Question

The first four ionization energy values of an element are 120, 240, 520, and 6420 k cal. The number of valence electrons in the element is:

Answer 1

There is a very high difference between third ionization energy (520 kcal) and fourth ionization energy (6420 kcal), thus it has three electrons in its valence shell. This is because when three electrons are lost, the noble gas electronic configuration is obtained. When the fourth electron is lost, the noble gas electronic configuration is to be broken.