3: Elements and Compounds
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There were 2 categories of matter introduced in chapter 2 that we will look at in more detail here: elements and compounds. Elements are made up of atoms and are the building blocks of matter. We need to understand a little bit about atoms in order to understand how the rest of matter works. Section 3.1 details the properties of atoms, including the even smaller particles that they are made up of, and why those particles affect properties of the atom itself. Section 3.2 provides a shortened summary of the ideas introduced in section 3.1. Sections 3.3 through 3.5 discuss some of the different possibilities for compounds. It is important to understand the 2 major types of compounds (ionic and covalent) before learning how to name compounds. The naming of compounds is covered in section 3.6.
- 3.1: Properties of Atoms
- You learned earlier how all matter in the universe is made out of tiny building blocks called atoms. All modern scientists accept the concept of the atom, but when the concept of the atom was first proposed about 2,500 years ago, ancient philosophers laughed at the idea. It has always been difficult to convince people of the existence of things that are too small to see. We will spend some time considering the evidence (observations) that convince scientists of the existence of atoms.
- 3.2: Summary of Atomic Theory and the Construction of Atoms
- Chemistry is based on the modern atomic theory, which states that all matter is composed of atoms. Atoms themselves are composed of protons, neutrons, and electrons. Each element has its own atomic number, which is equal to the number of protons in its nucleus. Isotopes of an element contain different numbers of neutrons. Elements are represented by an atomic symbol. The periodic table is a chart that organizes all the elements.
- 3.3: Compounds
- A compound is a substance that contains two or more elements chemically combined in a fixed proportion. The elements carbon and hydrogen combine to form many different compounds. One of the simplest is called methane, in which there are always four times as many hydrogen particles as carbon particles. Methane is a pure substance because it always has the same composition. However, it is not an element because it can be broken down into simpler substances—carbon and hydrogen.
- 3.4: A Molecular View of Elements and Compounds
- Most elements exist with individual atoms as their basic unit. It is assumed that there is only one atom in a formula if there is no numerical subscript on the right side of an element’s symbol. There are many substances that exist as two or more atoms connected together so strongly that they behave as a single particle. These multi-atom combinations are called molecules. A molecule is the smallest part of a substance that has the physical and chemical properties of that substance.
- 3.5: Ionic Compounds
- Ionic compounds are formed by balancing charges of ions. In order to determine the formulas of ionic compounds, we need to understand the charges of ions as well as how to balance them when they come together.
- 3.6: How to Name Compounds
- Knowing how to name a compound correctly is an important skill both for the remainder of this course, and as you move into other fields. The most important step in naming a chemical is determining which category that chemical belongs in. As you read the sections on how to name different types of chemicals, please recognize the importance of these chemical classifications.
- 3.E: Atoms, Molecules, and Ions (Exercises)
- The following questions are related to the material covered in this chapter, however they may not be presented in the same order that they were in your chapter.