Electronegativity: Its Periodicity Across the Table

Electronegativity is often taught as one of those "authoritative" numbers. Someone in authority has measured it, and put it into text books, and teachers use those numbers to make tests more difficult.

I think that there is a better way to handle this. In this post over in The Ross Model of the Atom, I present a simple, intuitive model of the essence of electronegativity. With this approach, electronegativity is actually a solution to a problem that students encounter when they try to predict the behaviour of the atom.

As you probably know, there are several electronegativity systems that differ slightly. To keep confusion to a minimum, I've decided to keep the electronegativity to just two significant figures instead of three. In addition, for the second row of the table, I've chosen to use numbers from two different systems, so that the electronegativity numbers are very easy to memorize. This might seem a little "royal" of me, but remember... this is a pedagogical table, a table for learning. To require a student to learn a few small adjustments in later studies seems to me to be a good price to pay for getting the basic idea right in the first place.

So... In the second row, the electronegativity numbers are:

This series is very easy for beginning chemistry students to recall.

All of the atoms in the third row have valence shells that are just a little bit larger. Their valence electrons are a little farther away from the core charge, so all of the electronegativities are less than those for the atoms above them. This effect is greatest for the halogens ( chlorine is 1.0 less than fluorine ) and smallest for the alkali metals ( sodium is 0.1 less than lithium ). Comparing row three and row two elements, the difference increases monotonically across the row.

Now, memorizing all of the electronegativity numbers beyond row two is not the objective here. Making them appear logical and reasonable is the objective.