIA is less than the VIIA which is less than the VIIIA; VIIII is a completely filled second energy level and is very stable so removing an electron requires a great deal of energy. Then, the III A is less than the IV A which is less than the V A but the V A is more than the VI A due to the stability of the half filled p subshell.

Within a family, the first ionization energy typically decreases with increasing atomic number; the easier it is to remove an electron from the nucleus, the lower the first ionization energy.

The first ionization energy is the amount of energy required to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous ions with a single positive charge.