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

Chemical Foundations: Elements, Atoms, and Ions
The Elements

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Foyer at the National Theatre in San Jose, Costa Rica, showing ornate gold decorations.

The chemical elements are very important to each of us in our daily lives. Although certain elements are present in our bodies in tiny amounts, they can have a profound impact on our health and behavior. As we will see in this chapter, lithium can be a miracle treatment for someone with bipolar disorder, and our cobalt levels can have a remarkable impact on whether we behave violently.

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Lithium is administered in the form of lithium carbonate pills.

Since ancient times, humans have used chemical changes to their advantage. The processing of ores to produce metals for ornaments and tools and the use of embalming fluids are two applications of chemistry that were used before 1000 B.C.

The Greeks were the first to try to explain why chemical changes occur. By about 400 B.C. they had proposed that all matter was composed of four fundamental substances: fire, earth, water, and air.

The next years of chemical history were dominated by alchemy. Some alchemists were mystics and fakes who were obsessed with the idea of turning cheap metals into gold. However, many alchemists were sincere scientists, and this period saw important events: the elements mercury, sulfur, and antimony were discovered, and alchemists learned how to prepare acids.

The first scientist to recognize the importance of careful measurements was the Irishman Robert Boyle (1627—1691). Boyle is best known for his pioneering work on the properties of gases, but his most important contribution to science was probably his insistence that science should be firmly grounded in experiments. For example, Boyle held no preconceived notions about how many elements there might be. His definition of the term element was based on experiments: a substance was an element unless it could be broken down into two or more simpler substances. For example, air could not be an element as the Greeks believed because it could be broken down into many pure substances.

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Portrait of the Hon. Robert Boyle, c. 1689 by Sir Peter Lely

As Boyle’s experimental definition of an element became generally accepted, the list of known elements grew, and the Greek system of four elements died. But although Boyle was an excellent scientist, he was not always right. For some reason he ignored his own definition of an element and clung to the alchemists’ views that metals were not true elements and that a way would be found eventually to change one metal into another.

The Elements

Objectives

· To learn about the relative abundances of the elements.

· To learn the names of some elements.

In studying the materials of the earth (and other parts of the universe), scientists have found that all matter can be broken down chemically into about different elements. At first it might seem amazing that the millions of known substances are composed of so few fundamental elements. Fortunately for those trying to understand and systematize it, nature often uses a relatively small number of fundamental units to assemble even extremely complex materials. For example, proteins, a group of substances that serve the human body in almost uncountable ways, are all made by linking together a few fundamental units to form huge molecules. A nonchemical example is the English language, where hundreds of thousands of words are constructed from only letters. If you take apart the thousands of words in an English dictionary, you will find only these fundamental components. In much the same way, when we take apart all of the substances in the world around us, we find only about fundamental building blocks—the elements. Compounds are made by combining atoms of the various elements, just as words are constructed from the letters of the alphabet. And just as you had to learn the letters of the alphabet before you learned to read and write, you need to learn the names and symbols of the chemical elements before you can read and write chemistry.

Presently about different elements are known,* of which occur naturally. (The rest have been made in laboratories.) The elements vary tremendously in abundance. In fact, only elements account for most of the compounds found in the earth’s crust. In Table 4.1, the elements are listed in order of their abundance (mass percent) in the earth’s crust, oceans, and atmosphere. Note that nearly half of the mass is accounted for by oxygen alone. Also note that the most abundant elements account for over of the total mass.

Table 4.1. Distribution (Mass Percent) of the Most Abundant Elements in the Earth’s Crust, Oceans, and Atmosphere

Oxygen, in addition to accounting for about of the earth’s atmosphere (where it occurs as molecules), is found in virtually all the rocks, sand, and soil on the earth’s crust. In these latter materials, oxygen is not present as molecules but exists in compounds that usually contain silicon and aluminum atoms. The familiar substances of the geological world, such as rocks and sand, contain large groups of silicon and oxygen atoms bound together to form huge clusters.

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Footprints in the sand of the Namib Desert in Namibia.

The list of elements found in living matter is very different from the list of elements found in the earth’s crust. Table 4.2 shows the distribution of elements in the human body. Oxygen, carbon, hydrogen, and nitrogen form the basis for all biologically important molecules. Some elements found in the body (called trace elements) are crucial for life even though they are present in relatively small amounts. For example, chromium helps the body use sugars to provide energy.

Table 4.2. Abundance of Elements in the Human Body

One more general comment is important at this point. As we have seen, elements are fundamental to understanding chemistry. However, students are often confused by the many different ways that chemists use the term element. Sometimes when we say element, we mean a single atom of that element. We might call this the microscopic form of an element. Other times when we use the term element, we mean a sample of the element large enough to weigh on a balance. Such a sample contains many, many atoms of the element, and we might call this the macroscopic form of the element. There is yet a further complication. As we will see in more detail in Section 4.9, the macroscopic forms of several elements contain molecules rather than individual atoms as the fundamental components. For example, chemists know that oxygen gas consists of molecules with two oxygen atoms connected together (represented as or more commonly as ). Thus when we refer to the element oxygen we might mean a single atom of oxygen, a single molecule, or a macroscopic sample containing many molecules. Finally, we often use the term element in a generic fashion. When we say the human body contains the element sodium or lithium, we do not mean that free elemental sodium or lithium is present. Rather, we mean that atoms of these elements are present in some form. In this text we will try to make clear what we mean when we use the term element in a particular case.