Intermolecular and Interatomic Forces

TL;DR

Forces between molecules (intermolecular) are weaker than forces within molecules (interatomic/chemical bonds). These forces affect physical properties like boiling point and melting point, with stronger forces leading to higher values. Molecular size also influences intermolecular force strength, especially for non-polar molecules.

1. The Mental Model

Imagine molecules as tiny Lego bricks. Interatomic forces are the strong studs that hold a single Lego brick together. Intermolecular forces are like the weaker static cling or magnetic pull that makes several bricks stick together loosely.

2. The Core Material

Intermolecular Forces (Van der Waal's forces)

These are forces between molecules. Your notes highlight three main types:

• Dipole-dipole forces: These occur between two molecules that are polar. Polar molecules have a slight positive end and a slight negative end due to uneven sharing of electrons. The positive end of one molecule is attracted to the negative end of another.
• Induced dipole forces or London forces: These are present between all molecules, but they are the primary force between non-polar molecules. They arise from temporary, fluctuating dipoles caused by the random movement of electrons. The strength of these forces increases with molecular size.
• Hydrogen bonding: This is a special, strong type of dipole-dipole force. It happens when hydrogen is covalently bonded to a highly electronegative atom like nitrogen (N), oxygen (O), or fluorine (F). The hydrogen atom becomes very positive, and is strongly attracted to a lone pair of electrons on an N, O, or F atom of an adjacent molecule.

Interatomic Forces (Chemical Bonds) vs. Intermolecular Forces

It's crucial to understand the difference between forces within a molecule and forces between molecules.

• Interatomic Forces / Chemical Bonds (Intramolecular forces): These are the strong forces that hold atoms together within a single molecule. Examples include covalent bonds (like C-H in methane) or ionic bonds. These are much stronger than intermolecular forces.
• Intermolecular Forces: These are the weaker forces that exist between separate molecules. Breaking these forces (e.g., when boiling water) doesn't break the individual molecules themselves.

Let's look at methane (CH₄) to visualize this:

graph TD
    subgraph "Single Methane Molecule"
        C1[Carbon] --- H2[Hydrogen]
        C1 --- H3[Hydrogen]
        C1 --- H4[Hydrogen]
        C1 --- H5[Hydrogen]
        style C1 fill:#f9f,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H2 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H3 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H4 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H5 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
    end

    subgraph "Another Methane Molecule"
        C6[Carbon] --- H7[Hydrogen]
        C6 --- H8[Hydrogen]
        C6 --- H9[Hydrogen]
        C6 --- H10[Hydrogen]
        style C6 fill:#f9f,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H7 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H8 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H9 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
        style H10 fill:#ccf,stroke:#333,stroke-width:2px,rx:5px,ry:5px;
    end

    C1 --- C6("Intermolecular Forces (weak)")
    linkStyle 10 stroke-dasharray: 5 5;
    linkStyle 0,1,2,3,4,5,6,7,8,9 stroke-width:4px,stroke:red;

In the diagram, the red solid lines within each methane molecule represent strong interatomic forces (covalent bonds). The dashed line connecting the two methane molecules represents much weaker intermolecular forces.

Effect of Intermolecular Forces on Physical Properties

The strength of intermolecular forces directly impacts several physical properties:

• Boiling Point: This is the temperature at which a liquid changes to a gas. To boil a substance, you need enough energy to overcome the intermolecular forces holding the molecules together in the liquid state.
  • Stronger intermolecular forces → higher boiling point.
• Melting Point: This is the temperature at which a solid changes to a liquid. Similarly, you need energy to overcome the intermolecular forces holding molecules in a rigid solid structure.
  • Stronger intermolecular forces → higher melting point.
• Vapour Pressure: This is the pressure exerted by the gas molecules above a liquid. If molecules are held together weakly, more of them can escape into the gas phase.
  • Stronger intermolecular forces → lower vapour pressure.

Relationship between Intermolecular Forces and Molecular Size

For non-polar molecules (which primarily experience induced dipole/London forces), there's a clear relationship:
* Larger molecular size → stronger induced dipole forces. This is because larger molecules have more electrons, which can be more easily displaced to create temporary dipoles, leading to stronger attractions.

3. Worked Example

Let's compare the boiling points of methane (CH₄), ethane (C₂H₆), and methanol (CH₃OH).

1. Identify the intermolecular forces present:

• Methane (CH₄): A non-polar molecule. Only experiences induced dipole forces (London forces).
• Ethane (C₂H₆): A non-polar molecule. Only experiences induced dipole forces (London forces). Ethane is larger than methane.
• Methanol (CH₃OH): This molecule has a hydrogen atom bonded directly to an oxygen atom (-OH group). Therefore, it experiences hydrogen bonding, as well as dipole-dipole and induced dipole forces.

2. Compare the strength of these forces:

• Methane and Ethane both have induced dipole forces. Since Ethane has a larger molecular size (more electrons), its induced dipole forces are stronger than Methane's.
• Methanol has hydrogen bonding, which is a much stronger intermolecular force than induced dipole forces.

3. Predict the boiling points:

Based on the strength of intermolecular forces:

graph LR
    M[Methane (CH4) - Smallest, Induced Dipole Forces] --> Eth[Ethane (C2H6) - Larger, Stronger Induced Dipole Forces]
    Eth --> Mth[Methanol (CH3OH) - Similar size to Ethane, but with Hydrogen Bonding]

    M -- "Lowest Boiling Point" --> BP1("BP: -161.5 °C")
    Eth -- "Higher Boiling Point" --> BP2("BP: -88.6 °C")
    Mth -- "Highest Boiling Point" --> BP3("BP: 64.7 °C")

    style M fill:#add8e6,stroke:#333,stroke-width:2px;
    style Eth fill:#90ee90,stroke:#333,stroke-width:2px;
    style Mth fill:#ffb6c1,stroke:#333,stroke-width:2px;
    style BP1 fill:#fff,stroke:#333,stroke-width:1px;
    style BP2 fill:#fff,stroke:#333,stroke-width:1px;
    style BP3 fill:#fff,stroke:#333,stroke-width:1px;
    linkStyle 0,2,4,6 stroke-width:0px;
    linkStyle 1,3,5 stroke:#000,stroke-width:2px,fill:none;

As you can see, methanol has a significantly higher boiling point due to hydrogen bonding, even though its molecular size is similar to ethane. Ethane has a higher boiling point than methane due to its larger size and thus stronger induced dipole forces.

4. Key Takeaways

• Intermolecular forces are attractions between separate molecules, while interatomic forces (chemical bonds) are within a molecule.
• Intermolecular forces are much weaker than interatomic forces.
• The three main types of intermolecular forces are dipole-dipole, induced dipole (London), and hydrogen bonding.
• Hydrogen bonding is the strongest type of intermolecular force and occurs when H is bonded to N, O, or F.
• Stronger intermolecular forces lead to higher boiling points, higher melting points, and lower vapour pressures.
• For non-polar molecules, larger molecular size means stronger induced dipole forces.

Common Mistakes to Avoid:

• Confusing intermolecular forces with chemical bonds; they are fundamentally different in strength and nature.
• Assuming all polar molecules participate in hydrogen bonding; hydrogen must be directly bonded to N, O, or F.
• Forgetting that induced dipole forces exist in all molecules, though they are usually negligible compared to stronger forces if present.
• Mixing up the effect of molecular size with polarity; size primarily affects induced dipole forces, while polarity dictates dipole-dipole potential.

5. Now Try It

Choose two of the following substances: H₂O (water), CH₄ (methane), NH₃ (ammonia), HCl (hydrogen chloride). For your chosen pair:
1. Identify all relevant intermolecular forces for each substance.
2. Compare the expected relative strengths of these forces.
3. Predict which substance would have a higher boiling point and explain your reasoning based on the intermolecular forces.

What success looks like: You can clearly identify the dominant intermolecular forces for each molecule and use this information to accurately compare their boiling points with a sound explanation.