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Unit 5: The Structure of Molecules—Lewis Structures and Molecular Geometries

Section 1: Introduction

While the universe is made up of over a hundred elements that can be organized into a helpful periodic table, what matters most for us to understand the chemical world is how these atoms organize together into the substances we see every day. It turns out that the electrons in the atoms, which were the focus of much of Units 3 and 4, are the stars of the show; these electrons are what will keep these atoms bound together in the substances that make up everything around us, including ourselves.

Over the course of this unit, we will see that by looking at some of the basic rules for how electrons behave and interact with each other will help us understand how atoms are held to each other. These interactions that hold two or more atoms together are called "bonds." Two specific types of bonding, ionic and covalent bonding will be the focus of this unit, and a third type of bonding, metallic bonding, will be addressed in Unit 13.

Representation of a Collection of Molecules

Figure 5-1. Representation of a Collection of Molecules

© A and B: Science Media Group. C: Wikimedia Commons, Public Domain.

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Representation of a Collection of Molecules

Figure 5-1. Representation of a Collection of Molecules

A collection of increasingly complex representations of molecules and their structures is shown here. A. This is a Lewis structure of benzene. B. This is a picture of the structure of taxol as an organic line structure, to give a better idea of its overall shape. C. This is a representation of the enzyme catalase, where the colored shapes and spirals are a way of representing the 3-dimensional structure of large biological molecules.

In addition to the forces holding atoms together in molecules, we'll also take a look at some intermolecular forces, that is, the forces that exist between different molecules. Intermolecular forces can cause molecules to stick together by mutual attraction, like water droplets.

To examine the question of molecules' physical structure—their geometry—we'll delve into the key methods for representing the structure of atoms and molecules: Lewis dot symbolism, VSEPR models, and hybrid orbitals. We'll then consider how, just like the pieces in a 3-D puzzle, a substance's geometry determines its macroscopic properties—how it will behave. In Figure 5-1, we can see how molecules range from what looks like a simple structure for benzene, a carcinogenic but common laboratory solvent, up to more complicated biological molecules. Figure 5-1A shows a Lewis structure of benzene and Figure 5-1B shows the compound taxol, an anticancer drug that is isolated from the Pacific yew tree. Here, taxol is drawn as an organic line structure, a Lewis structure where the carbons and hydrogens are hidden to make the pictures less cluttered. Lastly, Figure 5-1C shows a representation of catalase, which is the enzyme found in our cells that protects us against dangerous peroxides. This enzyme is what causes the bubbling to happen when we put hydrogen peroxide on a cut to disinfect it. In this case, this molecule is so large, with thousands of atoms, that chemists use pictures that show the 3-dimensional shapes of the molecule, rather than displaying all of the individual atoms.



An association between two atoms in which they remain close together in space due to some type of electron-based interaction.

Intermolecular forces

Forces that exist between different molecules.

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