Elements: What is matter made of? Why is one kind of matter different from another kind of matter? Educated people in ancient Greece debated these questions. Around 450 b.c., a Greek philosopher named Empedocles proposed that all matter was made of four “elements”—air, earth, fire, and water. He thought that all other matter was a combination of two or more of these four elements. The idea of four elements was so convincing that people believed it for more than 2,000 years.
What Is an Element? In the late 1600s, experiments by the earliest chemists began to show that matter was made up of many more than four elements. Now, scientists know that all matter in the universe is made of slightly more than 100 different substances, still called elements. An element is a pure substance that cannot be broken down into any other substances by chemical or physical means. Elements are the simplest substances. Each element can be identified by its specific physical and chemical properties.
You are already familiar with some elements. Aluminum, which is used to make foil and outdoor furniture, is an element. Pennies are made from zinc, another element. Then the pennies are given a coating of copper, also an element. With each breath, you inhale the elements oxygen and nitrogen, which make up 99 percent of Earth’s atmosphere. Elements are often represented by one- or two-letter symbols, such as C for carbon, O for oxygen, and H for hydrogen.
Particles of Elements—Atoms: What is the smallest possible piece of matter? Suppose you could keep tearing a piece of aluminum foil in half over and over again. Would you reach a point where you have the smallest possible piece of aluminum? The answer is yes. Since the early 1800s, scientists have known that all matter is made of atoms. An atom is the basic particle from which all elements are made. Different elements have different properties because their atoms are different. Experiments in the early 1900s showed that an atom is made of even smaller parts. Look at the diagram of a carbon atom in Figure 5. The atom has a positively charged center, or nucleus, that contains smaller particles. It is surrounded by a “cloud” of negative charge.
When Atoms Combine: Atoms of most elements have the ability to combine with other atoms. When atoms combine, they form a chemical bond, which is a force of attraction between two atoms. In many cases, atoms combine to form larger particles called molecules (mahl uh kyoolz)—groups of two or more atoms held together by chemical bonds. A molecule of water, for example, consists of an oxygen atom chemically bonded to two hydrogen atoms. Two atoms of the same element can also combine to form a molecule. Oxygen molecules consist of two oxygen atoms. Figure 6 shows models of three molecules. You will see similar models throughout this book.
What Is an Element? In the late 1600s, experiments by the earliest chemists began to show that matter was made up of many more than four elements. Now, scientists know that all matter in the universe is made of slightly more than 100 different substances, still called elements. An element is a pure substance that cannot be broken down into any other substances by chemical or physical means. Elements are the simplest substances. Each element can be identified by its specific physical and chemical properties.
You are already familiar with some elements. Aluminum, which is used to make foil and outdoor furniture, is an element. Pennies are made from zinc, another element. Then the pennies are given a coating of copper, also an element. With each breath, you inhale the elements oxygen and nitrogen, which make up 99 percent of Earth’s atmosphere. Elements are often represented by one- or two-letter symbols, such as C for carbon, O for oxygen, and H for hydrogen.
Particles of Elements—Atoms: What is the smallest possible piece of matter? Suppose you could keep tearing a piece of aluminum foil in half over and over again. Would you reach a point where you have the smallest possible piece of aluminum? The answer is yes. Since the early 1800s, scientists have known that all matter is made of atoms. An atom is the basic particle from which all elements are made. Different elements have different properties because their atoms are different. Experiments in the early 1900s showed that an atom is made of even smaller parts. Look at the diagram of a carbon atom in Figure 5. The atom has a positively charged center, or nucleus, that contains smaller particles. It is surrounded by a “cloud” of negative charge.
When Atoms Combine: Atoms of most elements have the ability to combine with other atoms. When atoms combine, they form a chemical bond, which is a force of attraction between two atoms. In many cases, atoms combine to form larger particles called molecules (mahl uh kyoolz)—groups of two or more atoms held together by chemical bonds. A molecule of water, for example, consists of an oxygen atom chemically bonded to two hydrogen atoms. Two atoms of the same element can also combine to form a molecule. Oxygen molecules consist of two oxygen atoms. Figure 6 shows models of three molecules. You will see similar models throughout this book.
Compounds: All matter is made of elements, but most elements in nature are found combined with other elements. A compound is a pure substance made of two or more elements chemically combined in a set ratio. A compound may be represented by a chemical formula, which shows the elements in the compound and the ratio of atoms. For example, part of the gas you exhale is carbon dioxide. Its chemical formula is CO2. The number 2 below the symbol for oxygen tells you that the ratio of carbon to oxygen is 1 to 2. (If there is no number after the element’s symbol, the number 1 is understood.) If a different ratio of carbon atoms and oxygen atoms are seen in a formula, you have a different compound. For example, carbon monoxide—a gas produced in car engines—has the formula CO. Here, the ratio of carbon atoms to oxygen atoms is 1 to 1.
When elements are chemically combined, they form compounds having properties that are different from those of the uncombined elements. For example, the element sulfur is a yellow solid, and the element silver is a shiny metal. But when silver and sulfur combine, they form a compound called silver sulfide, Ag2S. You would call this black compound tarnish. Table sugar (C12H22O11) is a compound made of the elements carbon, hydrogen, and oxygen. The sugar crystals do not resemble the gases oxygen and hydrogen or the black carbon you see in charcoal.
When elements are chemically combined, they form compounds having properties that are different from those of the uncombined elements. For example, the element sulfur is a yellow solid, and the element silver is a shiny metal. But when silver and sulfur combine, they form a compound called silver sulfide, Ag2S. You would call this black compound tarnish. Table sugar (C12H22O11) is a compound made of the elements carbon, hydrogen, and oxygen. The sugar crystals do not resemble the gases oxygen and hydrogen or the black carbon you see in charcoal.
Elements and compounds are pure substances, but most of the materials you see every day are not. Instead, they are mixtures. A mixture is made of two or more substances—elements, compounds, or both—that are together in the same place but are not chemically combined. Mixtures differ from compounds in two ways. Each substance in a mixture keeps its individual properties. Also, the parts of a mixture are not combined in a set ratio.
Think of a handful of moist soil such as that in Figure 8. If you look at the soil through a magnifier, you will find particles of sand, bits of clay, maybe even pieces of decaying plants. If you squeeze the soil, you might force out a few drops of water. A sample of soil from a different place probably won’t contain the same amount of sand, clay, or water.
Think of a handful of moist soil such as that in Figure 8. If you look at the soil through a magnifier, you will find particles of sand, bits of clay, maybe even pieces of decaying plants. If you squeeze the soil, you might force out a few drops of water. A sample of soil from a different place probably won’t contain the same amount of sand, clay, or water.
