High School Chemistry/Lewis Electron Dot Diagrams

This chapter will explore yet another shorthand method of representing the valence electrons. The method explored in this lesson will be a visual representation of the valence electrons. We will, as we observed in the previous lesson, finish the lesson with a look at how this visual representation flows in a pattern throughout the Periodic Table.

Lesson Objectives

 * Explain the meaning of an electron dot diagram.
 * Draw electron dot diagrams for given elements.
 * Describe the patterns of electron dot diagrams in the Periodic Table.

A Simplified Way to Show Valence Electrons
As defined earlier in this chapter, core electrons are all the electrons except the valence electrons and valence electrons are the electrons in the outermost energy level. Valence electrons are the electrons responsible for chemical reactions. Here is the electron configuration for sodium.


 * Electron configuration sodium.svg

The electron configuration is: 1s22s22p63s1. The core electrons are 1s22s22p6. The valence electron is 3s1. One way to represent this valence electron, visually, was developed by Gilbert N. Lewis. These visual representations were given the name Lewis electron dot diagrams. Lewis suggested that since the valence electrons are responsible for chemical reactions and the core electrons are not involved, we should use a diagram that shows just the valence electrons for an atom.

To draw a Lewis electron dot diagram for sodium you can picture the symbol for sodium in a box with the box having four sides. Each side of the box represents either the s or one of the three p orbitals in the outermost energy level of the atom. The first and second valence electrons are placed on the side representing the s orbital and the next electrons are placed in the p orbitals. Just as the electrons are placed singly before being doubled up in the orbital representation, so they are placed one at a time in the p orbitals of an electron dot formula. A single dot represents one valence electron. Thus, the Lewis dot formula for sodium is:


 * Lewis dot Na.svg

Look at the electron configuration for magnesium. Magnesium is element #12.


 * Electron configuration magnesium.svg

To draw the Lewis electron dot diagram we picture in our minds the symbol for Mg in a box with all of its core electrons (i.e., 1s22s22p6). Then we place the valence electrons around the sides of the box with each side representing an orbital in the outermost energy level. How many valence electrons does magnesium have? There are 2 valence electrons, 3s2.

Therefore the Lewis electron dot formula for magnesium is:


 * Lewis dot Mg.svg

Look at the electron configuration for chlorine.


 * Electron configuration chlorine.svg

The electron configuration for chlorine could be written as: 1s22s22p63s23p5. The core electrons would be 1s22s22p6 while the valence electrons would be in the third shell (or where n = 3). Therefore, chlorine has 7 valence electrons. The Lewis electron dot diagram would look like the following:


 * Lewis dot Cl.svg

Notice that the electrons are in groups of two. Think of the chlorine in a box and the box has 4 sides. Each side can have 2 electrons on it. Therefore there can be a maximum of 2 × 4 = 8 electrons normally on any Lewis electron dot diagram.

Write the Lewis electron dot formula for:
 * (a) Oxygen
 * (b) Sulfur
 * (c) Potassium
 * (d) Carbon

Solution:

(a) Oxygen has the electron configuration: 1s22s22p4, therefore there are 2 core electrons and 6 valence electrons. The Lewis electron dot formula is:
 * Lewis dot O.svg

(b) Sulfur has the electron configuration: 1s22s22p63s23p4, therefore there are 10 core electrons and 6 valence electrons. The Lewis electron dot formula is:
 * Lewis dot S.svg

(c) Potassium has the electron configuration: 1s22s22p63s23p64s1, therefore there are 18 core electrons and 1 valence electron. The Lewis electron dot formula is:
 * Lewis dot K.svg

(d) Carbon has the electron configuration: 1s22s22p2, therefore there are 2 core electrons and 4 valence electrons. The Lewis electron dot formula is:
 * Lewis dot C.svg

All the Elements in a Column Have the Same Electron Dot Diagram
In the previous lesson, it was shown that using the earlier version for numbering the periodic table, we could see a pattern for finding the number of valence electrons in each of the groups in the main group elements. Since the Lewis electron dot diagrams are based on the number of valence electrons, it would hold true that the elements in the same group would have the same electron dot diagram. In other words, if every element in Group 1A has 1 valence electron, then every Lewis electron dot diagram would have one single dot. Take a look at the table below.

The same pattern holds for all elements in the main group. Look at Table 9.8 for the Lewis dot diagrams for Group 2A elements.

A similar pattern exists for the Lewis electron dot diagrams for Group 3A.

Lesson Summary

 * A Lewis Electron Dot Diagram is used to visually represent only the valence electrons.
 * The core electrons are symbolized using the symbol of the element with the valence electron dots surrounding the element symbol.
 * The maximum number of valence electron dots in the Lewis electron dot diagram is 8. Two electrons can go on each side (top, bottom, left, and right).
 * Electrons are added to the electron dot formula by placing the first two valence electrons in the s orbital, then one in each p orbital until each p orbital has one electron, and then doubling up the electrons in the p orbitals.
 * Each element in a group has the same number of valence electrons and therefore has the same Lewis electron dot diagram. (Same number of dots, different symbol.)

Review Questions

 * 1) What is the advantage of the Lewis electron dot diagram?
 * 2) What is the maximum number of dots in a Lewis's electron dot diagram?
 * 3) Draw the Lewis electron dot diagram for lithium.
 * 4) Draw the Lewis electron dot diagram for calcium.
 * 5) Draw the Lewis electron dot diagram for bromine.
 * 6) Draw the Lewis electron dot diagram for selenium.
 * 7) Write the Lewis electron dot diagrams for each of the following. What observations can you make based of these diagrams?
 * (a) Oxygen 2- ion
 * (b) Sulfur 2- ion
 * (c) Antimony
 * (d) Aluminum
 * 1) Write the trend for the Lewis electron dot diagrams for Group 6A (or Group 16) by filling in the table below.

Vocabulary

 * Lewis electron dot diagram
 * A shorthand visual representation of the valence electrons for an element.