CHAPTER 12: CHEMICAL BONDING

Transcription

1 CHAPTER 12: CHEMICAL BONDING Problems: 1-26, 27c, 28, 33-34, 35b, 36(a-c), 37(a,b,d), 38a, 39-40, 41-42(a,c), 43-58, THE CHEMICAL BOND CONCEPT chemical bond: what holds atoms or ions together in a compound 1st type of chemical bond: IONIC BOND Ionic compounds are held together by ionic bonds. ionic bond: electrostatic attraction holding together positively charged metal cations and negatively charged nonmetal anions formula unit: most basic entity of an ionic compound (eg. NaCl, Al 2 O 3, etc.) gives the ratio of ions (not actual #) present In the 3D representation of NaCl at the right, Na + ions are shown in purple and Cl ions are shown in green Note that the formula, NaCl, indicates a 1-to-1 ratio of Na + ions and Cl ions present, not the actual number of each ion in the compound 2nd type of chemical bond: COVALENT BOND Molecules (or molecular compounds) are held together by covalent bonds. covalent bond: sharing of a pair of electrons by 2 nonmetal atoms molecule: basic unit of a compound of covalently bonded atoms Consider the CO 2, H 2 O, and HCN molecules below Note how the formula for each gives the actual number of each atom present in compound CO 2 H 2 O HCN CHM 130 Chapter 12 page 1 of 16

2 12.2 IONIC BOND Metal atoms lose valence electrons to achieve noble gas electron config. They become positively charged ions (metal cations) Nonmetal atoms gain valence electrons to achieve noble gas electron config. They become negatively charged ions (nonmetal anions) Ex. 1 Give the Lewis electron-dot symbol below each of the following atoms: sodium magnesium chlorine oxygen Ex. 2 Give the Lewis electron-dot symbol below each of the following ions: sodium ion magnesium ion chloride ion oxide ion Example: Let s say that metals and nonmetal molecules first separate into individual atoms before they react. Draw the electron-dot symbols representing each of the following reactions: a. sodium atom + chlorine atom form ions to produce sodium chloride b. magnesium atom + oxygen atom form ions to produce magnesium oxide CHM 130 Chapter 12 page 2 of 16

3 In reality, metal atoms transfer valence electrons to nonmetal atoms positively charged cations and negatively charged anions Ions come together ionic compound = 3D network of ions An ionic compound is actually a network of ions, with each cation surrounded by anions, and vice versa. A 3-dimensional image (at right) is shown on Fig on p Because every bond between all of the ions must be broken requiring extremely high temperatures to melt the substance, ionic compounds exist as solids at room temperature and have very high melting points IONIC BONDS IONIC RADII: distance from the nucleus to the outermost electrons in an ion an atom loses electrons to form a cation cations have smaller radius than their corresponding atom an atom gains electrons to form a anion anions have larger radius than their corresponding atom 11 p + 11 e loses 1 e 11 p + 10 e 17 p + 17 e gains 1 e 17 p + 18 e Na atom Na + ion Cl atom Cl ion 12.3 COVALENT BOND Nonmetal atoms form bonds to achieve a Noble Gas electron configuration However, instead of taking electrons away from one another to form ions, they simply share the electrons in a covalent bond. covalent bond: sharing of a pair of electrons between two nonmetal atoms achieved by overlapping outermost subshells with the valence electrons CHM 130 Chapter 12 page 3 of 16

4 Ex. 1: Use electron dot formulas to represent the reaction described. hydrogen atom + chlorine atom HCl molecule Note in HCl, H has 2 e (like He) and Cl has 8 valence e (like other Noble gases). We can also represent the HCl molecule as follows: H Cl This overlapping region is the covalent bond where electrons are shared. bond length: Actual distance from one nucleus to another when 2 atoms share electrons in a molecule less than the sum of the individual radii distance from nucleus to nucleus if two atoms are sitting side by side H atom Cl atom HCl molecule r 1 = nm r 2 =0.099 nm bond length=0.127 nm r 1 + r 2 = nm bond energy: amount of energy required to break a bond in a mole of gas recognize that breaking a bond always requires energy HCl(g) + bond energy H(g) + Cl(g) Because of conservation of energy, the amount of energy required to break the bond is equal to the amount of energy released when the same bond is formed between the atoms: H(g) + Cl(g) HCl(g) + bond energy CHM 130 Chapter 12 page 4 of 16

5 Multiple Bonds single bond: the sharing of one pair of electrons by two atoms (H H in H 2 ) double bond: the sharing of two pairs of electrons by two atoms (O=O in O 2 ) triple bond: the sharing of three pairs of electrons by two atoms (N N in N 2 ) Note: Single bonds are the longest and weakest, double bonds are shorter and stronger than single bonds, and triple bonds are the shortest and strongest ELECTRON DOT FORMULAS OF MOLECULES octet rule (rule of eight): atoms bond in such a way that each atom gets eight electrons (an octet) in its outer shell, except hydrogen which only needs 2 electrons GUIDELINES for Electron Dot Formulas of Molecules 1. Calculate the total # of valence electrons for all atoms in the compound Number should be even 2. Divide the total number by 2 to get the number of electron pairs 3. Write symbol for central atom (usually underlined or indicated in some way), then put all the other atoms around it. 4. Connect all the atoms together using single bonds. 5. Distribute remaining electrons around outer atoms so they all have an octet (8 e ), except hydrogen (H) only needs 2 electrons. bonding electrons: electron pairs shared between two atoms nonbonding (lone pair) electrons: unshared electron pairs belonging to a single atom 5. If there are not enough electron pairs for all atoms to have an octet, move a nonbonding electron pair from outer atom to a position between the outer atom and the central atom A single pair of e s between atoms = single bond between the atoms Two pairs of e s between atoms = double bond between the atoms Three pairs of e s between atoms = triple bond between the atoms CHM 130 Chapter 12 page 5 of 16

6 Example: Draw the Electron dot formula for each of the following molecules: a. H 2 S: b. CF 4 : e. CH 2 Br 2 : c. SO 3 : f. Cl 2 O: d. PCl 3 : g. HCN: Note: The textbook covers molecules with more than one central atom. You don t need to know how to draw Electron dot formulas for any molecule with more than one central atom. CHM 130 Chapter 12 page 6 of 16

7 12.4 ELECTRON DOT FORMULAS OF POLYATOMIC IONS polyatomic ion: a group of atoms bonded together with an overall charge 1. Calculate the total number of electrons (e s) for all the atoms 2. Account for # of electrons associated with charge: If ion is positively charged, subtract # of electrons from total If ion has +2 charge subtract 2 electrons from total # of electrons If ion is negatively charged, add # of electrons to total If ion has 3 charge add 3 electrons to total # of electrons 3. Divide new total by 2 to get total # of electrons pairs 4. Write symbol for central atom (usually underlined), then put all the other atoms around it. Connect all the atoms together using single bonds. 5. Distribute remaining electrons around outer atoms so they all have an octet (8 electrons), except hydrogen (H) only needs 2 electrons. 6. If any atom does not have an octet, move nonbonding electrons from central atom to a position between atoms, forming double and triple bonds until all atoms have an octet. 7. Put brackets around all the atoms, and put charge on upper right-hand side to indicate the charge belongs to whole entity, not just to a single atom in the ion. Example: Draw the Electron dot formula for each of the following polyatomic ions: a. NH 4 + : c. PO 4 3 : b. NO 3 : d. CO 3 2 : CHM 130 Chapter 12 page 7 of 16

8 12.9 SHAPES OF MOLECULES Repulsion between electrons causes them to spread out in a molecule valence-shell electron-pair repulsion (VSEPR) model accounts for shape of molecule by considering repulsion between electron pairs around a central atom in a molecule Molecular Shape (or Molecular Geometry): refers to three-dimensional arrangement of atoms in molecule responsible for many physical and chemical properties (m.p., b.p., density, etc.) MOLECULES WHERE CENTRAL ATOM HAS NO LONE PAIRS Consider a molecule composed of only two types of atoms, A and B A=central atom B=outer atoms If there are only two atoms, the molecule must be linear. For molecules with three or more atoms, determine a general formula in the form AB x (for x=2, 3, or 4) to determine the molecular geometry (or shape). AB 2 : linear the two outer atoms are 180 from each other Ex. Give the electron dot formula, shape, and bond angles for CO 2, where both carbon-oxygen bonds are equivalent. CHM 130 Chapter 12 page 8 of 16

9 AB 3 : trigonal planar three outer atoms at the corners of an equilateral triangle each outer atom is 120 from the other two outer atoms Ex. Give the electron dot formula, shape, and bond angles for CH 2 O AB 4 : tetrahedral (tetra = four) since four-sided, or four faces Ex. Give the Electron dot formula, shape, and bond angles for CH 4. MOLECULES WHERE CENTRAL ATOM HAS ONE OR MORE LONE PAIRS Lone pairs of electrons take up more space than bonded pairs of electrons because the bonded pair are held by two atoms whereas the lone pair is held only by one atom. A=central atom B=outer atoms E=lone pairs AB 2 E: bent (or angular) (central atom and 2 outer atoms have a bent shape) start with AB 3 molecule (tetrahedral) and replace one B atom w/ a lone pair of electrons (E) AB 2 E Ex. Give the electron dot formula, shape, and bond angles for SO 2. CHM 130 Chapter 12 page 9 of 16

10 AB 3 E: trigonal pyramid (central atom and 3 outer atoms make a pyramid) start with AB 4 molecule (tetrahedral) and replace one B atom w/ a lone pair of electrons (E) AB 3 E Ex. Give the electron dot formula, shape, and bond angles for NH 3. AB 2 E 2 : bent (or angular) (central atom and 2 outer atoms have a bent shape) start with AB4 molecule (tetrahedral) and replace 2 B atoms with 2 lone pairs of electrons (E) AB 2 E 2 Ex. Give the electron dot formula, shape, and bond angles for for H 2 O. Given any molecule or polyatomic ion, be able to determine the ELECTRON DOT FORMULA. Given the tables of shapes and bond angles, identify the shape and bond angles corresponding to a given or determined ELECTRON DOT FORMULA. CHM 130 Chapter 12 page 10 of 16

11 CHM130 Table of Molecular Shapes and Bond Angles General formula AB 2 MOLECULAR GEOMETRY NAME of SHAPE 180 linear Bond Angles 180 AB trigonal planar 120 AB tetrahedral <120 AB 2 E bent or angular <120 AB 3 E trigonal pyramid <109.5 < AB 2 E 2 bent or angular <109.5 < CHM 130 Chapter 12 page 11 of 16

12 Example For the following molecules (where the central atom is underlined): i. Draw the Electron dot formula. ii. Determine the shape of the molecule. iii. Determine the approximate bond angles. a. CH 2 F 2 b. OF 2 Electron dot formula Electron dot formula ii. shape of CH 2 F 2 : iii. bond angles in CH 2 F 2 : ii. shape of OF 2 : iii. bond angles in OF 2 : 2. Draw the Electron dot formula for the following polyatomic ions: a. phosphite ion, PO 3 3 Electron dot formula b. azide ion, N 3 (where one N is central atom) Electron dot formula ii. shape of PO 3 3 : ii. shape of N 3 : iii. bond angles in PO 3 3 : iii. bond angles in N 3 : CHM 130 Chapter 12 page 12 of 16

13 12.7 NONPOLAR COVALENT BONDS Nonpolar covalent bond: Bond between 2 atoms with equal electronegativity values Electronegativity (EN): Ability of an atom in a chemical bond to attract e s F is the most electronegative (EN) element Elements are less electronegative the further away from F EN increases Best example is found between two identical atoms: H 2, O 2, N 2, Cl 2, F 2, I 2, Br 2 Periodic Table Except for H, which has EN between B and C. Nonpolar covalent bonds can also occur between different atoms which have identical EN values: (See Fig. 12.9, p. 325) e.g. N Cl, C S, Si Ge, etc. EN increases 12.6 POLAR COVALENT BONDS In some covalent bonds, one of the two atoms holds electrons more tightly polar covalent bond results between two atoms Delta (δ) Notation for polar bonds: Electrons concentrate around the more EN atom in a molecule Atom gains a partial negative charge, indicated with δ Since electrons spend less time around the other atom Other atom gains a partial positive charge, indicated with δ+ Examples: For each of the bonds below: i. Use delta notation (δ + and δ ) to indicate which atom in each bond is more electronegative, and ii. Use an arrow to point from the less electronegative atom to the more electronegative atom. C Cl N O H O CHM 130 Chapter 12 page 13 of 16

14 METALLIC BONDS Metals exist as nuclei surrounded by a sea of electrons The electrons in a metal are shared among all the nuclei, so the electrons are delocalized (i.e., they are not fixed to a specific atom) The electrons can shift throughout the entire metal. The electrons freedom to move throughout the material gives metals their unique properties e.g. metals conduct heat and electrical because electrons flow through the metal; metals are malleable and ductile because electrons act as a glue, holding the positively charged nuclei together Example: Identify the type of bond described for each of the following: A. ionic bond C. polar covalent B. metallic bond D. nonpolar covalent i. The C O bonds in CO 2. ii. The bonds in F 2. iii. The bonds in K 2 O. iv. The C C bonds in C 3 H 8. v. The bonds in Ba. v. The bonds in MgS. vi. The bonds in H 2 O. CHM 130 Chapter 12 page 14 of 16

15 12.9 SHAPES OF MOLECULES POLARITY OF MOLECULES For diatomic molecules: nonpolar molecules: when the 2 atoms have equal EN values polar molecules: when the 2 atoms have different EN values have dipole (+ve and ve ends) For molecules of three of more atoms: polarity depend on the individual bonds and geometry around central atom Polar molecules have an overall dipole (positive end and negative end) In nonpolar molecules, there may be individual dipoles that cancel no overall dipole Guidelines for Determining if a Molecule is Polar or Nonpolar 1. Use Delta Notation and an arrow to indicate which atom in a polar bond is more electronegative. 2. Determine if there is an overall dipole: If two arrows point in opposite directions, all arrows point in, or all arrows point out, then the dipoles cancel nonpolar molecule. If all arrows point in the same direction and don t cancel, there is an overall dipole for the molecule polar molecule. Ex. 1: Determine whether the following molecules are polar or nonpolar: CO 2 : H 2 O: SO 3 S CHM 130 Chapter 12 page 15 of 16

16 Ex. 2: Determine whether the following molecules are polar or nonpolar: H CCl 4 CHCl 3 Ex. For the following molecules (where the central atom is underlined): i. Draw the electron-dot formula. iv. Sketch the molecule with dipoles. ii. Determine the shape of the molecule. v. Indicate if the molecule is polar/nonpolar. iii. Determine the approximate bond angles. i. COCl 2 i. CH 3 F electron-dot formula electron-dot formula ii. shape of COCl 2 : iii. bond angles in COCl 2 : iv. Sketch the 3D shape of COCl 2 drawing arrows to show the dipoles on each polar bond. ii. shape of CH 3 F: iii. bond angles in CH 3 F: iv. Sketch the 3D shape of CH 3 F drawing arrows to show the dipoles on each polar bond. v. The SO 3 molecule is. v. The CH 3 F molecule is. (Circle one) polar nonpolar (Circle one) polar nonpolar CHM 130 Chapter 12 page 16 of 16