Chapter 9 Chemical Bonds, Compounds, and Naming 1Chapter 9(S 20)CHEMICAL BONDS, COMPOUNDS, AND NAMINGTHREE TYPES OF BONDS Atoms react by either gaining electrons, losing electrons, or sharing electrons. Metals like to lose electrons, and nonmetals like to gain electrons. There are the only three possible combinations of metals and nonmetals: 1. Ionic Bonds: metal + nonmetal ®metal+nonmetal-The ionic bond forms when opposite charges attract. An ionic compound has at least one ionic bond, but may also have covalent bonds. The formation of an ionic compound happens as a result of an oxidation-reduction (redox) reaction: Metals like to lose electrons (oxidation) to become isoelectronic to a noble gas, s2p6, if they can. Nonmetals like to gain electrons (reduction) to become isoelectronic to a noble gas, s2p6, if they can. Here is an example of the formation of an ionic between magnesium and chlorine to form magnesium chloride, MgCl2 .. .. Mg: + 2 :Cl®Mg+2[:Cl:]-12¨¨[Ne]3s2[Ne]3s23p5[Ne] [Ne]3s23p6or [Ar] 2. Covalent Bonds: nonmetal + nonmetal ®nonmetal-nonmetal Covalent means to share valence electrons. A covalent compound contains only covalent bonds. In a covalent bond both atoms are attracted to the same electron cloud. The atoms themselves are not attracted to each other but are repelled by the negative surfaces of one another. The bond forms because the shared electrons help each atom obtain a more stable electron configuration, like a noble gas, and the repulsive forces are overpowered by the lowered electronic energy state provided by the bond. .. .. .. .. :Cl+ Cl: ®:Cl ¾Cl: + energy ¨¨¨¨s2p5s2p5“s2p6” “s2p6” atomic orbitals molecular orbitals - + + - higher energy, no bonding, repulsionthese are only theoretical - + - + lower energy, bonds form and less repulsionrepresents significant electron clouds between nuclei Thus, the two chlorine atoms repel one another but are attracted to the same electron cloud and thus form the covalent bond. Note that chlorine atoms have atomic orbitals and that chlorine molecules have molecular orbitals. Molecular orbitals represent the overlapping of the atomic orbitals. The figure only shows the 2pzorbitals, the only ones involved in formation of this bond. The periodic table gives both the atomic radius and the covalent radius for the elements. Sometimes the sum of the two covalent radii, the bond length, is longer than the sum of the two atomic radii, and sometimes it is shorter. For molecular chlorine the bond length is slightly longer,
Chapter 9 Chemical Bonds, Compounds, and Naming 2but it would require 243 kJ to break the bond, Cl2+ 243 kJ ®2 Cl. Another way of saying this is that forming the bond releases 243 kJ, 2 Cl ®Cl2+ 243 kJ. Thus, forming Cl2is an exothermic process. The strength and length of a covalent bond is a result of the net attractive and repulsive forces between atoms. Double bonds are shorter and stronger than single bonds, and triple bonds are even shorter and stronger. While double bonds are not unusual, triple bonds are rather rare. We may study molecular orbitals in more detail later in Biochemistry, but not in Chemistry 100.