Question

Which of the following compounds or ions exhibit resonance? a. \(\mathrm{O}_{3} \quad\) d. \(\mathrm{CO}_{3}^{2-}\) b. \(\mathrm{CNO}^{-} \quad\) e. \(\mathrm{AsF}_{3}\) c. \(\mathrm{AsI}_{3}\)

Short answer

The compounds and ions that exhibit resonance are: a. \(\mathrm{O}_{3}\) d. \(\mathrm{CO}_{3}^{2-}\)

Step-by-step solution

Draw Lewis structures for the given compounds and ions

For each of the given compounds and ions, draw their respective Lewis structures. Remember the rules for drawing Lewis structures: count the total number of valence electrons, place the least electronegative atom in the center, and complete the octets of the surrounding atoms. a. \(\mathrm{O}_3\): Draw a single molecule of ozone. There are 18 valence electrons, so place a double bond between one oxygen atom and the central oxygen atom, and a single bond between the central oxygen atom and the third oxygen atom. Place lone pairs on each oxygen atom to complete their octet. b. \(\mathrm{CNO}^-\): This ion has 14 valence electrons. Place carbon in the center and nitrogen and oxygen atoms on either side. Put a triple bond between carbon and nitrogen, a single bond between carbon and oxygen and 2 lone pairs on the nitrogen atom and 3 lone pairs on the oxygen atom. c. \(\mathrm{AsI}_3\): There are 26 valence electrons. Undergo the same procedure as for ozone. d. \(\mathrm{CO}_3^{2-}\): The carbonate ion has 24 valence electrons. Place carbon in the center, and arrange the oxygen atoms around it. Fill the octet for the central carbon atom by a single bond and a double bond on two other oxygen atoms accordingly. Place lone pairs on each oxygen atom. e. \(\mathrm{AsF}_3\): There are 26 valence electrons. Place a single bond between the central arsenic atom and each fluorine atom. Add 3 lone pairs on each fluorine atom and one lone pair on the central arsenic atom.

Identify resonance structures

Now check each drawn Lewis structure to see if there are alternative structures with identical atom positions but differing electron pair positions: a. \(\mathrm{O}_3\): There are resonance structures as the double bond between one oxygen atom and the central oxygen atom can be replaced with a single bond, and the single bond between the third oxygen and the central oxygen atom can become a double bond, exchanging the positions of lone pairs. b. \(\mathrm{CNO}^-\): There are no resonance structures, as any change in the placement of electron pairs would result in a change of atom positions. c. \(\mathrm{AsI}_3\): No resonance structures, as there are only single bonds and no adjacent double bonds or lone pairs. d. \(\mathrm{CO}_3^{2-}\): There are resonance structures, as we can move the double bond between carbon and one oxygen atom to another oxygen atom, redistributing the positions of electron pairs, while maintaining the same atom positions. e. \(\mathrm{AsF}_3\): No resonance structures, as there are only single bonds and no adjacent double bonds or lone pairs. In conclusion: a. \(\mathrm{O}_{3}\) exhibits resonance. b. \(\mathrm{CNO}^{-}\) does not exhibit resonance. c. \(\mathrm{AsI}_{3}\) does not exhibit resonance. d. \(\mathrm{CO}_{3}^{2-}\) exhibits resonance. e. \(\mathrm{AsF}_{3}\) does not exhibit resonance.

Key concepts

Lewis Structures

Lewis structures are a valuable tool in understanding the arrangement of electrons in a molecule or ion. They show how valence electrons are distributed among atoms and help determine the types and number of chemical bonds.

• Count all valence electrons in the molecule or ion.
• Place the least electronegative atom at the center.
• Draw single bonds to connect surrounding atoms to the central atom.
• Add lone pairs to complete octets for each atom where needed.
• Use double or triple bonds if there are leftover electrons or uncompleted octets.

In a structure without complete octets, electrons might need to be shared more than once, leading to the formation of multiple bonds.

Valence Electrons

Valence electrons are the electrons in the outermost shell of an atom. They play a crucial role in chemical bonding because they can be gained, lost, or shared to form compounds.
To determine the number of valence electrons:

• Check the group number of the element in the periodic table.
• Elements in the same group have the same number of valence electrons.
• For example, oxygen has six valence electrons because it is in Group 16.

Once you know the valence electrons, you can predict how an element will bond in a molecule.

Chemical Bonds

Chemical bonds are forces that hold atoms together in a compound. Based on electron sharing or exchange, there are several types crucial to understanding molecular structures.

• Covalent Bonds: Atoms share electrons, as seen in the ozone molecule (\(\mathrm{O}_3\)), where oxygen atoms bond to fulfill the octet rule.
• Ionic Bonds: Transfer of electrons from one atom to another, forming charged ions, not commonly seen in resonance structures.

Covalent bonds can vary in strength and are influenced by bond length and electron sharing. Single bonds share one electron pair, double and triple bonds share two or three, respectively.

Ozone Molecule

The ozone molecule, \(\mathrm{O}_3\), is a great example of resonance in chemistry.
Ozone has 18 valence electrons distributed in a way that allows for alternating bonding structures.

• Consists of three oxygen atoms bonded with one double bond and one single bond.
• Lone pairs on each oxygen complete the octet rule.
• The resonance involves shifting double bond positions without altering the atom layout.

This electron delocalization within \(\mathrm{O}_3\) explains its stability despite having different bond types among the same atoms.

Carbonate Ion

The carbonate ion, denoted as \(\mathrm{CO}_3^{2-}\), is another typical example of resonance.

• It contains 24 valence electrons, with carbon at the center.
• One double bond and two single bonds are used to connect with the oxygen atoms initially.
• By moving the double bond to another oxygen, equivalent resonance structures can be formed, redistributing electrons while maintaining atom positions.

Resonance structures in \(\mathrm{CO}_3^{2-}\) contribute to the molecule's equivalence in bond lengths and angles, underlining resonance's role in chemical stability.