Problem 132 - General Chemistry: Principles and Modern Applications

Question

Briefly describe each of the following ideas: (a) formal charge; (b) resonance; (c) expanded valence shell; (d) bond energy.

Step-by-Step Solution

Answer

Formal charge is the electrical charge difference between the isolated atom and the one in the molecule. Resonance refers to multiple possible structures for a molecule. Expanded valence shell refers to central atoms in period 3 and beyond in the periodic table having more than 8 electrons in their valence shell due to the presence of d orbitals for bonding. Bond energy is the energy required to break a bond under standard conditions.

Step 1: Formal Charge

Formal charge of an atom in a molecule is the electrical charge difference between the isolated neutral atom and the one in the molecule. It can be calculated as follows: Formal charge = (valence electrons in free atom) - (non-bonding valence electrons) - 1/2(bonding electrons).

Step 2: Resonance

Resonance refers to the capability of certain molecules or ions to be represented more than one Lewis structure. These structures are known as resonance forms. The actual structure of the molecule is a hybrid or blend of these resonance forms.

Step 3: Expanded Valence Shell

The expanded valence shell refers to the concept that central atoms (in period 3 and beyond in the periodic table) can have more than an octet, more than 8 electrons, in their valence shell. This happens due to the presence of d orbital's available for bonding in these elements.

Step 4: Bond Energy

Bond energy or bond dissociation energy is the amount of energy required to break one mole of a bond in a chemical compound under standard conditions. It is often measured in kJ/mol. A high bond energy implies a strong bond.

Key Concepts

Understanding Resonance Structures

Resonance structures occur when a molecule can be represented by two or more valid Lewis structures. These different arrangements, known as resonance forms, depict the same arrangement of atoms but different placements of electrons. In reality, none of these resonance structures are actually correct on their own. Instead, the true form of the molecule is a resonance hybrid—an average of all resonance forms.

• This hybrid tends to be more stable than any individual resonance form.
• Resonance increases the molecule’s stability by delocalizing electrons over multiple atoms.

Given the example of a molecule like benzene, this is visually represented by alternate double and single bonds, even if in reality, all bonds are the same strength and length. This depiction of delocalized electrons helps in predicting physical properties, stability, and reactivity of the molecules.

Expanded Valence Shell Explained

The expanded valence shell is a fascinating phenomenon observed in elements found in the third period of the periodic table and beyond. These elements have access to d orbitals that are energetically accessible and allow the atom to hold more than the typical eight electrons in its valence shell.
This ability enables these elements to form more than four bonds, which is why molecules like phosphorus pentachloride (PCl₅) and sulfur hexafluoride (SF₆) are stable even though they technically violate the octet rule.

• The presence of d orbitals offers extra space for electrons, hence accommodating more bonds.
• Expanded valence shells are essential in many chemical reactions and are often found in hypervalent molecules.

Understanding the expanded valence shell is crucial for predicting molecular geometry and potential reactivity in unusual environments.

The Significance of Bond Energy

Bond energy is a measure of the strength of a bond in a molecule, defined as the amount of energy needed to break one mole of bonds in gaseous molecules into individual gaseous atoms. It serves as an indicator of molecular stability—higher bond energies represent stronger bonds and typically, a more stable molecule.
Bond energy is crucial in understanding chemical reactions. During a reaction, energy is generally required to break existing bonds, and energy is released when new bonds are formed.

• High bond energy means more energy is required to break the bond, which signifies a stronger bond.
• Conversely, bonds with low bond energies are weaker and more reactive.

Calculating bond energies helps chemists predict the enthalpy changes in reactions and design molecules with desired energy properties.