Chemistry Question?

Electronic configuration of As : [Ar] 3d¹⁰ 4s² 4p³
Electronic configuration of Se : [Ar] 3d¹⁰ 4s² 4p⁴

In ionization of both As and Se, an electron is removed from 4p subshell of each atom.
As 4p subshell of As atom is half-filled and exhibits extra stability, thus more energy is needed to remove one electron from 4p subshell of As.
On the other hands, removal an electron from the 4p shell of a Se atom will lead to a half-filled 4p subshell which exhibits extra stability, and thus less energy is needed to remove one electron from Se atom.

Conclusively, As has a higher ionization energy than Se.

Ionization energy....

As has a slightly greater ionization energy than Se for the same reason that N is greater than O, and P is greater than S.

The ionization energy drops from As to Se because it requires less energy to remove an electron from an atom which is already at a higher energy state.

Arsenic
.....4p ↑_ ↑_ ↑_ ............ three unpaired electrons
4s ↑↓

Selenium
.....4p ↑↓ ↑_ ↑_ .......... two unpaired electrons. The pair of electrons
4s ↑↓ ......................... in the 4p orbital are at a higher energy because
................................... there is electron-electron repulsion.

Therefore, less additional energy is needed to remove one of those electrons from the atom, and the ionization energy is lower.

because it's more poisonous

I know that's ot the answer: The higher ionization MAKES it it more poisonous for humans. But why it is, idk.