Notes
1.3 Molecular Lewis Structures
Sections
Molecular Lewis Structures
Steps for drawing Lewis structures of molecules:
- Calculate the number of electrons that each atom will contribute
- Determine the predicted valency of each atom
- Place the atom with the highest valency in a central position
- Show the connected electrons as lines
- Determine if each atom has eight electrons in their valence shell (except for hydrogen, which should have two electrons in its valence shell)
As examples, draw Lewis structures of carbon disulfide (CS2) and carbonic acid (H2CO3).
Full-Length Text
- Here, we will learn how to draw Lewis structures of molecules.
- Modeling molecules as Lewis structures allows us to show the contributions of individual atoms to the structural entity as a whole.
- Show the first 18 elements of the periodic table. The periodic table contains the electron count for each atom.
We will move through the drawing of each compound step-by-step.
Example 1.
Write that the molecular formula of carbon disulfide is CS2.
- We want to draw one carbon atom and two sulfur atoms connected by their shared electrons.
- Step 1: calculate the number of electrons that each atom will contribute.
- We know the number of electrons in an atom is the same as its group number in the periodic table.
- 4 for carbon
- 6 for sulfur.
- Step 2: determine the predicted valency of each atom.
- This will help us propose the best arrangement for the three atoms.
- Show that:
- Carbon is tetravalent. It forms 4 bonds.
- Sulfur is divalent. It forms 2 bonds.
- We know sulfur should be divalent because is in the same group as oxygen: it has the same number of valence electrons, thus it follows the same bonding pattern.
- Step 3: Place the atom with the highest valency in a central position.
- So, surround the carbon with its four valence electrons.
- On either side of the carbon, put two S's to represent the sulfur atoms.
- Surround each S with its 6 valence electrons.
- Indicate with a dotted line that we can form a bond between the carbon and the sulfur on the right. And the sulfur on the left.
- There are still available electrons on both the carbon and the two sulfur atoms, so connect two more pairs of electrons.
- Step 4: Show the connected electrons as lines.
- Lines indicate bonds. Single bonds are shown as one line, double bonds as two lines, and triple bonds as three lines.
- Dots indicate nonbonding electrons (lone pairs).
- Step 5: Determine if each atom has an octet (eight electrons in their valence shell), which gives second and third row elements a stable electron configuration.
- First-row elements have only two electrons in their valence shell.
- Write that carbon has access to 4 bonds x 2 electrons/bond = 8 electrons, so it obeys the octet rule.
- Write that each sulfur has access to 2 bonds x 2 electrons/bond + 4 nonbonding electrons = 8 electrons, so they also obey the octet rule.
Example 2.
As a second example, write that we will draw the Lewis structure for the molecule with the formula H2CO3, carbonic acid.
- Step 1. The number of electrons contributed by each atom is:
- 1 for hydrogen
- 4 for carbon
- 6 for oxygen
- Step 2. The valency of the atoms can be ranked as follows:
- Carbon, tetravalent
- Oxygen, divalent; and
- Hydrogen, monovalent
- Let's start drawing the Lewis structure with the carbon atom in the center.
- Next, add the three oxygens above, to the left, and to the right so that they surround the carbon.
- Lastly, connect the two hydrogens at peripheral locations to the oxygens on the left and right sides of the carbon.
- Step 3. Let's first connect electrons between the atoms that can form multiple bonds:
- 1 single bond from the central carbon to the oxygen on the left
- 1 single bond to the oxygen on the right
- 1 double bond to the oxygen above
- Next, connect the hydrogen atoms with single bonds to their adjacent oxygen atoms.
- Step 4.
- Re-draw the structure with lines between the connected atoms.
- Step 5. Calculate the number of electrons on each atom:
- Carbon has 4 bonds x 2 electrons/bond = 8 electrons
- The oxygen above the carbon has 2 bonds x 2 electrons/bond + 4 nonbonding electrons = 8 electrons
- The oxygens to the right and left have: 2 bonds x 2 electrons/bond + 4 nonbonding electrons
- Therefore, all carbon and oxygen atoms follow the octet rule.
- The hydrogens have 1 bond x 2 electrons/bond = 2 electrons
- Therefore, all hydrogen atoms follow the duet rule.
Full Text
Here, we will learn how to draw Lewis structures of molecules. Modeling molecules as Lewis structures allows us to show the contributions of individual atoms to the structural entity as a whole.
Show the first 18 elements of the periodic table. The periodic table contains the electron count for each atom.
We will move through the drawing of each compound step-by-step.
Example 1.
Write that the molecular formula of carbon disulfide is CS2.
We want to draw one carbon atom and two sulfur atoms connected by their shared electrons.
Step 1: calculate the number of electrons that each atom will contribute.
We know the number of electrons in an atom is the same as its group number in the periodic table.
4 for carbon
6 for sulfur. ¬¬¬¬
Step 2: determine the predicted valency of each atom. This will help us propose the best arrangement for the three atoms.
Show that:
Carbon is tetravalent. It forms 4 bonds.
Sulfur is divalent. It forms 2 bonds.
We know sulfur should be divalent because is in the same group as oxygen: it has the same number of valence electrons, thus it follows the same bonding pattern.
Step 3: Place the atom with the highest valency in a central position.
So, surround the carbon with its four valence electrons.
On either side of the carbon, put two S's to represent the sulfur atoms.
Surround each S with its 6 valence electrons.
Indicate with a dotted line that we can form a bond between the carbon and the sulfur on the right.
And the sulfur on the left.
There are still available electrons on both the carbon and the two sulfur atoms, so connect two more pairs of electrons.
Step 4: Show the connected electrons as lines.
Lines indicate bonds. Single bonds are shown as one line, double bonds as two lines, and triple bonds as three lines.
Dots indicate nonbonding electrons (lone pairs).
Step 5: Determine if each atom has an octet (eight electrons in their valence shell), which gives second and third row elements a stable electron configuration. First-row elements have only two electrons in their valence shell.
Write that carbon has access to:
4 bonds x 2 electrons/bond = 8 electrons, so it obeys the octet rule.
Write that each sulfur has access to:
2 bonds x 2 electrons/bond + 4 nonbonding electrons = 8 electrons, so they also obey the octet rule.
Example 2.
As a second example, write that we will draw the Lewis structure for the molecule with the formula H2CO3, carbonic acid.
Step 1.
The number of electrons contributed by each atom is:
1 for hydrogen
4 for carbon
6 for oxygen
Step 2.
The valency of the atoms can be ranked as follows:
Carbon, tetravalent;
Oxygen, divalent; and
Hydrogen, monovalent.
Let's start drawing the Lewis structure with the carbon atom in the center.
Next, add the three oxygens above, to the left, and to the right so that they surround the carbon.
Lastly, connect the two hydrogens at peripheral locations to the oxygens on the left and right sides of the carbon.
Step 3.
Let's first connect electrons between the atoms that can form multiple bonds:
1 single bond from the central carbon to the oxygen on the left
1 single bond to the oxygen on the right
1 double bond to the oxygen above
Next, connect the hydrogen atoms with single bonds to their adjacent oxygen atoms.
Step 4.
Re-draw the structure with lines between the connected atoms.
Step 5.
Calculate the number of electrons on each atom:
Carbon has 4 bonds x 2 electrons/bond = 8 electrons
The oxygen above the carbon has 2 bonds x 2 electrons/bond + 4 nonbonding electrons = 8 electrons
The oxygens to the right and left have: 2 bonds x 2 electrons/bond + 4 nonbonding electrons
Therefore, all carbon and oxygen atoms follow the octet rule.
The hydrogens have 1 bond x 2 electrons/bond = 2 electrons
Therefore, all hydrogen atoms follow the duet rule.