DAT Biology: Bonding and Molecular Forces

DAT High-Yield Concept

Intra- vs. Inter-: Intramolecular forces hold atoms together within a molecule (Ionic, Covalent). Intermolecular forces (IMFs) act between separate molecules.
Strength Hierarchy: Covalent Bond > Ionic Bond > Hydrogen Bond > Dipole-Dipole > London Dispersion Forces. (Note: Ionic bonds are extremely strong in a lattice, but weak in aqueous solution).
Physical Property Correlations: Stronger IMFs result in higher Boiling Points, higher Melting Points, higher Viscosity, and lower Vapor Pressure.
Biological Relevance: Hydrogen bonds are critical for DNA base pairing (A-T = 2 bonds, G-C = 3 bonds) and Protein Secondary Structure (Alpha helices/Beta sheets).

1. Intramolecular Forces (Within the Molecule)

Intramolecular forces are the chemical bonds that hold atoms together to form molecules. These forces must be broken by chemical reactions, not by physical changes (like boiling).

A. Ionic Bonds

Formed by the complete transfer of one or more electrons from a metal to a non-metal. This creates charged ions (cations and anions) that are attracted by electrostatic forces.

Example: NaCl (Table Salt).
Biology Context: "Salt bridges" in proteins contribute to tertiary and quaternary structures.

B. Covalent Bonds

Formed by the sharing of electrons between non-metals. These are the strongest bonds in a biological context.

Nonpolar Covalent: Equal sharing of electrons (e.g., C-H bonds in lipids).
Polar Covalent: Unequal sharing of electrons due to differences in electronegativity (e.g., O-H bonds in water).

Key Definition: Electronegativity
The measure of an atom's ability to attract shared electrons.
Trend: Increases up and to the right on the periodic table. Fluorine is the most electronegative element.
Large difference = Ionic; Moderate difference = Polar Covalent; Negligible difference = Nonpolar Covalent.

2. Intermolecular Forces (Between Molecules)

Intermolecular forces (IMFs), often called Van der Waals forces, are attractions between separate molecules. These determine the physical state (solid, liquid, gas) of a substance.

A. London Dispersion Forces (LDF)

Also known as induced dipole-induced dipole forces. These act on ALL molecules, including nonpolar ones.

Mechanism: Random electron movement creates a temporary dipole in one atom, which induces a dipole in a neighbor.
Strength: Generally the weakest IMF, but strength increases with molecular weight (size) and surface area.

Schematic showing two iodine molecules with electron clouds shifting to create temporary positive and negative ends, resulting in attraction.

London dispersion forces (induced dipole-induced dipole) in iodine molecules. Even in non-polar molecules, temporary fluctuations in electron distribution create temporary dipoles that induce attraction between neighbors. (Source: Wikimedia Commons)

B. Dipole-Dipole Interactions

Occur between polar molecules. The positive end of one polar molecule is attracted to the negative end of another.

Mechanism: Permanent dipoles align (electrostatic attraction).
Strength: Stronger than LDF but weaker than hydrogen bonds.

Diagram of two HCl molecules aligned such that the H of one is near the Cl of the other, with delta plus and delta minus signs indicating partial charges.

Dipole-dipole interactions between hydrogen chloride (HCl) molecules. The partially positive hydrogen atom of one molecule is attracted to the partially negative chlorine atom of a neighboring molecule. (Source: Wikimedia Commons)

C. Hydrogen Bonding

A specific, unusually strong type of dipole-dipole interaction. It is crucial for life.

Requirement: A Hydrogen atom covalently bonded to a highly electronegative atom (Fluorine, Oxygen, or Nitrogen) is attracted to a lone pair on an F, O, or N of a neighboring molecule.
Biological Importance: Responsible for water's unique properties, DNA double helix stability, and protein folding.

Mnemonic: "Hydrogen bonds are FON"
Hydrogen bonding only happens when H is bonded to Fluorine, Oxygen, or Nitrogen.

2D diagram showing multiple water molecules. Solid lines connect Oxygen and Hydrogen within molecules; dotted lines connect Hydrogen of one molecule to Oxygen of another.

Diagram of water molecules illustrating both intramolecular (solid lines) and intermolecular (dotted lines) forces. (Source: Wikimedia Commons)

3. Comparison of Bonding Forces

Bond Type	Category	Relative Strength	Basis of Attraction	Biological Example
Covalent	Intramolecular	Strongest	Sharing electron pairs	Peptide bond, Phosphodiester bond
Ionic	Intramolecular	Strong (Solid) / Weak (Aq)	Charge transfer (+/-)	Salt bridges in proteins
Hydrogen Bond	Intermolecular	Moderate	H connected to F, O, N	DNA base pairs, Water lattice
Dipole-Dipole	Intermolecular	Weak	Permanent dipoles	Interactions between polar amino acid side chains
London Dispersion	Intermolecular	Weakest	Temporary dipoles	Hydrophobic interactions in lipid bilayers

4. How IMFs Affect Physical Properties

On the DAT, you will often be asked to rank molecules based on physical properties derived from their IMFs.

Boiling Point and Melting Point

Rule: Stronger IMFs require more energy (heat) to break.
Result: Molecules with Hydrogen bonding have significantly higher boiling points than similar-sized molecules with only LDF.
Example: Water (H₂O) boils at 100°C, while Methane (CH₄), which has similar mass but only LDF, boils at -161°C.

Vapor Pressure

Rule: Vapor pressure is the pressure exerted by a vapor in equilibrium with its liquid.
Result: Strong IMFs hold molecules in the liquid phase tightly. Therefore, High IMFs = Low Vapor Pressure.
Inverse relationship to Boiling Point.

Viscosity and Surface Tension

Viscosity: Resistance to flow. Stronger IMFs = Higher Viscosity (think honey vs. water).
Surface Tension: Energy required to increase surface area. Stronger IMFs = Higher Surface Tension (allows insects to walk on water).

Critical Thinking Questions

Use these questions to test your application of the concepts:

DNA Stability: A DNA segment with a high percentage of Guanine-Cytosine (G-C) pairs melts at a higher temperature than an Adenine-Thymine (A-T) rich segment. Based on intermolecular forces, why is this the case?
(Hint: Count the hydrogen bonds.)
Protein Folding: When a protein folds in an aqueous environment, nonpolar amino acids bury themselves in the core. Which intermolecular force is primarily driving this "hydrophobic effect," and how does entropy play a role?
Evaporative Cooling: Why does sweating cool the body down efficiently? Explain in terms of kinetic energy and the breaking of intermolecular forces.

Bonding and Molecular Forces

Summary

Practice Questions

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