I want to talk to you guys about a basic rule in chemistry called the octet rule. So we know that atoms love to be stable and the way an atom can achieve stability is by having a complete number of electrons, valence electrons, in the outer energy level. These valence electrons will allow atoms to have different properties. The octet rule states that an atom will gain or lose electrons to obtain eight electrons in its outer shell.
In other words, elements want to reach an electron configuration of the nearest noble gas. The word octet means a group of eight. For example, an octopus is an animal with eight legs. An octagon is a shape with eight sides. So let's look at a model of an atom.
We know that an atom has a nucleus, where the protons and neutrons are located. Around the nucleus, we know that there are electron shells, where all the electrons are located. The first electron shell in an atom can hold two electrons maximum.
The electrons in the outermost shell are called valence electrons, which determine the properties of the atom. The second electron shell can hold a maximum of eight electrons. In this case, the electrons in the second electron shell are called valence electrons. The third electron shell can also hold a maximum of eight electrons.
In this case, the electrons in the third electron shell are called valence electrons, and so on. So let's look at the element lithium. Lithium is in group 1 of the periodic table, and since it's in group 1, it has one electron in its outermost shell.
Another example is an element that is in group 16 of the periodic table. It will have 6 valence electrons. Elements in the same column as group 16 will also have 6 valence electrons. This is a general way to figure out the number of electrons for an element. Elements like to be in their most stable electron configuration.
So the easiest thing for lithium to do is lose that one electron in its outermost shell to obtain the electron configuration of the noble gas helium. Helium is a very stable element because of its electron configuration. Sodium, like lithium, is also in group 1 and has one balanced electron.
Notice the lonely valence electron in the third shell of the atom. This shell needs seven more electrons to satisfy the octet rule, but the easiest thing for sodium to do is lose that electron if it can. And when it does, sodium will now have the electron configuration of the noble gas neon.
Now let's look at chlorine. Chlorine is in group 17, which indicates it has seven valence electrons. So looking at the chlorine atom, notice how the outermost shell needs one more electron to satisfy the octet rule and have eight electrons to be stable.
That's why chlorine is a very reactive element, because it just needs one more electron. And when it does, it will have the stable electron configuration of noble gas argon. And that is the octet rule.