The octet rule is just an observation of how main-group elements tend to combine in certain ways to fill their valence shell and become stable with 8 electrons, in order to gain the same electron configuration of the noble gases. A Lewis-Dot Structure can be used to show the bonding between atoms and how many electrons are in their valence layer. Some of the ways an atom can become fully stable with 8 valence electrons are through covalent bonding and ionic bonding. Covalent bonding is when two or more atoms share their electrons to fill their valence shell. We can use carbon dioxide as an example. There is one carbon atom with 4 valence electrons and 2 oxygen atoms with 6 each. In this case, the carbon atom can share 2 electrons to each oxygen atoms to fill their outer layers, and each oxygen can also share 2 of their electrons with carbon to give it the 4 electrons it needs to have 8. Now, both oxygen atoms and the carbon atoms are filled, stable, and have created water. Ionic bonding is when 2 or more atoms, usually metals with low electronegativity and non-metals with high electronegativity, trade their valence electrons in a way that they’ll all end up with 8. Electronegativity is the atoms’ attraction or ability to take in electrons. If an atom has over 4 valence electrons, they will take the amount needed to fill. If they have fewer than 4, they will lose the necessary to become stable. For example, sodium fluoride requires 1 sodium atom and 1 fluorine atom. Sodium has 1 valence electron and fluorine has 7. In this case, since sodium’s electronegativity is so low, it will lose its only valence electron to fluorine. When sodium loses that last electron, its previous layer becomes the new valence shell. When that lost electron goes to fluorine, it becomes stable with its full valence shell, just like sodium.
There are different ways to know how many valence electrons an element has. The easiest ways are looking at a Lewis Dot Diagram or simply at the periodic table. A Lewis Dot Diagram is the abbreviation of an element and a number of dots around it. Each dot around the abbreviation represents one valence electron. To figure it out looking at a periodic table all you have to do is find the element and look at what group it’s in. The columns in the periodic table are known as groups. They are numbered 1-18 but we’ll only be paying attention to numbers 1-2 and 13-18 because these are the main group elements. The group number symbolizes how many valence electrons it has, but for the double-digit groups 13-18, it only contains the amount shown in the second digit. For example, elements in group 14 have 4, elements in group 15 have 5, and so on. The downside to the periodic table is that the number of valence electrons in metals cannot be figured. A third way this could be figured out is through electron configurations. Electron configurations show the amount of electrons in each layer. The coefficients represent the layer, or energy levels, and the superscripts represent the amount of electrons. The electron configuration of sodium is 1s22s22p63s1. Its first energy level has a full layer, as well as the second. But the third only has 1 electron. This method works for any atom, including metals.