Chemical Bonds and Electronegativity in Biology

Chemical Bonds in Biological Molecules

Types of Chemical Bonds

Chemical bonds are the forces that hold atoms together in molecules and compounds. The type of bond formed depends on the difference in electronegativity between the atoms involved.

• Covalent Bonds: Atoms share electrons to achieve a full outer shell. Covalent bonds can be nonpolar or polar depending on the electronegativity difference.

• Polar Covalent Bonds: Electrons are shared unequally due to a moderate difference in electronegativity (0.4 – 2.0). This creates partial charges (δ+ and δ-) on the atoms.

• Nonpolar Covalent Bonds: Electrons are shared equally between atoms with similar electronegativity (difference < 0.4).

• Ionic Bonds: Electrons are transferred from one atom to another, resulting in the formation of ions. This occurs when the electronegativity difference is large (greater than 2.0).

Electronegativity and Bond Type

Electronegativity is a measure of an atom's ability to attract shared electrons in a chemical bond. The difference in electronegativity between two atoms determines the bond type:

• Nonpolar Covalent: Difference < 0.4

• Polar Covalent: Difference between 0.4 and 2.0

• Ionic: Difference > 2.0

For example, in a water molecule (H2O), the difference in electronegativity between oxygen and hydrogen leads to polar covalent bonds, with oxygen carrying a partial negative charge (δ-) and hydrogen a partial positive charge (δ+).

Electronegativity Table (Selected Elements)

Element	Electronegativity
H	2.20
C	2.55
N	3.04
O	3.44
F	3.98

Additional info: Table values inferred from standard electronegativity scales.

Visual Representation of Bond Types

• Nonpolar Covalent: Electron cloud is evenly distributed between atoms.

• Polar Covalent: Electron cloud is closer to the more electronegative atom, creating partial charges.

• Ionic: Complete transfer of electrons creates positive and negative ions.

Examples and Applications

• Water (H2O): Polar covalent bonds between hydrogen and oxygen result in a molecule with distinct partial charges, leading to hydrogen bonding and unique properties of water.

• Sodium Chloride (NaCl): Ionic bond forms between sodium (Na+) and chloride (Cl-) due to a large difference in electronegativity.

Announcements (Course Logistics)

• Pre-reading assignments and study deadlines are listed for student reference.

• Preparation for functional groups and related textbook figures is required for upcoming classes.

Additional info: Announcements are included for context but are not part of the core biology content.