Combining Matter

INKAtoms combine through electric forces, forming molecules and compounds.

Real-World Reading Link Is there a rusty mailbox or bicycle on your street? Nearly everywhere you look, you can see iron objects that have become rusty. Rust forms when iron is exposed to water and oxygen in the air.


Can you identify the materials in Figure 3.7? The greenish gas in the flask is the element chlorine, which is poisonous. The solid, silvery metal is the element sodium, which is highly reactive. These two elements combine chemically to form the third material in the photograph—table salt. How can two dangerous elements combine to form a material that you sprinkle on your popcorn?

Table salt is a compound, not an element. A compound is a substance that is composed of atoms of two or more different elements that are chemically combined. Water is another example of a compound because it is composed of two elements—hydrogen and oxygen. Most compounds have different properties from the elements of which they are composed. For example, both oxygen and hydrogen are highly flammable gases at room temperature, but in combination they form water—a liquid.

Chemical formulas Compounds are represented by chemical formulas. These formulas include the symbol for each element followed by a subscript number that stands for the number of atoms of that element in the compound. If there is only one atom of an element, no subscript number follows the symbol. Thus, the chemical formula for table salt is NaCl. The chemical formula for water is H2O.

Figure 3.7 Sodium is a silvery metal that is soft enough to cut with a knife. Chlorine is a green, poisonous gas. When they react, they produce sodium chloride, a white solid.

Figure 3.7 Sodium is a silvery metal that is soft enough to cut with a knife. Chlorine is a green, poisonous gas. When they react, they produce sodium chloride, a white solid.

Chemical Structure Sodium Chlorite

66 Chapter 3 • Matter and Change

Stephen Frisch/Stock Boston

Covalent Bonds

Recall that an atom is chemically stable when its outermost energy level is full. A state of stability is achieved by some elements by forming chemical bonds. A chemical bond is the force that holds together the elements in a compound. One way in which atoms fill their outermost energy levels is by sharing electrons. For example, individual atoms of hydrogen each have just one electron. Each atom becomes more stable when it shares its electron with another hydrogen atom so that each atom has two electrons in its outermost energy level. Figure 3.8 shows an example of this bond. How do these two atoms stay together? The nucleus of each atom has one proton with a positive charge, and the two positively charged protons attract the two negatively charged electrons. This attraction of two atoms for a shared pair of electrons that holds the atoms together is called a covalent bond.

Molecules A molecule is composed of two or more atoms held together by covalent bonds. Molecules have no overall electric charge because the total number of electrons equals the total number of protons. Water is an example of a compound whose atoms are held together by covalent bonds, as illustrated in Figure 3.9. The chemical formula for a water molecule is H2O because, in this molecule, two atoms of hydrogen, each of which need to gain an electron to become stable, are combined with one atom of oxygen, which needs to gain two electrons to become stable. A compound comprised of molecules is called a molecular compound.

Polar molecules Although water molecules are held together by covalent bonds, the atoms do not share the electrons equally. As shown in Figure 3.9, the shared electrons in a water molecule are attracted more strongly by the oxygen atom than by the hydrogen atoms. As a result, the electrons spend more time near the oxygen atom than they do near the hydrogen atoms. This unequal sharing of electrons results in polar molecules. A polar molecule has a slightly positive end and a slightly negative end.

Covalent bond i Figure 3.8 In this covalent bond example, notice the positions of the electrons in the outermost energy levels. They can now be considered as part of each atom.


Science usage v. Common usage Polar

Science usage: the unequal sharing of • electrons

Common usage: locations of or near ; the north or south pole, or the ends of a magnet ;

Covalent Bond Examples
Figure 3.9 Polar molecules are similar to bar magnets. At one end of a water molecule, the hydrogen atoms have a positive charge, while at the opposite end, the oxygen atom has a negative charge.
Ionic Bond Hydrogen And Chlorine
Figure 3.10 The single valence electron in a sodium atom is used to form an ionic bond with a chlorine atom. Once an ionic bond is formed, the negatively charged ion is slightly larger than the positively charged ion.


Interactive Figure To see an animation of ionic bonds, visit

Figure 3.11 Metallic bonds are formed when valence electrons are shared equally among all the positively charged atoms. Because the electrons flow freely among the positively charged ions, you can visualize electricity flowing through electrical wires.


Interactive Figure To see an animation of electron flow, visit

Metallic bond

Ionic Bonds

As you might expect, positive and negative ions attract each other. An ionic bond is the attractive force between two ions of opposite charge. Figure 3.10 illustrates an ionic bond between a positive ion of sodium and a negative ion of chlorine called chloride. The chemical formula for common table salt is NaCl, which consists of equal numbers of sodium ions (Na+) and chloride ions (Cl) . Note that positive ions are always written first in chemical formulas.

Within the compound NaCl, there are as many positive ions as negative ions; therefore, the positive charge on the sodium ion equals the negative charge on the chloride ion, and the net electric charge of the compound NaCl is zero. Magnesium and oxygen ions combine in a similar manner to form the compound magnesium oxide (MgO)—one of the most common compounds on Earth. Compounds formed by ionic bonding are called ionic compounds. Other ionic compounds have different proportions of ions. For example, oxygen and sodium ions combine in the ratio shown by the chemical formula for sodium oxide (Na2O), in which there are two sodium ions to each oxygen ion.

^p Reading Check Describe how ionic bonds form.

Metallic Bonding

Most compounds on Earth are held together by ionic or covalent bonds, or by a combination of these bonds. Another type of bond is shown in Figure 3.11. In metals, the valence electrons are shared by all the atoms, not just by adjacent atoms as they are in covalent compounds. You could think of a metal as a group of positive ions surrounded by a sea of freely moving negative electrons. The positive ions of the metal are held together by the attraction to the negative electrons between them. This type of bond, known as a metallic bond, allows metals to conduct electricity because the electrons can move freely throughout the entire solid metal.

Metallic bonding also explains why metals are so easily deformed. When a force is applied to a metal, such as the blow of a hammer, the electrons are pushed aside. This allows the metal ions to move past each other, thus deforming or changing the shape of the metal. Figure 3.12 summarizes how valence electrons are used to form the three different types of bonds.

Metallic bond

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  • emma
    How sodium and chlorine form ions?
    8 years ago
  • jan-erik
    What has a positive charge?
    8 years ago
  • Balbo
    What is the diagram of atoms for hydrogen?
    8 years ago
  • publio
    How does NA2O form from sodium and oxygen?
    6 years ago

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