Lewis Dot Structure
Learning Objectives
By the end of this section, you will be able to:
- Write Lewis symbols for neutral atoms and ions
A Lewis structure is a graphic representation of the electron distribution around atoms. The reason for learning to draw Lewis structures is to predict the number and type of bonds that may be formed around an atom. A Lewis structure also helps to make a prediction about the geometry of a molecule.
Lewis Symbols of Monoatomic Elements
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 electron dot diagram (or electron dot diagram or a Lewis diagram or a Lewis 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.)
- Lewis Dot Structures - Definition and Example Lewis structure is basically a graphic representation of the electron distribution around an atom. The major reason why learning Lewis dot structure is important is that it helps in predicting the number and type of bonds which can be formed around an atom.
- Methanol, H 3 COH, is used as the fuel in some race cars. Ethanol, C 2 H 5 OH, is used extensively as motor fuel in Brazil. Both methanol and ethanol produce CO 2 and H 2 O when they burn. Write the chemical equations for these combustion reactions using Lewis structures instead of chemical formulas.
- Lewis Dot Structures can be produced by following a sequence of steps. Let’s produce a Lewis Dot Structure for: NH 4 + (the ammonium ion). Step 1: Count valence electrons: N = 5 4 x H = 4 x 1 = 4 “+” = -1 Total = 5+4-1= 8 electrons = 4 bonds and lone pairs. Step 2:!Arrange the atoms (identify a central atom, if possible).
For example, the Lewis electron dot diagram for calcium is simply
Figure 1 shows the Lewis symbols for the elements of the third period of the periodic table.
Figure 1. Lewis symbols illustrating the number of valence electrons for each element in the third period of the periodic table.
Lewis symbols can also be used to illustrate the formation of cations from atoms, as shown here for sodium and calcium:Likewise, they can be used to show the formation of anions from atoms, as shown below for chlorine and sulfur:Figure 2 demonstrates the use of Lewis symbols to show the transfer of electrons during the formation of ionic compounds.Figure 2. Cations are formed when atoms lose electrons, represented by fewer Lewis dots, whereas anions are formed by atoms gaining electrons. The total number of electrons does not change.
Example 1: Writing Lewis DoT SYmbols of Elements
What is the Lewis electron dot diagram for each element?
- aluminum
- selenium
The valence electron configuration for aluminum is 3s23p1. So it would have three dots around the symbol for aluminum, two of them paired to represent the 3s electrons (or three single dots around the atom):
The valence electron configuration for selenium is 4s24p4. In the highest-numbered shell, the n = 4 shell, there are six electrons. Its electron dot diagram is as follows:
Check Your Learning
What is the Lewis electron dot diagram for each element?
- phosphorus
- argon
Example 2: Writing Lewis DoT SYmbols of Ions
What is the Lewis electron dot diagram for each ion?
Lewis Dot Structure For Co2
- Ca2+
- O2−
Having lost its two original valence electrons, the Lewis electron dot diagram is just Ca2+.
Ca2+
The O2− ion has gained two electrons in its valence shell, so its Lewis electron dot diagram is as follows:
Check Your Learning
The valence electron configuration of thallium, whose symbol is Tl, is 6s25d106p1. What is the Lewis electron dot diagram for the Tl+ ion?
Key Takeaways
- Lewis electron dot diagrams use dots to represent valence electrons around an atomic symbol.
- Lewis electron dot diagrams for ions have fewer (for cations) or more (for anions) dots than the corresponding atom.
Exercises
1. Explain why the first two dots in a Lewis electron dot diagram are drawn on the same side of the atomic symbol.
2. Is it necessary for the first dot around an atomic symbol to go on a particular side of the atomic symbol?
3. What column of the periodic table has Lewis electron dot diagrams with two electrons?
4. What column of the periodic table has Lewis electron dot diagrams that have six electrons in them?
5. Draw the Lewis electron dot diagram for each element.
a) strontium
b) silicon
6. Draw the Lewis electron dot diagram for each element.
a) krypton
b) sulfur
7. Draw the Lewis electron dot diagram for each element.
a) titanium
b) phosphorus
8. Draw the Lewis electron dot diagram for each element.
a) bromine
b) gallium
9. Draw the Lewis electron dot diagram for each ion.
a) Mg2+
b) S2−
10. Draw the Lewis electron dot diagram for each ion.
a) In+
b) Br−
11. Draw the Lewis electron dot diagram for each ion.
a) Fe2+
b) N3−
12. Draw the Lewis electron dot diagram for each ion.
a) H+
b) H−
Show Select Answer1. The first two electrons in a valence shell are s electrons, which are paired.
3. The second column of the periodic table
5.
a)
b)
7.
a)
b)
9.
a) Mg2+
b)
11.
a) Fe2+
b)
Every chemistry student has to learn how to draw Lewis Dot Structures. The key is to understand the steps and practice. Lewis Structures are important to learn because they help us predict:
- the shape of a molecule.
- how the molecule might react with other molecules.
- the physical properties of the molecule (like boiling point, surface tension, etc.).
That helps us understand and predict interactions with things like medicine and our body, materials used to make buildings and airplanes, and all sorts of other substances. Lewis structures don't tell us everything, but along with molecule geometry and polarity they are hugely informative.
Lewis Dot Structure 03
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