Problem 49 - Chemistry: The Central Science in SI Units, Global Edition

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

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

Step 1: Option (a): Formal charge is the same as oxidation number.

Formal charge and oxidation number are different concepts. Formal charge is the charge assigned to an atom in a molecule, assuming that electrons are shared equally between atoms. Oxidation number is the number of electrons that an atom loses or gains in a chemical reaction. Therefore, option (a) is false.

Step 2: Option (b): To draw the best Lewis structure, you should minimize formal charge.

Minimizing formal charge helps in obtaining a more accurate representation of the molecule in the Lewis structure. By minimizing the formal charge, one can account for the most stable electron distribution within the molecule. Therefore, option (b) is true.

Step 3: Option (c): Formal charge takes into account the different electronegativities of the atoms in a molecule.

Formal charge does not take into account the electronegativity of the atoms within a molecule. Electronegativity is a measure of the tendency of an atom to attract a bonding pair of electrons, while formal charge is calculated based on the assumption that electrons are equally shared between atoms. So, option (c) is false.

Step 4: Option (d): Formal charge is most useful for ionic compounds.

Ionic compounds have a transfer of electrons between atoms, resulting in ions with whole number charges. Formal charge is more useful for covalent compounds, where electrons are shared between atoms. Option (d) is false.

Step 5: Option (e): Formal charge is used in calculating the dipole moment of a diatomic molecule.

Dipole moment in a diatomic molecule is determined by the product of the charge difference between the two atoms and the distance between their nuclei. Formal charge does not directly determine the dipole moment as it focuses on the electron distribution around the atoms in a molecule. So, option (e) is false. From the analysis of the various options, we can conclude that: The correct answer is option (b): To draw the best Lewis structure, you should minimize formal charge.

Key Concepts

Lewis structure

Lewis structures are diagrams that represent the bonding between atoms of a molecule, showcasing how the valence electrons are distributed. They act as a roadmap for understanding the basic framework of a molecule.
To draw a Lewis structure, follow these simple steps:

• Count the total number of valence electrons in the molecule.
• Draw the skeleton of the molecule using lines to represent shared electron pairs, also known as covalent bonds.
• Distribute the remaining electrons to fulfill the octet rule, where most atoms are stable when they have eight electrons in their outer shell.
• Minimize formal charge, ensuring that atoms have charges as close to zero as possible for more stability.

The best Lewis structure minimizes formal charges and reflects a stable electron distribution, making it a vital tool in chemistry for predicting molecular shapes and reactivity.

Oxidation number

Oxidation numbers are theoretical charges assigned to atoms in molecules to help keep track of electron transfers in chemical reactions. They are essential for understanding redox reactions, which involve the transfer of electrons.

• The oxidation number of a free element is always zero. For example, N in N extsubscript{2} has an oxidation number of 0.
• For ions composed of a single atom, the oxidation number is equal to the charge of the ion.
• In compounds, the oxidation number of Hydrogen is usually +1 and for Oxygen, it is generally -2.
• The sum of oxidation numbers in a neutral compound must be zero, while in polyatomic ions, it equals the ion's overall charge.

Understanding oxidation numbers is crucial for balancing chemical equations and recognizing the direction of electron flow in reactions, distinguishing them clearly from formal charges.

Electronegativity

Electronegativity refers to an atom's ability to attract and hold onto electrons within a chemical bond. It varies across the periodic table and can greatly affect molecular properties.

• Electronegativity increases across a period from left to right and decreases down a group in the periodic table.
• Fluorine is the most electronegative element, while Cesium and Francium are among the least.
• The difference in electronegativity between two bonded atoms determines bond polarity; greater differences lead to more polar bonds.

Understanding electronegativity helps predict bond types and molecular interactions, illustrating why certain atoms in a molecule attract electrons more strongly than others.

Dipole moment

A dipole moment occurs in a molecule when there is an uneven distribution of electron density, resulting in one end of the molecule being slightly negative and the other end slightly positive. It's a measure of the polarity of a molecule.

• The dipole moment is calculated by multiplying the amount of charge difference and the distance between the charges.
• Molecules with large differences in electronegativity between bonded atoms often have significant dipole moments.
• Nonpolar molecules can sometimes become polar if they have an asymmetric shape, even with similar electronegativities.

Dipole moments are crucial for understanding molecular interactions, affecting physical properties like boiling points, solubility, and even reactivity. They help in explaining why molecules attract or repel each other based on their charge distributions.