Chapter 4 Compounds and Their Bonds 4.5 Covalent Compounds 1 Copyright © 2009 by Pearson Education, Inc.

Covalent Bonds Covalent bonds form when atoms share electrons to complete octets. between two nonmetal atoms. between nonmetal atoms from Groups 4A (14), 5A (15), 6A (16), and 7A (17). 2

Hydrogen Molecule A hydrogen molecule is stable with 2 electrons (helium). has a shared pair of electrons. 3

Forming Octets in Molecules In a fluorine, F 2,, molecule, each F atom shares 1 electron. attains an octet. 4

Carbon Forms 4 Covalent Bonds In a CH 4 (methane) molecule, 1 C atom shares electrons with 4 H atoms to attain an octet. each H atom shares 1 electron to become stable, like helium. 5

Multiple Bonds In a nitrogen molecule, N 2, each N atom shares 3 electrons. each N attains an octet. the bond is a multiple bond called a triple bond. the name is the same as the element. 6

Naming Covalent Compounds In the names of covalent compounds, prefixes are used to indicate the number of atoms (subscript) of each element. (mono is usually omitted) 7

Naming Covalent Compounds What is the name of SO 3 ? 1. The first nonmetal is S sulfur. 2. The second nonmetal is O, named oxide. 3. The subscript 3 of O is shown as the prefix tri. SO 3 -> sulfur trioxide The subscript 1 (for S) or mono is understood. 8

Naming Covalent Compounds Name P 4 S The first nonmetal, P, is phosphorus. 2. The second nonmetal, S, is sulfide. 3. The subscript 4 of P is shown as tetra. The subscript 3 of O is shown as tri. P 4 S 3 -> tetraphosphorus trisulfide 9

Formulas and Names of Some Covalent Compounds 10

Examples Select the correct name for each compound. A.SiCl 4 1) silicon chloride 2) tetrasilicon chloride 3) silicon tetrachloride B. P 2 O 5 1) phosphorus oxide 2) phosphorus pentoxide 3) diphosphorus pentoxide C.Cl 2 O 7 1) dichlorine heptoxide 2) dichlorine oxide 3) chlorine heptoxide 11

Examples Write the name of each covalent compound. CO_____________________ CO 2 _____________________ PCl 3 _____________________ CCl 4 _____________________ N 2 O_____________________ 12

Example: Writing Formulas for Covalent Compounds Write the formula for carbon disulfide. STEP 1: Elements are C and S STEP 2: No prefix for carbon means 1 C Prefix di = 2 Formula: CS 2 13

Examples Write the correct formula for each of the following: A. phosphorus pentachloride B. dinitrogen trioxide C. sulfur hexafluoride 14

Examples Identify each compound as ionic or covalent, and give its correct name. A. SO 3 B. BaCl 2 C. (NH 4 ) 3 PO 3 D. Cu 2 CO 3 E. N 2 O 4 15

Examples Determine if each is ionic (I) or covalent (C ), and write the formula. A. calcium nitrate B. boron trifluoride C. aluminum carbonate D. dinitrogen tetroxide E. copper(I) phosphate 16

Examples Identify each compound as ionic or covalent and give its correct name. A. Ca 3 (PO 4 ) 2 B. FeBr 3 C. SCl 2 D. Cl 2 O E. N 2 17

Electronegativity The electronegativity value indicates the attraction of an atom for shared electrons. increases from left to right going across a period on the periodic table. is high for the nonmetals, with fluorine as the highest. is low for the metals. 18

Some Electronegativity Values for Group A Elements 19 Low values High values ` Electronegativity increases ` Electronegativity decreases Copyright © 2009 by Pearson Education, Inc.

Nonpolar Covalent Bonds A nonpolar covalent bond occurs between nonmetals. is an equal or almost equal sharing of electrons. has almost no electronegativity difference (0.0 to 0.4). Examples: Electronegativity Atoms Difference Type of Bond N-N = 0.0 Nonpolar covalent Cl-Br = 0.2 Nonpolar covalent H-Si = 0.3 Nonpolar covalent 20

Polar Covalent Bonds A polar covalent bond occurs between nonmetal atoms. is an unequal sharing of electrons. has a moderate electronegativity difference (0.5 to 1.8). Examples: Electronegativity Atoms DifferenceType of Bond O-Cl = 0.5Polar covalent Cl-C = 0.5Polar covalent O-S = 1.0Polar covalent 21

Comparing Nonpolar and Polar Covalent Bonds 22 Copyright © 2009 by Pearson Education, Inc.

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: Electronegativity Atoms Difference Type of Bond Cl-K 3.0 – 0.8 = 2.2 Ionic N-Na 3.0 – 0.9 = 2.1 Ionic S-Cs2.5 – 0.7= 1.8 Ionic 23

Electronegativity and Bond Types 24

Examples Use the electronegativity difference to identify the type of bond [nonpolar covalent (NP), polar covalent (P), or ionic (I)] between the following: A. K-N B. N-O C. Cl-Cl D. Si-Cl 25

4.7Shapes and Polarity of In the valence-shell electron-pair repulsion theory (VSEPR), the electron groups around a central atom are arranged as far apart from each other as possible. have the least amount of repulsion of the negatively charged electrons. have a geometry around the central atom that determines molecular shape. 26

Steps to determine the shape Steps to determine the shape To determine shape, 1. draw the electron-dot structure. 2. count the electron pairs around the central atom. 3. count the bonded atoms to determine shape. 27

Four Electron Groups In a molecule of CH 4, there are 4 electron groups around C. repulsion is minimized by placing 4 electron groups at angles of 109°, which is a tetrahedral arrangement. the shape with four bonded atoms is tetrahedral. 28 Copyright © 2009 by Pearson Education, Inc.

Three Bonding Atoms and One Lone Pair In a molecule of NH 3, 3 electron groups bond to H atoms, and the fourth one is a lone (nonbonding) pair. repulsion is minimized with 4 electron groups in a tetrahedral arrangement. with 3 bonded atoms, the shape is pyramidal. 29

Two Bonding Atoms and Two Lone Pairs In a molecule of H 2 O, 2 electron groups are bonded to H atoms and 2 are lone pairs (4 electron groups). 4 electron groups minimize repulsion in a tetrahedral arrangement. the shape with 2 bonded atoms is bent. 30

Shapes with 4 Electron Groups Electron Pairs Bonded Atoms Lone Pairs Molecular Shape Example 440TetrahedralCH 4 431PyramidalNH 3 422BentH2OH2O 31

Examples State the number of electron groups, lone pairs, and use VSEPR theory to determine the shape of the following molecules or ions. 1) tetrahedral 2) pyramidal3) bent A. PF 3 B. H 2 S C. CCl 4 32

Polar Molecules A polar molecule contains polar bonds. has a separation of positive and negative charge called a dipole, indicated with  + and  -. has dipoles that do not cancel.  +  - H–Cl H — N — H dipole H dipoles do not cancel 33

Nonpolar Molecules A nonpolar molecule contains nonpolar bonds. Cl–Cl H–H or has a symmetrical arrangement of polar bonds. O=C=O Cl Cl–C–Cl Cl dipoles cancel 34

Determining Molecular Polarity STEP 1: Write the electron-dot formula. STEP 2: Determine the shape. STEP 3: Determine if dipoles cancel or not. Example: H 2 O H 2 O is polar dipoles do not cancel 35

Examples Identify each of the following molecules as 1) polar or 2) nonpolar. Explain. A. PBr 3 B. HBr C. Br 2 D. SiBr 4 36

Chapter 4 Compounds and Their Bonds 4.8 Attractive Forces in Compounds 37 °

4.8 Attractive Forces in Compounds In ionic compounds, ionic bonds are strong attractive forces. hold positive and negative ions together. 38

Dipole-Dipole Attractions 39 In covalent compounds, polar molecules exert attractive forces called dipole-dipole attractions. form strong dipole attractions called hydrogen bonds between hydrogen atoms bonded to F, O, or N, and other very electronegative atoms. Dipole-dipoleHydrogen bonds

Dispersion Forces Dispersion forces are weak attractions between nonpolar molecules. caused by temporary dipoles that develop when electrons are not distributed equally. 40

Attractive Forces 41

Melting Points and Attractive Forces Ionic compounds require large amounts of energy to break apart ionic bonds. Thus, they have high melting points. Hydrogen bonds are the strongest type of dipole-dipole attractions. They require more energy to break than other dipole-dipole attractions. Dispersion forces are weak interactions and very little energy is needed to change state. 42

Examples Identify the main type of attractive forces for each: 1) ionic 2) dipole-dipole 3) hydrogen bonds 4) dispersion A. NCl 3 B. H 2 O C. Br-Br D. KCl E. NH 3 43