Where Would The Electron Go?

The first task using the Ross model that I give students is to make a decision on a selection of puzzles. These puzzles introduce the students to the concept of electronegativity, or its first cousin, electron affinity. 

Each puzzle consists of a representation of two atoms. 

Imagine that two different atoms are sitting close to each other. Imagine that you can drop a single electron into the space between the two atoms. Now imagine that the electron will move in such a way as to:

• occupy a vacancy in one of the valence shells, so that
• it is as close as possible
• to the largest core charge available

 Try it yourself, in the eight pairs of atoms in the diagram below. A short discussion follows the diagram. But hey! You are all teachers... So don't cheat!

1. The electron will be most strongly attracted to the place where it can get as close as possible to the strongest core charge possible. That is, it will occupy the only vacancy in the valence shell of the fluorine atom.
2. The two core charges, at 7+, are equally strong. The electron will move to the atom whose single vacancy is closest to the strong core charge. The electron is most strongly attracted to fluorine.
3. The two core charges, at 1+, are equally weak, in fact, the weakest possible core charge. Once again, the electron will occupy a vacancy in the atom with the smallest radius, that is, lithium. 
4. Neon has the smallest radius and the largest core charge. An electron in the neon atom's valence shell will be more strongly attracted than a valence electron in an oxygen atom. But neon's valence shell is already full! there is no more room there. The electron moves to occupy one of the two vacancies in the oxygen valence shell.
5. A sodium atom and a hydrogen atom are competing for the "free" electron. Both atoms have a weak 1+ core charge. The electron can get extremely close to the core of the hydrogen atom, and the attraction there will be much larger. The electron moves to the hydrogen atom.
6. Finally... carbon and phosphorus. Which atom will attract the electron the strongest? Will it be phosphorus, with its greater core charge? Or will it be carbon, with its smaller radius?  

It's interesting, isn't it, that most human beings (and certainly your students) experience the intense ambivalence of that last case. Why is that? My answer is that the powerful emotional tension of the ambivalence is caused by the very reasoning structures that students most often use. I really must write another blog about that.

This is the place to introduce the experimentally measured quantity electronegativity. That quantity is best introduced as one of the periodic trends, over at the periodic table.