29 7.2 Lewis Dot Structures
Learning Objective
By the end of this section, you will be able to:
- Draw a Lewis electron dot diagram for any atom or a monatomic ion with an atomic number of less than 20.
In almost all cases, chemical bonds are formed by interactions of valence electrons in atoms. To facilitate our understanding of how valence electrons interact, a simple way of representing those valence electrons would be useful.
A Lewis dot structure is a representation of the valence electrons of an atom that uses dots around the symbol of the element. The number of dots equals the number of valence electrons in the atom. These dots are arranged to the right and left and above and below the symbol, with no more than two dots on a side. (It does not matter what order the positions are used.) For example, the Lewis electron dot diagram for hydrogen is simply
Because the side is not important, the Lewis electron dot diagram could also be drawn as follows:
The electron dot diagram for helium, with two valence electrons, is as follows:
By putting the two electrons together on the same side, we emphasize the fact that these two electrons are both in the first shell. The next atom, lithium, has three electrons total but only one electron in its valence shell. Its electron dot diagram resembles that of hydrogen, except the symbol for lithium is used:
Carbon has four valence electrons. We draw the dots for the electrons on different sides. As such, the electron dot diagram for carbon is as follows:
With nitrogen, we distribute the five valence electrons around the atom, using four positions and not pairing up any electrons until necessary:
Oxygen, with a total of six valence electrons, ends up with two unpaired electrons and two sets of paired electrons.
Fluorine and neon have seven and eight dots, respectively:
With the next element, sodium, the process starts over with a single electron because sodium has a single electron in its highest-numbered shell, the n = 3 shell.
Example 1
What is the Lewis electron dot diagram for each element? a) phosphorous b) argon
Solution
a) Phosphorous has five valence electrons, and argon has eight. 
Elements in the same column of the periodic table have similar Lewis electron dot diagrams because they have the same valence shell electron configuration. Thus the electron dot diagrams for the first column of elements are as follows:
Lewis dot structures are particularly useful for describing covalent bonding in compounds. They are somewhat but less helpful for describing ions. As a reminder, covalent bonding occurs when nonmetal elements form bonds, so drawing covalent compounds with Lewis structures only requires drawing atoms of these nonmetal elements. Unless you want to delve deeper, there is no need to learn about drawing Lewis structures for metals and ions.
Key Concepts and Summary
Lewis dot structures represent atoms with their atomic symbol surrounded by valence electrons, which are represented as dots. This type of symbolic representation can help describe compound formation, especially for covalent compounds.
Review-Reflect, Extend
Review-Reflect
1. What column of the periodic table has Lewis electron dot diagrams with two electrons?
2. Draw the Lewis electron dot diagram for silicon.
Extend
We know that water can be broken down into elemental hydrogen and elemental oxygen, and that the ratio of these gases that form is 2:1. Since both of these gases are diatomic (H2 and O2), water is made of H and O atoms in a 2:1 ratio as well. The observable properties of water indicate that these atoms are bonded together by covalent, rather than ionic bonds.
Draw Lewis dot structures for two hydrogen atoms and one oxygen atom. Attempt to arrange these three atoms so that they are sharing electrons. A finished “correct” structure should have every atom in the structure, once the sharing arrangements are made, with an electron arrangement that could be seen as “complete” or a “full shell.”
Answers to Review-Reflect
1. the second column of the periodic table
2.
