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1 Objectives Compare and contrast ionic bonds and covalent bonds. Describe various ways to represent molecules. Summarize what happens in a chemical reaction. Key Terms ionic bond ion covalent bond molecule chemical reaction reactant product Reactions between atoms result in filled outer energy levels. One atom may transfer electrons to another, or two atoms may share electrons. This process of transferring or sharing electrons creates an attraction a chemical bond that holds the atoms together. Ionic Bonds One type of chemical bond, an ionic bond, occurs when an atom transfers an electron to another atom. Table salt, or sodium chloride (NaCl), is a compound formed as a result of electron transfer between sodium (Na) atoms and chlorine (Cl) atoms. You can follow the reaction between these atoms in Figure 4-8. Notice that at the beginning, sodium has only 1 electron in its highest energy level, whereas chlorine has 7. At this point, each atom has an equal number of protons and electrons and therefore is electrically neutral. 1 of 5 10/5/11 6:48 AM

2 Figure 4-8 The transfer of an electron from a sodium atom to a chlorine atom results in oppositely charged ions. The attraction between these ions is an ionic bond. When the two atoms collide, the chlorine atom strips away sodium's outer electron. In the process, chlorine's highest energy level, now with 8 electrons, becomes filled. In losing an electron, the sodium atom's second energy level, which already has 8 electrons, becomes the highest. It, too, is filled. Look at how the balance of electric charges has changed. One unit of negative charge (1 electron) moved from sodium to chlorine. The two atoms are now referred to as ions atoms (or groups of atoms) that have become electrically charged as a result of gaining or losing electrons. Losing an electron leaves the sodium ion with a charge of 1+, while gaining an electron gives chlorine (now called chloride) a charge of 1-. The attraction holding the oppositely charged Na + and Cl - ions together is the ionic bond. Covalent Bonds In contrast to the transfer of electrons that results in an ionic bond, a covalent bond forms when two atoms share electrons. Electron sharing can be modeled using element symbols, with dots representing the atoms' outermost electrons (Figure 4-9). In the first example, each hydrogen atom (H) shares its electron with another hydrogen atom. This shared pair of electrons is one covalent bond. In the second example, an oxygen atom (O) shares a pair of electrons with each of two hydrogen atoms, forming two covalent bonds. 2 of 5 10/5/11 6:48 AM

3 Figure 4-9 A pair of electrons (dots) between atoms represents a covalent bond. The electron pair counts in the total of outermost electrons for both atoms. You can count 8 electrons around the oxygen atom on the bottom right. Its highest energy level is filled as a result of sharing electrons with the hydrogen atoms. Numbers of Bonds The number of bonds an atom can form usually equals the number of additional electrons that will fill its highest energy level. A hydrogen atom can accept one additional electron, so it can form one bond. In contrast, an oxygen atom can accept two electrons in its highest energy level, so it can form two bonds. Molecules Two or more atoms held together by covalent bonds form a molecule. Molecules can be modeled in different ways (Figure 4-10). A chemical formula tells you the number and types of atoms in a molecule. For example, the chemical formula H 2 O indicates that a water molecule contains two atoms of hydrogen and one atom of oxygen. (The absence of a subscript after the oxygen symbol is understood to mean "1.") A structural formula indicates how atoms in a molecule are linked by bonds. Each line between symbols represents a single covalent bond a shared pair of electrons. Double or even triple bonds form when two atoms share two or three pairs of electrons between them. The two lines in the structural formula for oxygen (O 2 ) indicate that an oxygen molecule contains a double bond. A space-filling model, in which color-coded spheres symbolize atoms, is a drawing that depicts a three-dimensional model of a molecule. You will see all three of these representations used in this book. 3 of 5 10/5/11 6:48 AM

4 Figure 4-10 Some molecules consist of atoms of only one element, as in molecules of hydrogen (H 2 ) and oxygen (O 2 ). However, most molecules are made of two or more different types of atoms, as in water (H 2 O). Chemical Reactions Within your cells certain molecules constantly become rearranged into other molecules as existing chemical bonds break (absorbing energy from the surroundings) and new ones form (releasing energy to the surroundings). Such changes, which result in the formation of one or more new substances, are called chemical reactions. Some chemical reactions absorb more energy than they release, while others release more energy than they absorb. An example of a reaction that has a net release of energy is the one that occurs between hydrogen and oxygen, forming water (Figure 4-11). In fact, this reaction can be so explosive that it is harnessed to help power the space shuttle. Figure 4-11 Space-filling models for molecules of hydrogen, oxygen, and water illustrate the chemical reaction that is described by the equation. A water molecule always consists of hydrogen atoms and oxygen atoms in a ratio of 2 to 1. 4 of 5 10/5/11 6:48 AM

5 The expression above, called a chemical equation, is a convenient way to describe a chemical reaction. Just like a math equation, such as = 2, you read this equation from left to right. It states that two molecules of hydrogen (2 x H 2 ) react with one molecule of oxygen (O 2 ), forming two molecules of water (2 x H 2 O). The starting materials for the reaction (hydrogen and oxygen) are called reactants. The ending materials (in this case, only water) are called products. Notice that the same number of hydrogen atoms (4) is present on both sides of the equation, although the atoms are combined in different molecules. The same is true of the oxygen atoms (2 on each side). Chemical reactions do not create or destroy atoms, but only rearrange them. These rearrangements usually involve breaking chemical bonds in reactants and forming new bonds in products. Concept Check Describe how an ionic bond forms. How is this process different from the formation of a covalent bond? 2. In a molecule represented by the chemical formula C 6 H 12 O 6, how many atoms of each element are present? 3. What information does a chemical equation provide about a chemical reaction? 4. A carbon atom has 4 electrons in its highest energy level. How many hydrogen atoms could become bonded to a single carbon atom? Copyright 2006 by Pearson Education, Inc., publishing as Pearson Prentice Hall. All rights reserved. 5 of 5 10/5/11 6:48 AM