When comparing the successive ionization energies of an element, an unusually big increase in ionization energy is seen when

a. the first valence electron is removed
b. the second valence electron is removed
c. the first core electron is removed
d. the eighth electron is removed
e. the last valence electron is removed

When comparing the successive ionization energies of an element, an unusually big increase in ionization energy is seen when the first core electron is removed.

The ionization energy refers to the energy required to remove an electron from an atom. The ionization energy increases across the period as atoms become smaller but decreases down the group as more shells are added.

An atom has as many ionization energies as there are electrons in the atom. However, these ionization energies increase smoothly for electrons on the same shell. An unusually big increase in ionization energy is seen when the first core electron is removed.

Learn more: link

The first valence electron is removed.

Explanation:

Ionization energy is the energy that is needed to tear out the last electron of an atom in its ground state or gaseous state.

A(g)  + IE > A+(g)    + 1e-

When we talk about starting an electron we refer to the last valence electron, since it is the one that is furthest from the atomic nucleus, the one that does not feel attracted. As the question asks us the opposite, the ionization energy is going to be extremely high, if the electron to be removed occupies the first valence layer, since it is totally attracted to the nucleus.