Chapter 5 Compounds and Their Bonds

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1 General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 5 Compounds and Their Bonds A closer look at bonding type the concept of electronegativity General Rules for Bond Type Negative complex ions Metal Ionic Bond Nonmetal Covalent Bond Nonmetal 2013 Pearson Education, Inc Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Ionic or Covalent? So far, we have been employing the following generalization for the type of bonding holding atoms together in a substance Generalization metal + nonmetal = ionic bond nonmetal + nonmetal = covalent bond Reality Bonding type is on a continuum, from 100% ionic to 100% covalent! Bonding Conundrum Arose in the 1930 s from American Chemist Linus Pauling s work on the nature of the chemical bond. Pauling realized that there was some inbetweeness with the bonding in many substances Measured the ability of elements to attract electrons to themselves when bonding Called this tendency Electronegativity 100% covalent 100% ionic 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Determining Predominant Bond type Electronegativity is a measure of the tendency for atoms of an element to attract electrons to themselves in a chemical bond. Originated with American chemist Linus Pauling ( ), a 2x Nobel Prize winner who did most of his work at Cal Poly Tech, but finished his career at Stanford 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

2 Electronegativity (EN) Values of the Elements Scale ranges from 0.7 to 4.0 Electronegativity Electronegativity is a measure of an atom s ability to attract electrons to itself in a chemical bond increases from left to right, going across a period on the periodic table. decreases going down a group on the periodic table. is high for the nonmetals, with fluorine as the highest. is low for the metals and transition metals Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Trends in Periodic table Periodic trends in electronegativity Electronegativity decreases down a group because: Atomic radius increases, valence e- further away from nucleus, decreases pulling power of nucleus Electronegativity increases from left to right across a period because: Atomic radius decreases, valence e- closer to the nucleus, increases pulling power of nucleus 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Bonding and Electronegativity According to Pauling, the difference in the electronegativity values of the two atoms involved in a chemical bond can be used to predict the type of bond that forms. Pauling Classified chemical bonds into the following three types, based on differences in electronegativity Ionic Polar Covalent Nonpolar Covalent Bond Character Summary a) Nonpolar covalent bond electrons shared equally b) Polar covalent bond e- not shared equally, more electronegative atom has greater share of e- cloud c) Ionic bond e- transferred from one atom to other, creates a + and ion 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

3 Electronegativity and Bond Type It is the difference in the electronegativies ( EN) of two bonded atoms that determines the predominant bond type EN 0.4 Nonpolar covalent bond (e- shared equally by atoms) EN between 0.5 and 1.7 polar covalent bond Nonpolar Covalent Bonds A nonpolar covalent bond occurs between nonmetals. has an equal or almost equal sharing of electrons. has almost no electronegativity difference (0.0 to 0.4). Examples: Atoms Electronegativity Type of Bond Difference EN > 1.7 ionic bond (e- transferred) Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Example of Nonpolar Covalent Bonds Diatomic elements e.g. H 2 Bonds between P and H = =0 Polar Covalent Bonds A polar covalent bond occurs between nonmetal atoms that do not share electrons equally. has a moderate electronegativity difference (0.5 to 1.7). Examples: Atoms Electronegativity Type of Bond Difference 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Result of differences in electronegativity: More electronegative element has greater share of e- cloud, it is electron rich Comparing Nonpolar and Polar Covalent Bonds Less electronegative element is e- poor Creation of partial charges on the atoms in the bond (bond dipoles) δ + and δ Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

4 Example: Bond between H and Cl Electronegativity values H= 2.1, Cl = 3.0 e- cloud pulled towards Cl atom = Polar Covalent Bond electron poor region H electron rich region Cl e - rich H Cl δ + δ - e - poor Bond Polarity and Dipoles Bonds become more polar as the difference in electronegativity values of bonding atoms increases. Polar covalent bonds have a separation of charges called a dipole. The positive and negative ends of the dipole are indicated by the lowercase Greek letter delta with a positive or negative sign, δ+ and δ-, or an arrow that points from the positive to the negative charge. Direction e- cloud being pulled 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Ionic Bonds An ionic bond occurs between metal and nonmetal ions. is a result of electron transfer. has a large electronegativity difference (1.8 or more). Examples: Atoms Electronegativity Type of Bond Difference Determining Predominant Bond Type Look at the electronegativity values of atoms involved in bond Calculate electronegativity difference ( EN) If the bond is polar covalent, draw arrow in direction that e- cloud is pulled (more electronegative atom) Also use lower case Greek symbol for delta, δ, along with + or - sign 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Electronegativity and Bond Types Predicting Bond Types 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

5 Polar Molecules A polar molecule contains polar bonds. has a separation of positive and negative charge. called a dipole, indicated with δ + and δ. has dipoles that do not cancel. δ + δ H Cl NH 3 dipole Nonpolar Molecules A nonpolar molecule contains nonpolar bonds Cl Cl H H or has a symmetrical arrangement of polar bonds. Dipoles do not cancel Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Guide to Determination of Polarity Molecular Polarity, H 2 O Determine the polarity of the H 2 O molecule. Step 1 Determine if the bonds are polar covalent or nonpolar covalent. From the electronegativity table, O 3.5 and H 2.1 gives a difference of 1.4, which makes the O H bonds, polar covalent Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Molecular Polarity, H 2 O Determine the polarity of the H 2 O molecule. Step 2 If the bonds are polar covalent, draw the electron-dot formula and determine if the dipoles cancel or not. The four electron groups of oxygen are bonded to two H atoms. Thus, the H 2 O molecule has a net dipole, which makes it a polar molecule. Learning Check Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr 3 2. HBr 3. Br 2 4. SiBr Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

6 Solution Determine the shape of each of the following molecules and whether they are polar or nonpolar. Explain. 1. PBr 3 pyramidal; polar; dipoles don t cancel 2. HBr linear; polar; one polar bond (dipole) 3. Br 2 linear; nonpolar; nonpolar bond 4. SiBr 4 tetrahedral; nonpolar; dipoles cancel General, Organic, and Biological Chemistry Fourth Edition Karen Timberlake Chapter 5 Compounds and Their Bonds 5.9 Attractive Forces in Compounds 2013 Pearson Education, Inc. Lectures 2013 Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Ionic Compounds In ionic compounds, ionic bonds require large amounts of energy to break. hold positive and negative ions together. explain their high melting points. Covalent Compounds In covalent compounds, the attractive forces between solid and liquid molecules are weaker than ionic bonds. require less energy to break. explain why their melting points are lower than ionic compounds. These attractive forces include dipole-dipole attractions, dispersion forces, and hydrogen bonding Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Dipole-Dipole Attractions In covalent compounds, polar molecules exert attractive forces between molecules called dipole-dipole attractions. Dipole-Dipole Attractions, Hydrogen Bonds In covalent compounds, some polar molecules form strong dipole attractions called hydrogen bonds, which occur between the partially positive hydrogen atom of one molecule and a lone pair of electrons on a nitrogen, oxygen, or fluorine atom in another molecule Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

7 Dispersion Forces Dispersion forces are weak attractions between nonpolar molecules. caused by temporary dipoles that develop when electrons are not distributed equally. Comparison of Bonding and Attractive Forces Nonpolar molecules form attractions when they form temporary dipoles Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Melting Points and Attractive Forces The stronger the attractive force between ions or molecules, the higher the melting points. Ionic compounds, have the strongest attractive force and, therefore the highest melting points. Covalent molecules have less attractive forces than ionic compounds and, therefore lower melting points. Melting Points and Attractive Forces The attractive forces between covalent molecules vary in magnitude; the stronger the attractive force, the higher its melting point. Hydrogen bonds are the strongest type of dipole dipole attractions, requiring the most energy to break, followed by dipole dipole forces. Dispersion forces are the weakest, requiring even less energy to break them, and therefore have lower melting points than hydrogen bonds and dipole dipole forces Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section Melting Points of Selected Substances Learning Check Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole dipole, hydrogen bonds or dispersion. 1. NCl 3 2. H 2 O 3. Br 2 4. KCl 5. NH Pearson Education, Inc. Chapter 5, Section Pearson Education, Inc. Chapter 5, Section

8 Solution Identify the main type of attractive forces for each of the following compounds: ionic bonds, dipole dipole, hydrogen bonds or dispersion. 1. NCl 3 dipole dipole 2. H 2 O hydrogen bonds 3. Br 2 dispersion 4. KCl ionic bonds 5. NH 3 hydrogen bonds 2013 Pearson Education, Inc. Chapter 5, Section