Covalent Bonding and Electronegativity

Types of Covalent Bonds

Covalent bonds are formed when atoms share electrons. The nature of the bond depends on the difference in electronegativity between the bonded atoms.

• Nonpolar Covalent Bond: Electrons are shared equally between atoms. Occurs when the electronegativity difference is less than 0.5.

• Polar Covalent Bond: Electrons are shared unequally, resulting in partial charges. Occurs when the electronegativity difference is between 0.5 and 1.9.

• Ionic Bond: Electrons are transferred from one atom to another. Occurs when the electronegativity difference is greater than 1.9.

Electronegativity Values:

• Carbon (C): 2.6

• Oxygen (O): 3.5

• Chlorine (Cl): 3.0

Examples

• CO2: Each C–O bond is polar covalent (), but the molecule is linear and symmetric, so the dipoles cancel and the molecule is nonpolar.

• CCl4: Each C–Cl bond is polar covalent (), but the molecule is tetrahedral and symmetric, so the overall molecular dipole is zero (nonpolar molecule).

Molecular Dipoles and Polarity

Bond Dipole vs. Molecular Dipole

A bond dipole is the separation of charge across a single bond due to differences in electronegativity. A molecular dipole is the vector sum of all bond dipoles in a molecule.

• Bond Dipole: Pair of equal and opposite charges separated by distance.

• Molecular Dipole: Overall dipole moment of a molecule, resulting from the vector sum of all individual dipoles.

Examples

• CO2: Although each C–O bond is polar, the molecule is linear and the dipoles cancel, so CO2 is nonpolar.

• Formaldehyde (H2CO): The molecule is polar because the bond dipoles do not cancel.

Lewis Structures

Definition and Purpose

A Lewis structure is a diagram that represents the valence electrons of atoms within a molecule. Dots represent electrons, and lines represent covalent bonds. Carbon and hydrogen atoms are explicitly shown.

Steps to Draw a Lewis Structure

1. Determine the number of valence electrons for all atoms in the molecule.

2. Determine the arrangement of atoms (generally, the least electronegative atom is central; hydrogen is never central).

3. Connect the atoms by single bonds (each bond represents two shared electrons).

4. Arrange the remaining electrons so that each atom has a complete valence shell (octet for most atoms, duet for hydrogen). There are exceptions.

Examples

• Methane (CH4): Carbon is central, surrounded by four hydrogens. Each bond is a single covalent bond.

• Formaldehyde (H2CO): Carbon is central, double bonded to oxygen, single bonded to two hydrogens. Oxygen has two lone pairs.

Exceptions to the Octet Rule

Common Exceptions

• Group 13 elements (e.g., boron, aluminum): Often stable with only 6 valence electrons.

• Period 3 elements (e.g., phosphorus, sulfur): Can expand their valence shells to contain more than 8 electrons (up to 10 or 12).

Examples

• Boron trifluoride (BF3): Boron has only 6 electrons in its valence shell.

• Phosphorus pentachloride (PCl5): Phosphorus has 10 electrons in its valence shell.

Bond-Line Notation

Introduction and Rules

Bond-line notation is a simplified way to represent organic molecules. Carbon atoms are represented by the ends and intersections of lines, and hydrogen atoms attached to carbon are usually omitted.

• Each vertex or end of a line represents a carbon atom.

• Hydrogens attached to carbon are not shown unless necessary.

• Heteroatoms (atoms other than C and H) are always shown explicitly.

• Formal charges must be indicated if present.

Counting Atoms in Bond-Line Structures

• Count the number of vertices and ends to determine the number of carbon atoms.

• Each carbon forms four bonds; fill in hydrogens to satisfy this rule.

Example

• A zig-zag line with five vertices represents a five-carbon chain (pentane).

Functional Groups in Organic Chemistry

Importance of Functional Groups

Functional groups are specific groups of atoms within molecules that are responsible for characteristic chemical reactions. Molecules with similar functional groups have similar properties and reactivities.

• Molecules are classified and named based on their functional groups.

• Functional groups determine the physical and chemical properties of organic compounds.

Common Functional Groups (to Memorize)

Example: Catecholamine Neurotransmitters

• Catecholamines contain both amine and phenol functional groups, which influence their biological activity.

Summary Table: Bond Types and Electronegativity

Key Equations

• Electronegativity Difference:

• Bond Dipole Moment:

Additional info: The notes also reference the importance of IUPAC nomenclature and the use of formal charges in Lewis structures, which are foundational for organic chemistry students.