Introductory Chemistry: A Foundation - Zumdahl S.S., DeCoste D.J. 2019

Elements and Compounds


· To understand the definitions of elements and compounds.

As we examine the chemical changes of matter, we encounter a series of fundamental substances called elements . Elements cannot be broken down into other substances by chemical means. Examples of elements are iron, aluminum, oxygen, and hydrogen. All of the matter in the world around us contains elements. The elements sometimes are found in an isolated state, but more often they are combined with other elements. Most substances contain several elements combined together.

The atoms of certain elements have special affinities for each other. They bind together in special ways to form compounds , substances that have the same composition no matter where we find them. Because compounds are made of elements, they can be broken down into elements through chemical changes:

Water is an example of a compound. Pure water always has the same composition (the same relative amounts of hydrogen and oxygen) because it consists of molecules. Water can be broken down into the elements hydrogen and oxygen by chemical means, such as by the use of an electric current (Fig. 3.3).

As we will discuss in more detail in Chapter 4, each element is made up of a particular kind of atom: a pure sample of the element aluminum contains only aluminum atoms, elemental copper contains only copper atoms, and so on. Thus an element contains only one kind of atom; a sample of iron contains many atoms, but they are all iron atoms. Samples of certain pure elements do contain molecules; for example, hydrogen gas contains (usually written ) molecules, and oxygen gas contains molecules. However, any pure sample of an element contains only atoms of that element, never any atoms of any other element.

A compound always contains atoms of different elements. For example, water contains hydrogen atoms and oxygen atoms, and there are always exactly twice as many hydrogen atoms as oxygen atoms because water consists of molecules. A different compound, carbon dioxide, consists of molecules and so contains carbon atoms and oxygen atoms (always in the ratio ).

A compound, although it contains more than one type of atom, always has the same composition—that is, the same combination of atoms. The properties of a compound are typically very different from those of the elements it contains. For example, the properties of water are quite different from the properties of pure hydrogen and pure oxygen.