Lewis Structures 1) Write the element symbol. 2) Carbon is in the 4th group, so it has 4 valence electrons. 3) Starting at the right, draw 4 electrons, or dots, counter-clockwise around the element symbol. A Lewis dot structure is a drawing of a molecule. The drawing only “works” f0r stable molecules that actually exist. So it’s a nice tool to explore how atoms bond into more complex substances. A Lewis dot structure is also called a Lewis structure, a Lewis dot diagram, an electron dot structure, or a dot diagram.
- Lewis Dot Structure For H2o
- Lewis Dot Structure For Carbon
- Lewis Dot Structure For H2o
- Lewis Dot Structures
- Lewis Dot Structure Worksheet
Learning Objectives
By the end of this section, you will be able to:
- Write Lewis symbols for neutral atoms and ions
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.)
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?
- 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.
Lewis Dot Structure For H2o
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.
Lewis Dot Structure For Carbon
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)
A Lewis Structure is a very simplified representation of the valence shell electrons in a molecule. It is used to show how the electrons are arranged around individual atoms in a molecule. Electrons are shown as 'dots' or for bonding electrons as a line between the two atoms. The goal is to obtain the 'best' electron configuration, i.e. the octet rule and formal charges need to be satisfied.
Lewis Dot Structure For H2o
Note
Lewis structure does NOT attempt to explain the geometry of molecules, how the bonds form, or how the electrons are shared between the atoms. It is the simplest and most limited theory on electronic structure.
How to draw Lewis Diagrams
The following is an example of how to draw the 'best' Lewis structure for NO3- (learning by example).
- First determine the total number of valence electrons in the molecule. This will be the sum of the group number a of all atoms plus the charge.
| N | 5 |
| O (x 3) | 18 |
| charge | 1 |
| 24 |
Lewis Dot Structures
- Draw a skeletal structure for the molecule which connects all atoms using only single bonds. The central atom will be the one that can form the greatest number of bonds and/or expand its octet. This usually means the atom lower and/or to the right in the Periodic Table, N in this case.
- Now we need to add lone pairs of electrons. Of the 24 valence electrons available in NO3-, 6 were used to make the skeletal structure. Add lone pairs of electrons on the terminal atoms until their octet is complete or you run out of electrons.
- If there are remaining electrons they can be used to complete the octet of the central atom. If you have run out of electrons you are required to use lone pairs of electrons from a terminal atom to complete the octet on the central atom by forming multiple bond(s). In this case the N is short 2 electrons so we can use a lone pair from the left most O atom to form a double bond and complete the octet on the N atom.
- Now you need to determine the FORMAL CHARGES for all of the atoms. The formal charge is calculated by: (group number of atom) - (½ number of bonding electrons) - (number of lone pair electrons), i.e. see the figure below.
No Lewis structure is complete without the formal charges. In general you want:
Lewis Dot Structure Worksheet
- the fewest number of formal charges possible, i.e. formal charges of 0 for as many of the atoms in a structure as possible.
- the formal charges should match the electronegativity of the atom, that is negative charges should be on the more electronegative atoms and positive charges on the least electronegative atoms if possible.
- Charges of -1 and +1 on adjacent atoms can usually be removed by using a lone pair of electrons from the -1 atom to form a double (or triple) bond to the atom with the +1 charge. Note: the octet can be expanded beyond 8 electrons but only for atoms in period 3 or below in the periodic table. In our present example N can not expand beyond 8 electrons so retains a formal charge of +1, but the S atom below can expand its octet.
- You have determined the 'best' Lewis structure (octets completed and lowest formal charges) for NO3-, but there are a number of ways to show this structure. Although it is most common to use a line to indicate a bonding pair of electrons they can be shown as electrons, see the left most image below. It is also common to show only the net charge on the ion rather than all of the formal charges, i.e. see the right most figure below.
Why are there different ways for the 'same' Lewis structure? It depends what you want to show. While the most complete structure is more useful for the novice chemist, the simplest is quicker to draw and still conveys the same information for the experienced chemist. You should learn to recognize any of the possible Lewis structures.