Problem 49 - Chemistry The Central Science In Si Units

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.

Step-by-Step Solution

Answer

(b) is true: Minimize formal charge for the best Lewis structure.

Step 1: Understanding Formal Charge

Formal charge is a concept in chemistry used to determine the sharing of electrons in a molecule. It assumes equal sharing of electrons between atoms, ignoring differences in electronegativity. Unlike oxidation states, formal charge is not an indication of electron density or atom oxidation levels, but rather it helps in determining the most appropriate Lewis structure by minimizing these charges.

Step 2: Checking Each Statement

Let's examine each statement in the question: (a) "Formal charge is the same as oxidation number." This is false. While both are used in chemistry, formal charges assume equal sharing of electrons, unlike oxidation numbers which consider electronegativity differences. (b) "To draw the best Lewis structure, you should minimize formal charge." This is true. Minimizing formal charges generally leads to the most stable and plausible Lewis structure. (c) "Formal charge takes into account the different electronegativities of the atoms in a molecule." This is false. Formal charges ignore electronegativity. (d) "Formal charge is most useful for ionic compounds." This is false. Formal charge is particularly useful for covalent compounds, not ionic. (e) "Formal charge is used in calculating the dipole moment of a diatomic molecule." This is false. Formal charge is not directly used in dipole moment calculations, which depend on bond polarity.

Step 3: Identifying the Correct Answer

Among the given options, (b) is the only true statement. It is crucial to consider and minimize formal charges when drawing Lewis structures to ensure they are as accurate and representative of the actual molecular structure as possible.

Key Concepts

Lewis Structure

Lewis structures are diagrams that represent the bonds between atoms in a molecule and the arrangement of lone pairs of electrons. They are essential for understanding how molecules are constructed.
To draw a Lewis structure, follow these steps:

• Count the total number of valence electrons for all atoms involved.
• Place the least electronegative atom in the center (except H, which is always terminal).
• Use pairs of electrons to form bonds between the central atom and surrounding atoms.
• Distribute remaining electrons to complete octets on the outer atoms first and then the central atom if needed.
• Adjust for overall charge by adding or removing electrons accordingly.

Minimizing formal charges in a Lewis structure is key to finding the most stable structure. While drawing Lewis structures, remember that they only depict probable distribution of electrons, not the actual electronic arrangement in molecules.

Oxidation Number

The oxidation number (or oxidation state) of an atom in a compound provides insight into the electron transfer processes. It indicates the degree of oxidation of an atom, ideally reflecting electrons that are not equally shared in a chemical bond. When determining oxidation numbers, consider these rules:

• The oxidation number of a free element (uncombined) is always 0.
• For monoatomic ions, the oxidation number is equal to the ion's charge.
• In compounds, hydrogen is assigned an oxidation state of +1, and oxygen is usually -2, with exceptions like peroxides.
• The sum of oxidation states for all atoms in a neutral molecule must be zero, while in ions, it equals the ion charge.

Oxidation numbers are particularly useful for identifying the electron flow in redox reactions and determining how oxidation and reduction have occurred in chemical processes.

Electronegativity

Electronegativity is a measure of an atom's ability to attract shared electrons when forming a chemical bond. This tendency is crucial for predicting the nature of bonds between atoms. Some key points about electronegativity include:

• Elements with higher electronegativities tend to attract electrons more strongly, forming polar covalent bonds.
• The most electronegative element is fluorine, followed by oxygen, nitrogen, and chlorine.
• Differences in electronegativity can explain the type of bond formation: nonpolar covalent (similar electronegativities), polar covalent (moderate differences), and ionic (large differences).

Understanding electronegativity aids in predicting molecular behavior, reactivity, as well as the distribution of partial charges within molecules, which directly impacts physical properties and chemical reactions.