Question

Which statements are true about resonance contributing structures? (a) All contributing structures must have the same number of valence electrons. (b) All contributing structures must have the same arrangement of atoms. (c) All atoms in a contributing structure must have complete valence shells. (d) All bond angles in sets of contributing structures must be the same.

Short answer

a) All contributing structures must have the same number of valence electrons. b) All contributing structures must have the same arrangement of atoms. c) All atoms in a contributing structure must have complete valence shells. d) All bond angles in sets of contributing structures must be the same. Answer: The true statements about resonance contributing structures are (a), (b), and (d).

Step-by-step solution

Evaluate Statement (a)

All contributing structures must have the same number of valence electrons. This statement is true. In resonance contributing structures, the total number of valence electrons remains constant while the distribution of the electrons in the structure may change.

Evaluate Statement (b)

All contributing structures must have the same arrangement of atoms. This statement is also true. Resonance contributing structures only differ in the placement of electrons, not the arrangement of atoms. The atoms are connected in the same sequence and positions across all contributing structures.

Evaluate Statement (c)

All atoms in a contributing structure must have complete valence shells. This statement is false. It is not necessary for all atoms in a contributing structure to have complete valence shells. In resonance structures, some atoms may have less than an optimal number of electrons in their valence shell, while others have more to accommodate the electron movements between structures.

Evaluate Statement (d)

All bond angles in sets of contributing structures must be the same. This statement is true. Since the arrangement of atoms remains constant across different contributing structures, the bond angles must also remain the same. In summary, the true statements about resonance contributing structures are (a), (b), and (d).

Key concepts

Valence Electrons

Valence electrons play a pivotal role in chemistry, particularly in determining how atoms bond and form compounds. Every atom is surrounded by a set of electrons called valence electrons, which are found in the outermost electron shell of the atom, known as the valence shell. These electrons are responsible for an atom's chemical properties, as they can be lost, gained, or shared to form chemical bonds.

In the context of resonance structures, it's vital to understand that while electrons might shift between atoms, the total number of valence electrons remains the same. This consistent electron count across all resonance structures is essential because it maintains the overall charge and chemical stability of the molecule, as we've seen in statement (a) of our exercise.

Atom Arrangement in Resonance

When considering the arrangement of atoms in resonance structures, we must remember that the framework of the molecule remains fixed. Resonance involves the delocalization of pi (π) electrons or lone pairs across a series of atoms without altering the positions of those atoms. This is why statement (b) of our exercise is accurate.

However, the electrons involved in resonance can occupy different positions in the valence shells of the atoms. This shifting of electrons leads to multiple possible contributing structures, which are simply different ways to draw the same molecule to represent the delocalization of electrons. The atom arrangement being invariant in resonance structures is fundamental, enabling chemists to compare and understand different resonance forms while acknowledging they are just different depictions of the same underlying structure.

Valence Shells and Resonance

Delving into the concept of valence shells and their role in resonance, we encounter the misconception that often arises from statement (c). While the octet rule is a guideline suggesting that atoms prefer to have eight electrons in their valence shell, resonance structures can display atoms with incomplete valence shells. This variability can arise due to the aforementioned electron delocalization.

In some resonance forms, certain atoms may have more than an octet of electrons, known as expanded octets, particularly common for elements in the third period and beyond. Conversely, atoms may also have fewer than eight electrons, which is often the case in molecules with odd numbers of electrons or in species that possess positive charges. These exceptions to the octet rule are important in understanding the flexibility of electron sharing and the distribution in molecules with resonance.

Bond Angles in Resonance Structures

The bond angles in resonance structures are a subtle yet crucial aspect of molecular geometry. As we confirmed in statement (d), the bond angles remain consistent among all resonance contributing structures for a given molecule. Even though the electrons may shift and the lengths of some bonds can vary (single vs. double bonds, for example), the spatial arrangement of the atoms does not change.

This consistency in bond angles preserves the three-dimensional shape of the molecule, a factor that can greatly influence physical and chemical properties, including reactivity and interaction with other molecules. It is a common error to confuse resonance structures with different possible isomers, which can indeed have different bond angles. However, in resonance forms, all variations simply represent different electron arrangements rather than distinct molecular structures.