The Octet Rule: Help, Definition, and Exceptions

The octet rule is a very general rule that allows us to predict how atoms will form chemical bonds to make molecules. It's used extensively in drawing Lewis structures. When elements have eight electrons in their outer shell, they have an octet. The octet rule is based on observations of how atoms behave in the real world. It's a general rule, and there are numerous exceptions, but it works very well for some of the most common elements that we deal with in science.

So, how does the octet rule work? Let's look at hydrogen and chlorine and see how they bond to form HCl, hydrochloric acid. Since both are nonmetals, they share electrons and form a covalent bond. Chlorine has 7 valence electrons in its highest energy level, sometimes called the outer shell. To obtain an octet, it needs an additional electron. Hydrogen has one valence electron. Hydrogen is an important exception to the octet rule; hydrogen only needs two valence electrons to have a full outer shell.

Instead of an octet, think of it as a duet.

A covalent bond is formed when the hydrogen comes together with the chlorine and forms a chemical bond by sharing valence electrons. The hydrogen, which only had one, by sharing now has two. And you remember that hydrogen only needs two to fill its highest energy level. The chlorine had seven, but by sharing with the hydrogen, it now has eight. That's a full outer shell for chlorine, and it's called an octet.

But what if we had two hydrogen atoms bonding? For hydrogen, which only needs two valence electrons to have a full outer shell, the hydrogen would join with another hydrogen to form a chemical bond. Now both hydrogens would have two valence electrons in their highest energy level. If we had two chlorine atoms coming together, they have seven in their outer shell right now, but they come together and form a chemical bond. Now both have eight in their outer shell, an octet, by sharing valence electrons.

Back to our HCl molecule: hydrogen and chlorine normally exist as H2 and Cl2 in the real world, so this is a bit of an oversimplification. But it's how we think about the bond between the H and the Cl forming to make HCl. To recap, the octet rule works well for many of the molecules we use in science, especially biology. But be careful; hydrogen only needs two to have a full outer shell.

For ionic compounds, it's a little bit different.

Here we have a sodium and a chlorine atom. The sodium is a metal, and the chlorine is a non-metal, which means it's going to be an ionic compound. Ionic compounds don't share valence electrons; they transfer them. So in this case, the sodium is going to lose its valence electron to the chlorine. The chlorine had seven; now it has eight in an octet.

Now, when we look at the sodium, it doesn't have anything in that outer shell. In fact, that outer shell goes away, and underneath it are eight valence electrons in that new outer shell, so it has an octet. The sodium lost an electron; it lost a negative charge, so it's going to be positive. The chlorine gained an electron, and electrons are negative; therefore, the chlorine is going to have a negative charge. Because of that, these two atoms are going to be attracted to each other, and that is how the chemical bond forms in an ionic compound.

Often, we'll put brackets around the chlorine and a negative sign, and then a positive sign on the sodium to show that they're attracted and they form this ionic bond. A word of caution: NaCl is considered a formula unit of a larger crystalline structure. It doesn't really exist by itself, but the idea of the octet rule still holds in this lattice structure.

When we draw Lewis structures to describe the arrangement of atoms in molecules, the octet rule is a very useful guide. But just remember, it's only a guide. There are many exceptions, like hydrogen, and it doesn't always work for all elements. An octet isn't the only reason atoms bond; electrostatic attraction between atoms is important to consider as well as the energy changes involved when the bonds form.

But the octet rule is a useful guide in predicting how atoms bond, and it mostly matches how the main group elements form chemical bonds in the real world.

Finally, atoms form bonds to minimize energy and become more stable. That's a core idea that helps explain why the octet rule works for so many elements. This is Dr. B, and thanks for watching.