Why is Oxygen Electronegative?

Let’s face it, oxygen is a pretty important element. It creates the interesting properties of water, and fuels our bodies through the process of cellular respiration. It is a reactive element and likes to steal electrons from other atoms, releasing energy. This is the basis of many chemical reactions, from fire (a relatively fast chemical reaction) to rust (a much slower reaction). Oxygen’s fondness for electrons can be attributed to its electronegativity, which is the second highest on the periodic table. But this begs the question: why?

It’s a common misconception, namely in the context of water, that oxygen attracts electrons from the hydrogen atoms because it has more protons, and thus more positive charge, attracting the electrons with a much stronger pull than either hydrogen could muster. This isn’t quite the reason. It’s part of the reason, but not the whole story.

The main reason lies in the way oxygen’s electrons are configured. Neutral oxygen has 8 protons (straight from the periodic table) and therefore 8 electrons to balance out the positive charge. These electrons are arranged in shells of 2 and 6. Now, like all elements, oxygen would very much like to have 8 electrons in its outer shell so that it can look like a noble gas and reach a stable configuration. But it is 2 electrons short. So, oxygen is always trying to steal electrons from whatever atom is less willing to keep its electrons so that it can complete its outer valence shell.

So why isn’t nitrogen, which is missing 3 electrons from its valence shell, more electronegative than oxygen? That’s because nitrogen doesn’t have as many protons as oxygen does. The fact that oxygen has more protons allows it to keep a firmer grip on its electrons and the more positive charge on its nucleus allows it to attract electrons more easily than nitrogen. Likewise, fluorine (atomic number 9) has more positive charge in its nucleus, and with its one missing electron, it is trying to steal one from another atom, and its stronger nuclear charge makes that easier.

Electronegativity does not increase as the element becomes bigger, however. In fact, the opposite happens: it decreases. This may seem counterintuitive because just in the last paragraph I said that a larger nucleus increases electronegativity. But going down the periodic table, there are more inner electron shells that “muffle” the charge of the nucleus, weakening the pull the nucleus has on the outer shell. This decreases the electronegativity. So even though sulfur (atomic number 16) has 6 valence electrons, like oxygen, it has another electron shell between the nucleus and the valence shell, decreasing the nucleus’s pull. Oxygen doesn’t have much between its nucleus and valence shell, however, allowing the nucleus to pull harder on the electrons. Electronegativity still increases from left to right across the table, though, because of the aforementioned nucleus size and the electron configuration of the elements.

And that, in short, is why oxygen is so electronegative.