0.1 Atomic molecular theory

Introduction

In this study, our goal is to develop the concept of the Atomic Molecular Theory. This is the theory at the foundation of everything we understand about Chemistry, as it states that all matter is made up of individual particles called atoms, which combine in ways that are both simple and complex to form larger particles called molecules. When we understand these atoms and molecules, it changes the way that we look at the world around us. We can understand the properties of the substances we interact with, we can make predictions about the changes and reactions that these substances will undergo, and we can design materials with properties that would be useful to us.

The idea that everything is made of atoms is something we are told at a very early age, and for many students, it is hard then to imagine a world in which we don’t know that everything is made of atoms. On the other hand, this “particulate view” of matter does seem counter to almost all of our own observations. The desks in front of us, the air we breathe, the water we drink, and even our own flesh show no signs of these particles. Quite the opposite: they seem to be either very solid or quite fluid, and certainly not grainy like a collection of particles might be expected to be.

In this concept development study, then, we set aside our knowledge of these atoms and molecules and ask, quite skeptically, why do we believe that there are atoms that combine to form molecules? Or asked another way, if we believe that all matter is made up of atoms, how would we show that this is true? What is the evidence? Does the proof require us to “see” atoms, or is it possible to prove that they exist without actually seeing them?

Foundation

Chemistry is the study of matter , so it makes sense for us to agree on what we mean by matter and what we want to know about it. Technically, matter is anything that has mass, but more commonly, matter is what we regard as “stuff.” Anything that has physical properties and takes up space, whether a solid, a liquid, or gas, is matter. Matter can be anything from microscopic to galactic, or from rocks and air to butterflies and humans. But we can go further than this and focus on a specific type of matter called a pure substance . This is a material that is completely uniform in properties regardless of the size of the sample we take or from where we take the sample. It is easiest to understand a pure substance by comparing it to a mixture , which may or may not be uniform in its properties such as color, density, and texture and can vary depending on how we make the mixture or its origin. Showing that a substance is either a pure substance or a mixture requires a lot of experimentation, but we will assume for our foundation that we have already identified which samples of matter are pure substances and which are mixtures.

As of 2013, the most comprehensive list of chemical substances numbered over 70 million entries. This huge number of materials seems incomprehensible, far beyond our understanding. However, it turns out that these 70 million substances are all made up from a much smaller set of pure substances called elements. An element is a substance, which cannot be broken down into simpler substances. There are only about 90 commonly occurring elements on earth. The remaining 70 million pure substances are combinations of these elements called compounds , and these are distinguishable from the elements in that a compound can be broken down into the elements from which it is made. For example, metallic iron and gaseous oxygen are both elements, which cannot be reduced into simpler substances, but common iron rust, “ferrous oxide,” is a compound made up from iron and oxygen. Therefore, rust can be reduced to iron and oxygen, and rust can be created by combining iron and oxygen. But iron and oxygen are elements; they cannot be transformed into one another and are not composed of simpler or common materials.