Electron Affinity and Electronegativity

Electron affinity and electronegativity are the opposites of ionization energy. They measure how much you want an electron rather than how easily you lose an electron. The difference between the electron affinity and electronegativity is how we measure the “desire” of an element for electrons.

It’s All The Same

All of the trends are based on the same ideas and these two are no different. If you are a stronger element you are going to be able to grab onto additional electrons more easily. If you don’t have to reach very far to grab those electrons it is also easier to do so.

This means that as we add protons to the nucleus from left to right both the electronegativity and electron affinity increase. The noble gases are excluded and we see a really big drop in both electronegativity and electron affinity. As we have already seen they are their most stable selves adding another electron only messes this up.

Again as we remove shells it becomes easier and easier to grab up electrons. Thus as we move from the bottom of the periodic table to the top the electronegativity and electron affinity increase. With the trend looking just like Zeff and ionization energy.

Electron Affinity Versus Electronegativity

So what’s the difference? As I mentioned earlier it is a difference in how we measure the “desire” of an element. In electron affinity, we measure how much energy is released or gained when an electron is added to an element. Negative values mean that energy was released when an electron is added.

While negative energy might sound like a bad thing it is simply an indication of favorability. For example, consider element X that has an electron affinity of -400J and a different element Y that has an electron affinity of -200J. Adding the electron to element X was more favorable than adding an electron to element Y. As a result we would say that element X has a higher electron affinity.

In contrast, electronegativity describes the ability of an element to draw an electron involved in a bond towards itself. What we are measuring here is relative. It is a constructed scale analogous to measuring pain on a 0 to 10 scale. Why couldn’t it be from 0 to 5 or 50 to 100? For electronegativity, we call this the Pauling Scale and it ranges from 0.79 to 3.98.

Weirdly the AAMC does expect us to know the electronegativity (EN) values of period 2 and hydrogen. My best guess is that you won’t be expected to recall an exact value as an answer choice, but rather use this information in the course of answering a question about something else likely passage-based.

The easiest way to recall these is to focus on memorizing the rough values of hydrogen and lithium in addition to a trend in the EN increases across the period. Hydrogen has an EN of 2.0 and Li has an EN of 1.0. Then from there as we go across the second period the EN values of each successive element increase by 0.5.