Question

Which one of these statements about formal charge is true? (a) Formal charge is the same as oxidation number. (b) To draw the best Lewis structure, you should minimize formal charge. (c) Formal charge takes into account the different electronegativities of the atoms in a molecule. (d) Formal charge is most useful for ionic compounds. (e) Formal charge is used in calculating the dipole moment of a diatomic molecule.

Short answer

The correct statement about formal charge is: (b) To draw the best Lewis structure, you should minimize formal charge.

Step-by-step solution

(a) Formal charge is the same as oxidation number

Formal charge and oxidation numbers are different concepts in chemistry. Formal charge represents the charge an atom would have if the bonding electrons were shared equally, while oxidation number indicates the number of electrons an atom has theoretically gained or lost. Thus, statement (a) is incorrect.

(b) To draw the best Lewis structure, you should minimize formal charge

When drawing a Lewis structure, it is important to minimize the formal charges on each atom, as a structure with low or zero formal charges would be more stable. The most stable Lewis structures will generally result in a molecule or ion with the lowest possible formal charges on the individual atoms. Therefore, statement (b) is correct.

(c) Formal charge takes into account the different electronegativities of the atoms in a molecule

The concept of formal charge assumes that electrons in a covalent bond are shared equally between the bonded atoms, regardless of their electronegativity. Electronegativity is better represented through other concepts, such as bond polarity and dipole moments. Therefore, statement (c) is incorrect.

(d) Formal charge is most useful for ionic compounds

The concept of formal charge is more commonly used to evaluate covalent compounds' stability, rather than ionic compounds. In ionic compounds, the charge on each ion is more clearly defined, and the concept of oxidation number is more relevant. Therefore, statement (d) is incorrect.

(e) Formal charge is used in calculating the dipole moment of a diatomic molecule

The dipole moment of a molecule arises from the difference in electronegativities between the atoms involved and the distance between them. Formal charge does not take electronegativities into account and is not directly involved in calculating the dipole moment. Thus, statement (e) is incorrect. Based on the analysis, the correct answer is: (b) To draw the best Lewis structure, you should minimize formal charge.

Key concepts

Lewis Structure

A Lewis structure is a way to depict the arrangement of atoms within a molecule. It uses dots to represent valence electrons and lines to represent covalent bonds between atoms. The goal is to show how the outer shell electrons are shared or transferred between atoms.

• Valence electrons are depicted as dots around the chemical symbols.
• Shared pairs of electrons, or bonds, are shown by lines between the atoms.

To draw a successful Lewis structure, it's vital to minimize the formal charge across the molecule. This means assigning electron pairs to atoms such that their formal charge—calculated based on the assumption that bonds are equally shared—is close to zero. This approach helps in determining the most stable molecular structure possible.

Oxidation Number

Oxidation numbers provide an insight into the electron transfer in reactions. Unlike formal charge, which assumes equal sharing of electrons, the oxidation number estimates the charge by considering full transfer of electrons based on electronegativity differences.

• Each atom in a molecule or ion is assigned an oxidation number that helps to understand electron distribution.
• For pure elements, the oxidation number is always zero.
• In compounds, the sum of oxidation numbers must equal the overall charge of the compound or ion.

Understanding oxidation numbers is essential in redox reactions. It helps us understand how electrons are transferred and the resultant charges on atoms, offering insight into the chemical behavior of substances.

Electronegativity

Electronegativity is the ability of an atom to attract shared electrons in a chemical bond. It plays a significant role in determining bond polarity and can influence various properties of a molecule, including its dipole moment.

• Fluorine is the most electronegative element, while cesium and francium are among the least.
• The greater the difference in electronegativity between two atoms, the more polar the bond is.
• Highly electronegative atoms tend to attract electrons more strongly, which can lead to partial charges within a compound.

Although formal charges ignore electronegativity, it provides a critical understanding of how atoms interact within molecules, influencing their stability and reactivity.

Dipole Moment

Dipole moment is a measure of the separation of positive and negative charges within a molecule. It is a vector quantity, having both magnitude and direction. Dipole moments are a direct result of differences in electronegativity.

• A molecule with a larger difference in electronegativity between atoms and greater separation of charge will generally have a higher dipole moment.
• It is an important factor in determining the physical properties of substances, such as boiling and melting points.
• In polar molecules, dipole moments arise because of asymmetric charge distribution.

Dipole moments are not calculated using formal charges, but rather depend on the bond polarities and geometrical arrangement of molecules. They help explain behaviors like solubility, polarity, and interactions with other molecules.