1. Introduction
2. A Basic Principle – Coulomb's Law
3. The Atom Construction Kit – Electrons and Nuclei
4. The Hydrogen Atom
5. Monoelectronic (Hydrogen-like) Ions
6. Polyelectronic Atoms and Ions
7. Atomic Excited States
8. Ionization Energies and Electron Affinities

One of the great human discoveries is that all of the matter around us—of which we ourselves are made—is built from a limited set of building blocks called elements that exist as individual atoms. Atoms themselves are constructed from just three kinds of subatomic particles, electrons, protons, and neutrons. From atoms are built the molecules and more complex materials that make up the world and the universe. Molecules and materials have properties that are derived from their structures and the forces that hold their atoms together. Understanding what those properties are and where they come from takes us a long way toward being able to predict how matter will behave under various physical conditions or what will happen if we mix two or more substances together.

The realm of atoms and molecules is sometimes described as the nanoworld because of the length scale. We live at a scale where we measure things in terms of inches, feet, yards, and miles—or perhaps centimeters, meters, and kilometers. The ruler appropriate for measuring atoms would be marked in fractions of a nanometer, hence nanoworld. A nanometer is one-billionth of a meter.

In this module, we will describe the manner in which atoms come together to form molecules through the formation of what chemists call chemical bonds. 19th century scientists like John Dalton were the first to deduce that atoms existed and combined in certain proportions to form molecular compounds. It was understood for a long time that some form of chemical bonding held atoms together. But to understand why molecules form, scientists needed to understand a number of other things first. They needed to understand what makes up atoms and why atoms exist with the structure we now take for granted, a nucleus of protons and neutrons surrounded by electrons. Understanding the basic structure of atoms only occurred with the discovery of quantum mechanics, the physics that dominates interactions in the nanoworld. Understanding the nature of chemical bonding followed: quantum mechanics is responsible both for the structure of atoms and for the forces that bring them together to form molecules and every other assemblage of atoms that make up all the materials that exist in our world. If you as a student are to understand both atoms and bonding, you need to understand some of the basic principles of quantum chemistry, which is the application of quantum mechanics to the chemical behavior of atoms and molecules.

But this leads to a dilemma. The math of quantum chemistry and quantum mechanics is quite advanced. It is way beyond what we expect you to have mastered as an introductory student, probably in your first year as an college or university undergraduate student. So in this module we will focus on the implications of quantum behavior rather than on the equations. If you take advanced chemistry courses like Physical Chemistry, you will learn more about the mathematics of quantum chemistry. The fundamental concepts will not change. They will only be treated at a more sophisticated level.

Teaching introductory students about the quantum chemical nature of atoms and molecules without the math is challenging. Here's how we're going to do it. Quantum chemists have developed various computer programs that use the complicated math to characterize how atoms and molecules behave and what they look like. The programs have been tested very well and are able to reproduce results that have been measured with experiments. The programs are trustworthy. In this module we're going to use those programs as a black box to perform virtual experiments without showing you the math involved.

1.1

In the example above, we want to find out what happens if we put together a proton (p⁺) and an electron (e^–). The programs tell us that we get a hydrogen atom (H) when we do that. The programs also tell us that the electron in H looks like the set of concentric spheres centered on the proton that is shown above on the right.

In a number of places throughout the module, you will find the icon shown on the right. If you click on it, you will learn a bit more about the subject from an advanced perspective.

If, on the other hand, you find that you have difficulty with something in the module, you can click on the icon shown on the right, which will provide additional ways to explain something for both instructors and students.

If you've taken chemistry in high school, you may note that we omit some ideas that you've probably seen before, including the Bohr model, Lewis structures, the octet rule, the VSEPR model, and Pauling hybridization. In each case, there are good reasons to not use the older approach, because modern theories of quantum chemistry have shown us there is a better, more rigorous way to cover the same general idea. Click on the icon to see a summary of why we think our approach is better than these approaches that are traditionally employed.

One final note. We want you to learn why atoms and molecules behave the way they do and not just memorize a bunch of things in order to pass quizzes and exams (yes, we were once students, too). Part of our approach is to ask questions frequently in order to encourage you to think through the concepts and even try to anticipate what we're going to tell you next. Once we've deduced important concepts, they will be placed in boxes in a larger font size. You should know these and understand where they came from.