# Bonding and molecular geometry

Physics I · Chemistry · https://tryals.app/learn/physics-i/bonding-and-molecular-geometry

## Chemical Bonds and Lewis Structures

Atoms bond to reach a **lower energy state**. The type of bond depends on electronegativity difference ($\Delta\chi$).

| Bond | Description | Example |
| --- | --- | --- |
| **Ionic** | Electron transfer ($\Delta\chi > 1.7$) | NaCl |
| **Covalent** | Electron sharing ($\Delta\chi < 1.7$) | H$_2$O |
| **Metallic** | Delocalised electron sea | Cu |

**Lewis structures** map valence electrons as shared bonds and lone pairs, guided by the **octet rule** for main-group atoms.

$$\Delta H_{\text{rxn}} = \sum D_{\text{broken}} - \sum D_{\text{formed}}$$

A **negative** $\Delta H$ indicates an **exothermic** reaction.

## VSEPR Theory and Dipoles

**VSEPR theory** dictates that **molecular shape** stems from electron-pair repulsion. Calculate the **steric number**: $\text{Steric no.} = \text{bonding pairs} + \text{lone pairs}$.

| Steric no. | Geometry | Example |
| --- | --- | --- |
| 2 | Linear | CO$_2$ |
| 3 | Trigonal planar | BF$_3$ |
| 4 | Tetrahedral | CH$_4$ |

The total molecular dipole is the vector sum: $\vec{\mu}_{\text{total}} = \sum \vec{\mu}_i$.

> **Key insight:** Symmetric molecules like CO$_2$ can have polar bonds but **zero net dipole** because vectors cancel.

> **Common pitfall:** **All** electron groups set the shape, but named geometries describe only the **atoms**. Water has four tetrahedral groups yet is bent.

## Practice questions

6 of this lesson's 13 practice questions, with answers. The full set is in the app.

### 1. Water is H–O–H, yet the molecule is **bent** (104.5°), not straight. What bends it?

A. Two lone pairs on oxygen push the O–H bonds together
B. Gravity pulls the hydrogen atoms downward
C. Hydrogen atoms attract each other, closing the angle
D. The O–H bonds are different lengths

**Answer:** A. Two lone pairs on oxygen push the O–H bonds together

**Why:** VSEPR: four electron groups adopt a tetrahedron; the two invisible lone pairs claim their corners and shove the bonds together to 104.5°. That bend makes water polar, and water’s polarity underwrites solubility, hydrogen bonding, and life.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/water-is-h-o-h-yet-the-molecule-is-bent-104-5-not-straight-what

### 2. Sort each bond by its character, using electronegativity differences (ΔEN).

**Answer:**

- Nonpolar covalent: Cl–Cl (ΔEN = 0), C–H (ΔEN ≈ 0.4)
- Polar covalent: H–O (ΔEN ≈ 1.4), H–Cl (ΔEN ≈ 0.9)
- Ionic: Na–Cl (ΔEN ≈ 2.1)

**Why:** ΔEN measures how unevenly the bonding electrons are shared: even (nonpolar), lopsided (polar), or captured (ionic). C–H’s near-evenness is why oils repel water; H–O’s lopsidedness is why water dissolves salt.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/sort-each-bond-by-its-character-using-electronegativity-differences

### 3. Which molecular geometry is characterized by a bond angle of ~104.5° due to the strong repulsion of two lone pairs on the central atom?

A. Tetrahedral (~109.5°)
B. Trigonal Pyramidal (~107°)
C. Linear (180°)
D. Bent (~104.5°)

**Answer:** D. Bent (~104.5°)

**Why:** Two lone pairs push the bonding pairs closer together, compressing the ideal 109.5° tetrahedral angle down to a bent ~104.5°.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/which-molecular-geometry-is-characterized-by-a-bond-angle-of-104-5

### 4. A polar covalent bond forms when two atoms of different electronegativity share electrons unequally.

**Answer:** True

**Why:** The more electronegative atom attracts the shared electrons more strongly, creating a partial negative charge ($\delta$−) on itself and $\delta$+ on the other atom.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/a-polar-covalent-bond-forms-when-two-atoms-of-different

### 5. Every C=O bond in $CO_2$ is strongly polar, yet the molecule has no dipole moment at all. Why does bond polarity not settle molecular polarity?

A. Because the two bonds share electrons and so neutralise each other chemically
B. Because a molecule with more than one polar bond can never be polar
C. Because dipoles are vectors, and $CO_2$'s cancel by symmetry
D. Because carbon and oxygen have equal electronegativities in a double bond

**Answer:** C. Because dipoles are vectors, and $CO_2$'s cancel by symmetry

**Why:** Shape decides. This is why $CO_2$ does not dissolve well in water and $SO_2$, which is bent, does, the same polar bonds with a different geometry give opposite results.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/every-c-o-bond-in-co2-is-strongly-polar-yet-the-molecule-has-no

### 6. Order the steps to predict molecular geometry using VSEPR theory.

**Answer:**

1. Draw the Lewis structure showing all valence electrons
2. Count bonding pairs and lone pairs around the central atom
3. VSEPR: electron pairs arrange to minimise mutual repulsion
4. Name the molecular shape from the positions of bonded atoms only

**Why:** VSEPR (Valence Shell Electron Pair Repulsion) predicts shape from electron-pair geometry; lone pairs occupy space but are not counted in the molecular shape name.

Page: https://tryals.app/practice/physics-i/bonding-and-molecular-geometry/order-the-steps-to-predict-molecular-geometry-using-vsepr-theory