Video: Identifying the Electron Configuration of an Atom of the Halogen with the Highest First-Ionization Energy in a Set of Electron Configurations

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 [E] 1s² 2s² 2p⁶ 3s¹ 3p²

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Video Transcript

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.

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