2.5. Elements http://www.ck12.org
A:Carbon is the most common element in living things. It has the unique property of being able to combine with
many other elements as well as with itself. This allows carbon to form a huge number of different substances.
History of Elements
For thousands of years, people have wondered about the substances that make up matter. About 2500 years ago,
the Greek philosopher Aristotle argued that all matter is made up of just four elements, which he identified as earth,
air, water, and fire. He thought that different substances vary in their properties because they contain different
proportions of these four elements. Aristotle had the right idea, but he was wrong about which substances are
elements. Nonetheless, his four elements were accepted until just a few hundred years ago. Then scientists started
discovering many of the elements with which we are familiar today. Eventually they discovered dozens of different
elements. You can read how and when the different elements were discovered at this URL: http://www.nndc.bnl.
gov/content/origindc.pdf.
Particles of Elements
The smallest particle of an element that still has the properties of that element is theatom. Atoms actually consist
of smaller particles, including protons and electrons, but these smaller particles are the same for all elements. All
the atoms of an element are like one another, and are different from the atoms of all other elements. For example,
the atoms of each element have a unique number of protons.
Consider carbon as an example. Carbon atoms have six protons. They also have six electrons. All carbon atoms are
the same whether they are found in a lump of coal or a teaspoon of table sugar (Figure2.9). On the other hand,
carbon atoms are different from the atoms of hydrogen, which are also found in coal and sugar. Each hydrogen atom
has just one proton and one electron.
FIGURE 2.9
Carbon is the main element in coal (left).
Carbon is also a major component of
sugar (right).Q:Why do you think coal and sugar are so different from one another when carbon is a major component of each
substance?
A:Coal and sugar differ from one another because they contain different proportions of carbon and other elements.
For example, coal is about 85 percent carbon, whereas table sugar is about 42 percent carbon. Both coal and sugar
also contain the elements hydrogen and oxygen but in different proportions. In addition, coal contains the elements
nitrogen and sulfur.
Summary
- An element is a pure substance that cannot be separated into any other substances. There are 92 naturally
occurring elements.