Electron Dot Diagrams, Lewis Dot Symbols, and Lewis Structures
Electron dot diagrams, also known as Lewis dot symbols, are a way of quickly communicating how many electrons an atom has. Electron dot diagrams were first professionally used by Gilbert Lewis, who wrote an article titled "The Atom and the Molecule" in 1916, in hopes of explaining his scientific work with atomic particles to the rest of the world. He used electron dot diagrams to explain the electron placement in one atom, and then went on to show his very own "Lewis structures," which combine multiple atoms using electron dot diagrams, and show bonding electrons as well as lone pairs of electrons. It’s very important to know how to draw both electron dot symbols and diagrams, as well as Lewis structures, because you can get a lot of relevant information from these structures, including electron location, number and types of bonds, and geometric structure. In other words, you can tell what "shape" these atoms will take on by drawing its Lewis structure and counting the number of bonds you have placed. In this section, we’ll show you how to determine electron dot symbols and Lewis structures based on what we already know about electrons.
Electron Dot Diagrams
Electron dot diagrams, also known as electron dot structures or symbols, show electron location in an atom. In order to start an electron dot diagram, you need to write the element’s symbol (either one letter or two letters) like this:
Next, we are going to count valence electrons of the element. In our example, we listed helium, He. Helium has 2 valence electrons. We know this because we counted to see which group helium is in, which is IIA. Because it’s the second element in its row, we know the configuration would be 1s2, which means it has two valence electrons. Now, using a dot to represent each electron, we’re going to place them around the element’s symbol, like this:
Notice that we did not place the electrons directly next to each other on the same side. Electrons fill the orbitals according to the diagonal rule. The order in which electrons are written around the element symbol is this:
The numbers around the element symbol indicate the order in which electrons are placed around the atom. Notice that we placed helium’s two electrons in the spaces marked 1 and 2, and left the rest blank. There are 8 total spaces, because we know that an atom can have up to 8 valence electrons, with 8 being the most stable (and a noble gas). The following diagram shows the placement of electrons from 1-8. We’re going to use X as a generic element symbol. When you do this, you would use the element’s symbol rather than X.
Now that you know how the electron dot diagrams work, you can begin to put atoms of different elements together to form compounds. When we put two or more electron dot diagrams together, we call the resulting figure a Lewis structure. These can also be called Lewis dot structures or electron dot structures. For our purposes, we are going to call them Lewis structures. Lewis structures allow us to see how atoms bond and their shape after they’ve bonded. Lewis structures also allow us to easily look at the electron configuration of the compound. We’ll give you a couple of examples of what Lewis structures look like, and how to draw different structures in order to determine the shape of the molecule.
The first example we’ll show you is of the compound CH4. The first thing to do is determine the central atom, which we can recall is the atom with the least electronegative value, aside from hydrogen. So, we know that hydrogen will never be our central atom. If the compound only has one other element in addition to hydrogen, we know that other element has to be the central atom. In this case, we only have carbon in addition to hydrogen, so we know our central atom is going to be carbon. Then, we can arrange the electron dot diagrams, like this:
Now, we can see the electrons of each atom. We know that for covalent and hydrogen bonding, we need two electrons to pair in order to form a bond. We look at our diagram and see that each of the hydrogen atoms has one electron, and each "side" of the carbon atom only has one electron. Therefore, we can put bonds in between each of the electrons of carbon and hydrogen, like this:
The blue circles show bonding electrons. We can see here that all of carbon’s valence electrons easily bond with the hydrogen atoms surrounding it, which satisfies the octet rule for carbon. Once you know which electrons bond, you can replace the bonds with lines, like this:
This is called a Lewis structure.
VSEPR and Molecular Geometry
VSEPR stands for Valence Shell Electron Pair Repulsion, which is a theory that chemists use to predict the shape and bond angles of different molecules. VSPER is pronounced "ves-per" in everyday use. Basically, the theory says that the valence electron pairs around an atom repel each other mutually. This implies that the electrons will arrange themselves in such a way that this repulsion is minimized. The arrangement of electrons determines their molecular geometry (also known as shape). Keep in mind that this theory is a prediction, and though it is almost entirely accurate, you still must keep in mind that there is some room for variation in reality. This means that, if we could see all of the electrons in every atom, they may differ slightly from this predicted model.
Using the VSEPR theory, scientists have determined a user-friendly chart, which allows easy determination of molecular structure. In our chart, central atoms are in light blue, surrounding atoms are in gray, and lone pairs are in yellow. We’ve sorted our chart according to ascending number of bonds. In order to use this chart, you would first count the number of bonds the compound has. Then, you would count up the number of lone pairs it has. Please note that each lone pair = 1, when counting. The number of bonds plus the number of lone pairs equals the number of electron domains. Normally, compounds have four (or less) electron domains. However, some structures can accommodate five, six, or seven. Here’s a copy of the chart for you to use:
|Bonds||Lone Pairs||Electron Domains||Shape||Bond Angle||Image|
|3||2||5||T-shaped||90o, 180o; (87o, <>o)|
|5||0||5||trigonal bipyramidal||90o, 120o|
|7||0||7||pentagonal bipyramidal||90o, 72o|