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Chemical Bonding and Intermolecular Forces

CSCA Chemical Bonding and Intermolecular Forces study guide organized around the publicly available CSCA syllabus. Practice Chemistry questions on aicsca.com.

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Chemical Bonding and Intermolecular Forces

Core Concepts and Learning Objectives

This section explains how atoms combine to form molecules or crystals (**Chemical Bonds**) and how molecules interact with each other (**Intermolecular Forces**). This is key to understanding physical properties (melting point, boiling point, solubility) and chemical properties.

**Learning Objectives:**

1. **Distinguish Strength**: Understand the fundamental difference between chemical bonds (strong) and intermolecular forces (weak).

2. **Master Types**: Deeply understand Ionic, Covalent, and Metallic bonds, and the specificity of Hydrogen bonding.

3. **Infer Properties**: Compare melting points and hardness based on bond type and force strength.

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I. Chemical Bonds

Strong interactions between **adjacent** atoms or ions (Bond energy usually >100 kJ/mol).

#### 1. Ionic Bond

* **Definition**: Formed by **electrostatic attraction** between cations and anions.

* **Formation**: Electron transfer from reactive metals (e.g., Na, Mg) to reactive non-metals (e.g., Cl, O).

* **Characteristics**:

* **Non-directional, Non-saturable**.

* Forms **Ionic Crystals**: Hard, high melting point. Conducts electricity only when **molten or dissolved in water**.

* **Examples**: $NaCl$, $MgO$, $K_2SO_4$.

#### 2. Covalent Bond

* **Definition**: Formed by **sharing electron pairs** between atoms.

* **Formation**: Typically between non-metal elements (e.g., H-Cl, C-C).

* **Characteristics**:

* **Directional, Saturable**.

* **Classification**:

* **Non-polar**: Equal sharing (e.g., $Cl-Cl$).

* **Polar**: Unequal sharing (e.g., $H-Cl$).

* **Coordinate (Dative)**: Shared pair comes from one atom (e.g., in $NH_4^+$).

* **Examples**: $H_2O$, $CO_2$, $CH_4$.

#### 3. Metallic Bond

* **Definition**: Strong electrostatic attraction between metal cations and **delocalized electrons** (electron sea).

* **Characteristics**:

* Free-moving electrons give metals **conductivity, heat transfer, and ductility**.

* **Examples**: $Fe$, $Cu$, Alloys.

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II. Intermolecular Forces

**Weak interactions** between molecules, affecting **physical properties** (Melting/Boiling points).

#### 1. Van der Waals Forces

Universal among molecules. For most non-polar molecules ($He, N_2, CH_4$), **London Dispersion Force** is dominant.

* **Trend**: Similar structure $\rightarrow$ Higher molecular mass $\rightarrow$ More polarizable electron cloud $\rightarrow$ Stronger VdW force $\rightarrow$ Higher MP/BP (e.g., $F_2 < Cl_2 < Br_2 < I_2$).

#### 2. Hydrogen Bond

* **Definition**: A special, strong intermolecular force. H bonded to high electronegativity atoms (**N, O, F**) attracts another electronegative atom.

* **Notation**: $X—H \cdots Y$ ($X, Y$ = $N, O, F$).

* **Impact**: Abnormally high BP (e.g., $H_2O > H_2S$); High solubility (e.g., $NH_3$ in water).

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III. Summary and Comparison

| Type | Particles | Strength | Main Impact | Example |

| :--- | :--- | :--- | :--- | :--- |

| **Ionic** | Ions | Strong | Crystal stability, MP/BP | $NaCl$ |

| **Covalent** | Atoms | Strong | Molecular stability | $HCl$ |

| **Metallic** | Cations + Electrons | Variable | Conductivity, Ductility | $Cu$ |

| **H-Bond** | Molecules (H-N/O/F) | Weak (10-40 kJ) | High MP/BP, Solubility | $H_2O$ |

| **VdW** | Molecules | Very Weak (<10 kJ) | MP/BP, Liquefaction | $Ar, CO_2$ |

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IV. Typical Examples

**Ex 1: Identifying Bonds**

Identify bonds in $NH_4Cl$.

* **Solution**: Ionic compound ($NH_4^+$ and $Cl^-$). Contains **Ionic Bonds**. Within $NH_4^+$, N and H are linked by **Covalent Bonds** (including Coordinate Covalent). Total: **Ionic & Covalent**.

**Ex 2: BP Comparison**

Compare BP of $H_2O$ vs $H_2S$. Why?

* **Solution**: $H_2O > H_2S$. Despite smaller mass, water has **Hydrogen Bonds**, which are much stronger than standard Van der Waals forces.

**Ex 3: Molecular Geometry (VSEPR)**

Analyze the geometry and polarity of $NH_3$.

* **Solution**:

1. N has 5 valence e-, forms 3 bonds, has 1 **Lone Pair**.

2. Lone pair repulsion pushes bonds down.

3. Shape: **Trigonal Pyramidal**.

4. Asymmetric $\rightarrow$ **Polar Molecule**.