Chemical bonds between atoms involve electrons.

Transcription

1 Chapter 6, Section 2 Key Concept: Chemical bonds hold compounds together. BEFORE, you learned Elements combine to form compounds Electrons are located in a cloud around the nucleus Atoms can lose or gain electrons to form ions NOW, you will learn How electrons are involved in chemical bonding About the different types of chemical bonds How chemical bonds affect structure THINK ABOUT How do you keep things together? Think about the different ways the workers at this construction site connect materials. They may use nails, screws, or even glue, depending on the materials they wish to keep together. Why would they choose the method they do? What factors do you consider when you join two objects? Chemical bonds between atoms involve electrons. Water is a compound of hydrogen and oxygen. The air you breathe, however, contains oxygen gas, a small amount of hydrogen gas, as well as some water vapor. How can hydrogen and oxygen be water sometimes and at other times not? The answer is by forming chemical bonds. Chemical bonds are the glue that holds the atoms of elements together in compounds. Chemical bonds are what make compounds more than just mixtures of atoms. Remember that an atom has a positively charged nucleus surrounded by a cloud of electrons. Chemical bonds form when the electrons in the electron clouds around two atoms interact. How the electron clouds interact determines the kind of chemical bond that is formed. Chemical bonds have a great effect on the chemical and physical properties of compounds. Chemical bonds also influence how different substances interact. You ll learn more about how substances interact in a later chapter. Atoms can transfer electrons. Ions are formed when atoms gain or lose electrons. Gaining electrons

2 changes an atom into a negative ion. Losing electrons changes an atom into a positive ion. Individual atoms do not form ions by themselves. Instead, ions typically form in pairs when one atom transfers one or more electrons to another atom. An element s location on the periodic table can give a clue as to the type of ions the atoms of that element will form. The illustration to the left shows the characteristic ions formed by several groups. Notice that all metals lose electrons to form positive ions. Group 1 metals commonly lose only one electron to form ions with a single positive charge. Group 2 metals commonly lose two electrons to form ions with two positive charges. Other metals, like the transition metals, also always form positive ions, but the number of electrons they may lose varies. Nonmetals form ions by gaining electrons. Group 17 nonmetals, for example, gain one electron to form ions with a 1 charge. The nonmetals in Group 16 gain two electrons to form ions with a 2 charge. The noble gases do not normally gain or lose electrons and so do not normally form ions. check your reading What type of ions do metals form? Ionic Bonds What happens when an atom of an element from Group 1, like sodium, meets an atom of an element from Group 17, like chlorine? Sodium is likely to lose an electron to form a positive ion. Chlorine is likely to gain an electron to form a negative ion. An electron, therefore, moves from the sodium atom to the chlorine atom. Remember that particles with opposite electrical charges attract one another.when the ions are created, therefore, they are drawn toward one another by electrical attraction. This force of attraction between positive and negative ions is called an ionic bond. Electrical forces act in all directions. Each ion, therefore, attracts

3 all other nearby ions with the opposite charge. The next illustration shows how this all-around attraction produces a network of sodium and chloride ions known as a sodium chloride crystal. Notice how each positive ion is surrounded by six negative ions, and each negative ion is surrounded by six positive ions. This regular arrangement gives the sodium chloride crystal its characteristic cubic shape. You can see this distinctive crystal shape when you look at table salt crystals through a magnifying glass. Ionic bonds form between all nearby ions of opposite charge. These interactions make ionic compounds very stable and their crystals very strong. Although sodium chloride crystals have a cubic shape, other ionic compounds form crystals with different regular patterns. The shape of the crystals of an ionic compound depends, in part, on the ratio of positive and negative ions and the sizes of the ions. Names of Ionic Compounds The name of an ionic compound is based on the names of the ions it is made of. The name for a positive ion is the same as the name of the atom from which it is formed. The name of a negative ion is formed by dropping the last part of the name of the atom and adding the suffix -ide. To name an ionic compound, the name of the positive

4 ion is placed first, followed by the name of the negative ion. For example, the chemical name for table salt is sodium chloride. Sodium is the positive sodium ion and chloride is the negative ion formed from chlorine. Therefore, to name the compound with the chemical formula BaI2 First, take the name of the positive metal element: barium. Second, take the name of the negative, nonmetal element, iodine, and give it the ending -ide: iodide. Third, combine the two names: barium iodide. Similarly, the name for KBr is potassium bromide, and the name for MgF 2 is magnesium fluoride. Atoms can share electrons. In general, an ionic bond forms between atoms that lose electrons easily to form positive ions, such as metals, and atoms that gain electrons easily to form negative ions, such as nonmetals. Another way in which atoms can bond together is by sharing electrons. Nonmetal atoms usually form bonds with each other by sharing electrons. Covalent Bonds A pair of shared electrons between two atoms is called a covalent bond. In forming a covalent bond, neither atom gains or loses an electron, so no ions are formed. The shared electrons are attracted to both positively charged nuclei. The illustrations below show a covalent bond between two iodine atoms. In the first illustration, notice how the electron clouds overlap. A covalent bond is also often represented as a line between the two atoms, as in the second illustration. The number of covalent bonds that an atom can form depends on the number of electrons that it has available for sharing. For example, atoms of the halogen group and hydrogen can contribute only one electron to a covalent bond. These atoms, therefore, can form only one covalent bond. Atoms of group 16 elements can form two covalent

5 bonds. Atoms of the elements of Group 15 can form three bonds. Carbon and silicon in Group 14 can form four bonds. For example, in methane (CH 4), carbon forms four covalent bonds with four hydrogen atoms, as shown below. Ball-and-Stick model Space-filling model We don t always show the lines representing the covalent bonds between the atoms. The space-filling model still shows the general shape of the bonded atoms, but occupies far less space on the page. Each carbon-hydrogen bond in methane is a single bond because one pair of electrons is shared between the atoms. Sometimes atoms may share more than one pair of electrons with another atom. For example, the carbon atom in carbon dioxide (CO2) forms double bonds with each of the oxygen atoms. A double bond consists of four (two pairs of) shared electrons. Two nitrogen atoms form a triple bond, meaning that they share six (three pairs of) electrons. A group of atoms held together by covalent bonds is called a molecule. A molecule can contain from two to many thousand atoms. Most molecules contain the atoms of two or more elements. For example, water (H 2O), ammonia (NH 3), and methane (CH 4)

6 are all compounds made up of molecules. However, some molecules contain atoms of only one element. The following elements exist as two-atom molecules: H 2, N 2, O 2, F 2, Cl 2, Br 2, and I 2. check your reading What is a molecule? Polar Covalent Bonds In an iodine molecule, both atoms are exactly the same. The shared electrons therefore are attracted equally to both nuclei. If the two atoms involved in a covalent bond are very different, however, the electrons have a stronger attraction to one nucleus than to the other and spend more time near that nucleus. A covalent bond in which the electrons are shared unequally is called a polar covalent bond. The word polar refers to anything that has two extremes, like a magnet with its two opposite poles. In a water molecule (H 2O), the oxygen atom attracts electrons far more strongly than the hydrogen atoms do. The oxygen nucleus has eight protons, and the hydrogen nucleus has only one proton. The oxygen atom pulls the shared electrons more strongly toward it. In a water molecule, therefore, the oxygen side has a slightly negative charge, and the hydrogen side has a slightly positive charge. Chemical bonds give all materials their structures. The substances around you have many different properties. The structure of the crystals and molecules that make up these substances are responsible for many of these properties. For example, crystals bend rays of light, metals shine, and medications attack certain diseases in the body because their atoms are arranged in specific ways. Ionic Compounds Most ionic compounds have a regular crystal structure. Remember how the size, shape, and ratio of the sodium ions and chloride ions

7 give the sodium chloride crystal its shape. Other ionic compounds, such as calcium chloride, have different but equally regular structures that depend upon the ratio and sizes of the ions. One consequence of such rigid structures is that, when enough force is applied to the crystal, it shatters rather than bends. Covalent Compounds Unlike ionic compounds, covalent compounds exist as individual molecules. Chemical bonds give each molecule a specific, three-dimensional shape called its molecular structure. Molecular structure can influence everything from how a specific substance feels to the touch to how well it interacts with other substances. A few basic molecular structures are shown below. Molecules can have a simple linear shape, like iodine (I 2), or they can be bent, like a water molecule (H 2O). The atoms in an ammonia molecule (NH 3) form a pyramid, and methane (CH 4) molecules even have a slightly more complex shape. The shape of a molecule depends on the atoms it contains and the bonds holding it together. Iodine (I2) water (H2O) Ammonia (NH3) Methane (CH4) Molecular shape can affect many properties of compounds. For example, there is some evidence to indicate that we detect scents because molecules with certain shapes fit into certain smell receptors in the nose. Molecules with similar shapes, therefore, should have similar smells.molecular structure also plays an essential role in how our bodies respond to certain drugs. Some drugs work because molecules with certain shapes can fit into specific receptors in body cells. Questions for Chapter 6, Section 2 KEY CONCEPTS 1. What part of an atom is involved in chemical bonding?

8 2. How are ionic bonds and covalent bonds different? 3. Describe two ways that crystal and molecular structures affect the properties of ionic and covalent compounds. CRITICAL THINKING 4. Analyze Would you expect the bonds in ammonia to be polar covalent? Why or why not? 5. Infer What kind of bond would you expect atoms of strontium and iodine to form? Why? Write the formula and name the compound. CHALLENGE 6. Conclude Is the element silicon likely to form ionic or covalent bonds? Explain.