Which of the following is the
electron configuration for an atom of the halogen with the highest first-ionization
energy? A) 1s², B) 1s² 2s² 2p⁵, C) 1s² 2s²
2p⁶, D) 1s² 2s² 2p⁶ 3s¹, or E) 1s² 2s² 2p⁶ 3s¹ 3p².
Halogen is the collective term for
elements like fluorine, chlorine, bromine, and so on. The halogens are to be found in
group 17 of the periodic table, sometimes called group seven. The first-ionization energy is the
energy required to strip an atom of its outer electron. As we go up group 17, the
ionization energy of the element increases. It gradually takes more energy to
remove that outer electron. The complementary process is where
we add an electron to a neutral atom. And that’s called an electron
affinity. The relationship, as we move up and
down the group, with electron affinity is quite different.
So for this question, it’s vital
that you don’t get them confused. So the halogen with the highest
first-ionization energy is the halogen right near the top of the group,
fluorine. The atomic number of fluorine is
nine. So an atom of fluorine, being
neutral, must have nine electrons to balance out the nine protons in the
nucleus. So we start building up the
electron configuration for an atom of fluorine by adding two electrons to the 1s
subshell, leaving us seven electrons. A further two in the 2s subshell
gives us five electrons.
Remember that s subshells can only
fit two electrons each. The next subshell is the 2p
subshell, which has a maximum occupancy of six electrons. So we can put all five of our
remaining electrons in the 2p subshell. This corresponds with answer B.
The reason for this trend in the
first place is that as we go from the bottom to the top of group 17, we have smaller
and smaller atoms. And the outer electron gets closer
and closer to the nucleus, experiencing a greater force of attraction, requiring
more energy to remove. And the shielding from core
electrons lessens going from iodine to bromine to chlorine to fluorine. Meaning that the electron
configuration for an atom of the halogen with the highest first-ionization energy,
that’s fluorine, is 1s² 2s² 2p⁵.