# 4: The Basics of Chemistry

The chapters in this unit are absolutely essential for anyone embarking on the serious study of Chemistry. The material covered here will be needed in virtually every topic you will encounter in the remainder of your first-year course, as well as in subsequent Chemistry courses — so you might as well master it now!

• 4.1: Atoms, Elements, and the Nucleus
The parallel concepts of the element and the atom constitute the very foundations of chemical science. The concept of the element is a macroscopic one that relates to the world that we can observe with our senses. The atom is the microscopic realization of this concept; that is, it is the actual physical particle that is unique to each chemical element. Their very small size has long prevented atoms from being observable by direct means, so their existence was not universally accepted until the
• 4.2: Avogadro's Number and the Mole
The chemical changes we observe always involve discrete numbers of atoms that rearrange themselves into new configurations. These numbers are far too large in magnitude for us to count , but they are still numbers, and we need to have a way to deal with them. We also need a bridge between these numbers, which we are unable to measure directly, and the weights of substances, which we do measure and observe. The mole concept provides this bridge, and is key to all of quantitative chemistry.
• 4.3: Formulas and Their Meaning
At the heart of chemistry are substances — elements or compounds— which have adefinite composition which is expressed by a chemical formula. In this unit you will learn how to write and interpret chemical formulas both in terms of moles and masses, and to go in the reverse direction, in which we use experimental information about the composition of a compound to work out a formula.
• 4.4: Chemical Equations and Stoichiometry
A chemical equation expresses the net change in composition associated with a chemical reaction by showing the number of moles of reactants and products. But because each component has its own molar mass, equations also implicitly define the way in which the masses of products and reactants are related. In this unit we will concentrate on understanding and making use of these mass relations.
• 4.5: Introduction to Chemical Nomenclature
Chemical nomenclature is far too big a topic to treat comprehensively, and it would be a useless diversion to attempt to do so in a beginning course; most chemistry students pick up chemical names and the rules governing them as they go along. But we can hardly talk about chemistry without mentioning some chemical substances, all of which do have names— and often, more than one!
• 4.6: Significant Figures and Rounding
The numerical values we deal with in science (and in many other aspects of life) represent measurements whose values are never known exactly. Our pocket-calculators or computers don't know this; they treat the numbers we punch into them as "pure" mathematical entities, with the result that the operations of arithmetic frequently yield answers that are physically ridiculous even though mathematically correct.

## Contributors

Thumbnail: Spinning Buckminsterfullerene ($$\ce{C60}$$). Image used with permission (CC BY-SA 3.0; unported; Sponk).