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2.5: Elements

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As this mountain of trash suggests, there are many different kinds of matter. In fact, there are millions of different kinds of matter in the universe. Yet all kinds of matter actually consist of relatively few pure substances.

Pure Substances

A pure substance is called an element . An element is a pure substance because it cannot be separated into any other substances. Currently, 92 different elements are known to exist in nature, although additional elements have been formed in labs. All matter consists of one or more of these elements. Some elements are very common; others are relatively rare. The most common element in the universe is hydrogen, which is part of Earth’s atmosphere and a component of water. The most common element in Earth’s atmosphere is nitrogen, and the most common element in Earth’s crust is oxygen. Several other elements are described in the musical video at this URL:

http://www.youtube.com/watch?v=d0zION8xjbM (3:47)

Elemental Properties

Each element has a unique set of properties that is different from the set of properties of any other element. For example, the element iron is a solid that is attracted by a magnet and can be made into a magnet, like the compass needle shown in the Figure below . The element neon, on the other hand, is a gas that gives off a red glow when electricity flows through it. The lighted sign in the Figure below contains neon.

The needle of this compass is made of the element iron.

The red lights in this sign contain the element neon.

Q: Do you know properties of any other elements? For example, what do you know about helium?

A: Helium is a gas that has a lower density than air. That’s why helium balloons have to be weighted down so they won’t float away.

Q: Living things, like all matter, are made of elements. Do you know which element is most common in living things?

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 the atom . 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 ( Figure below ). 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.

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.
  • Each element has a unique set of properties that is different from the set of properties of any other element.
  • For about 2000 years, people accepted Aristotle’s idea that all matter is made up of just four elements: earth, air, water, and fire. Starting about 500 years ago, scientists began discovering all of the elements that are known today.
  • The smallest particle of an element that still has the properties of that element is the atom. All the atoms of an element are like one another, and are different from the atoms of all other elements.

Vocabulary

  • atom : Smallest particle of an element that still has the element’s properties.
  • element : Pure substance that cannot be separated into any other substances.

Practice

At the following URL, choose any three elements and learn about their properties, history, and uses. Then create a table comparing and contrasting the three elements.

http://www.webelements.com/

Review

  1. What is an element?
  2. Why can an element be identified by its properties?
  3. Explain why the following statement is either true or false: The idea that all matter consists of the elements was first introduced a few hundred years ago.
  4. How are atoms related to elements?

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Difficulty Level:

At Grade

Concept Nodes:

Grades:

7 , 8

Date Created:

Oct 31, 2012

Last Modified:

Sep 05, 2014
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