Question

Draw as many resonance structures as possible for each of the following molecules or ions, giving all atoms (except H) octets. Used curved arrows to show how one structure can be converted into another. (a) \(\mathrm{SO}_{2}\) (b) \(\mathrm{CO}_{3}^{2-}\) (c) \(\mathrm{BF}_{3}\)

Short answer

SO2 and CO3(2-) have multiple resonance structures, while BF3 primarily lacks resonance due to boron's incomplete octet.

Step-by-step solution

Drawing Resonance Structures for SO2

Start by drawing the Lewis structure for \( \text{SO}_2 \). The sulfur atom is central, connected to two oxygen atoms. Across the structures, try distributing electrons to give each atom an octet. The structure with a sulfur-oxygen single bond and another with a double bond is a valid resonance form. Then switch the double bond's position to the other oxygen to get the second resonance structure. Both structures have a total of 18 valence electrons.

Using Curved Arrows for SO2

Curved arrows indicate the movement of electrons to interconvert between resonance structures. Here, arrows are used to shift a pair of electrons from a sulfur-oxygen bond to the oxygen's lone pair and simultaneously form a new double bond with the other oxygen. Repeat this process to go back and forth between the resonance forms.

Drawing Resonance Structures for CO3(2-)

For \( \text{CO}_3^{2-} \), draw a central carbon atom single-bonded to three oxygen atoms, then distribute the remaining electrons to fulfill the octet rule (total of 24 electrons). Three resonance structures arise by forming a double bond with each oxygen, one at a time, and placing lone pairs appropriately. Each shift maintains the negative charge over the molecule.

Using Curved Arrows for CO3(2-)

To show electron movement, draw a curved arrow from a lone pair on one oxygen forming a double bond with carbon, and another arrow showing the breaking of the existing double bond to form another lone pair on the other oxygen. This illustrates the shift of the double bond around the carbon center, indicating resonance.

Drawing Resonance Structures for BF3

In \( \text{BF}_3 \), the central boron is connected to three fluorine atoms. Each fluorine has three lone pairs, and boron has an incomplete octet (excluding potential expansion). As such, the molecule does not readily participate in resonance with classical structures since boron holds an incomplete octet.

Key concepts

Lewis Structures

Lewis structures are diagrams that show how atoms in a molecule are bonded together and how valence electrons are distributed. These structures help in visualizing the arrangement of atoms and the electron pairs around them.
To draw a Lewis structure:

• Determine the total number of valence electrons in the molecule.
• Arrange atoms with the least electronegative atom usually in the center (e.g., sulfur in \( \text{SO}_2 \), carbon in \( \text{CO}_3^{2-} \)).
• Use lines to represent bonds and dots for lone pairs of electrons.

In resonance structures, we can draw different possible arrangements by shifting the position of electrons, maintaining connectivity between atoms.

Octet Rule

The octet rule is a principle stating that atoms are more stable with eight electrons in their valence shell. This rule is central in drawing Lewis structures and is seen in many compounds.
However, there are exceptions:

• Atoms like hydrogen seek only 2 electrons.
• Elements like boron (as in \( \text{BF}_3 \)) sometimes have incomplete octets.
• Elements beyond the second period can expand their octet by utilizing d orbitals, though such scenarios were not observed in our examples.

In \( \text{SO}_2 \) and \( \text{CO}_3^{2-} \), satisfying the octet rule is a key goal for each resonance structure.

Valence Electrons

Valence electrons are the outermost electrons of an atom and are crucial for determining how atoms bond and interact.

• They are represented in Lewis structures as either bonding pairs (lines) or lone pairs (dots).
• To calculate the number: add up the group number of each element for simple molecules.
• \( \text{SO}_2 \) has 18 valence electrons, while \( \text{CO}_3^{2-} \) has 24, due to its negative charge (adding two extra electrons).

Balancing valence electrons is essential for achieving structures that obey the octet rule, where possible.

Electron Movement

Electron movement describes how electrons are redistributed between various atoms in resonance and during chemical reactions.
This process creates alternative resonance structures without changing the actual placement of atoms, just their electron distribution.
In \( \text{SO}_2 \), for example, electrons shift between single and double bonds with sulfur and oxygen, explaining different resonance forms.
Using arrows, one can predict how structures interconvert and which atoms gain or lose electron density, helping to explain molecular reactivity and stability.

Curved Arrows in Chemistry

Curved arrows are a visual tool used to depict the movement of electrons in resonance structures and chemical reactions. This helps in understanding how bonds are formed or broken.

• Begin the arrow at the electron source (a lone pair or bond).
• Point it towards the electron recipient (an atom or bond where electrons move).

For \( \text{CO}_3^{2-} \), arrows show how a double bond can shift from one oxygen to another, maintaining overall charge distribution.
Mastering curved arrows allows students to visualize changes within molecules, making complex concepts more manageable.