The Shapes of Molecules

1. The Shapes of Molecules

2. The steps to follow in converting a molecular formula into LEDS. 1st: Place the atoms relative to each other. [Atom with the lowest electronegativity(EN)] 2nd: Determine the total number of valence electrons available. (Recall that the number of valence e- equals the A-groupnumber) 3rd: Draw a single bond from each surrounding atom to the central atom and subtract two valence electrons for each bond. 4th: Distribute the remaining electrons in pairs so that each atom ends up with 8 electrons (or 2 for H). First place the lone pairs to the surrounding (more electronegative) atoms to give each an octet.

3. The steps to follow in converting a molecular formula into LEDS. 5th : CASES INVOLVING MULTIPLE BONDS If after step 4, a central atom still does NOT have an octet, make MULTIPLE bond by changing a lone pair from one of the surrounding atoms into a bonding pair in the central atom.

4. Figure 10.1 The steps in converting a molecular formula into a Lewis structure. Place atom with lowest EN in center Molecular formula Step 1 Atom placement Add A-group numbers Step 2 Sum of valence e- Draw single bonds. Subtract 2e- for each bond. Step 3 Give each atom 8e- (2e- for H) Remaining valence e- Step 4 Lewis structure

5. Molecular formula NF3 F F N Atom placement N 5e- F F 7e- Sum of valence e- N 5e- X 1 = 5e- Total 26e- F 7e- X 3 = 21e- Remaining valence e- : : : F F : : .. N : Lewis structure F : : :

6. PROBLEM: Write a Lewis structure for CCl2F2, one of the compounds responsible for the depletion of stratospheric ozone. PLAN: Follow the steps outlined in Figure 10.1 . : : : Cl : : : Cl : C F : : : : F : SAMPLE PROBLEM 1 Writing Lewis Structures for Molecules with One Central Atom SOLUTION: Cl Step 1: Carbon has the lowest EN and is the central atom. The other atoms are placed around it. Cl C F F Steps 2-4: C has 4 valence e-, Cl and F each have 7. The sum is 4 + 4(7) = 32 valence e-. Make bonds and fill in remaining valence electrons placing 8e- around each atom.

7. PROBLEM: Write the Lewis structure for methanol (molecular formula CH4O), an important industrial alcohol that is being used as a gasoline alternative in car engines. SAMPLE PROBLEM 2 Writing Lewis Structure for Molecules with More than One Central Atom SOLUTION: Hydrogen can have only one bond so C and O must be next to each other with H filling in the bonds. There are 4(1) + 4 + 6 = 14 valence e-. C has 4 bonds and O has 2. O has 2 pair of nonbonding e-. H : H C O H : H

8. There are some compounds that undergo the process of chemical bonding that form more or less than 8 electrons are are considered EXCEPTIONS TO THE OCTET RULE. • Electron Deficient molecules - gaseous compounds containing Be or B as the central atom. Ex. BF3; BeCl2 2) Odd-Electron molecules -most have central atoms from an odd-numbered group. Ex. N (Group 5A -15); Cl (Group 7A -17) NO2 (free radical contain a lone electron.)

9. There are some compounds that undergo the process of chemical bonding that form more or less than 8 electrons are are considered EXCEPTIONS TO THE OCTET RULE. 3) Expanded Valence Shells - molecules having MORE than 8 valence electrons around the central atom. - occur around a central NON METAL atom from period 3 or higher,those in which d orbitals are available. Ex. SF6

10. PROBLEM: Write Lewis structures for (a)H3PO4(pick the most likely structure); (b)BFCl2. SAMPLE PROBLEM 3 Writing Lewis Structures for Octet Rule Exceptions PLAN: Draw the Lewis structures for the molecule and determine if there is an element which can be an exception to the octet rule. Note that (a) contains P which is a Period-3 element and can have an expanded valence shell. SOLUTION: (a) H3PO4 has two resonance forms and formal charges indicate the more important form. -1 0 +1 0 (b) BFCl2 will have only 1 Lewis structure. 0 0 0 0 0 0 0 0 0 0 more stable 0 0 lower formal charges

11. The valence-shell electron-pair repulsion (VSEPR) theory states that electron pairs repel each other whether they are in bond pairs or in lone pairs. Electron pairs will spread themselves as far from each other as possible to minimize repulsion. • VSEPR focuses not only on electron pairs it also focus on electron groups as a whole. • An electron group is an electron pair, a lone pair, a single unpaired electron, a double bond or a triple bond on the central atom.

12. E -nonbonding valence electron-group X -surrounding atom A - central atom integers VSEPR - Valence Shell Electron Pair Repulsion Theory has a its general formula. A Xm En Understanding the molecular structure of a compound can help determine the polarity, reactivity, phase of matter, color, magnetism, as well as the biological activity.

13. The actual shape of a molecule can be determined by the location of the nuclei and the distribution of electrons. ELECTRON GROUP GEOMETRY CATEGORIES: • Electron-group geometry is determined by the NUMBER of ELECTRON GROUPS 2) Molecular Geometry depends on the NUMBER OF LONE PAIRS.

18. linear tetrahedral trigonal planar trigonal bipyramidal octahedral Figure 10.2 Electron-group repulsions and the five basic molecular shapes.

19. Figure 10.3 The single molecular shape of the linear electron-group arrangement. Examples: CS2, HCN, BeF2

20. Class Shape Figure 10.4 The two molecular shapes of the trigonal planar electron-group arrangement. Examples: SO2, O3, PbCl2, SnBr2 Examples: SO3, BF3, NO3-, CO32-

21. 1220 Effect of Double Bonds 1160 real Effect of Nonbonding(Lone) Pairs Factors Affecting Actual Bond Angles Bond angles are consistent with theoretical angles when the atoms attached to the central atom are the same and when all electrons are bonding electrons of the same order. 1200 larger EN 1200 ideal greater electron density Lone pairs repel bonding pairs more strongly than bonding pairs repel each other. 950

22. The three molecular shapes of the tetrahedral electron-group arrangement. Figure 10.5 Examples: CH4, SiCl4, SO42-, ClO4- NH3 PF3 ClO3 H3O+ H2O OF2 SCl2

23. Lewis structures and molecular shapes. Figure 10.6

24. Figure 10.7 The four molecular shapes of the trigonal bipyramidal electron-group arrangement. PF5 AsF5 SOF4 SF4 XeO2F2 IF4+ IO2F2- XeF2 I3- IF2- ClF3 BrF3

25. Figure 10.8 The three molecular shapes of the octahedral electron-group arrangement. SF6 IOF5 BrF5 TeF5- XeOF4 XeF4 ICl4-

26. A summary of common molecular shapes with two to six electron groups. Figure 10.9

27. See Figure 10.1 The steps in determining a molecular shape. Figure 10.10 Molecular formula Step 1 Lewis structure Count all e- groups around central atom (A) Step 2 Electron-group arrangement Note lone pairs and double bonds Step 3 Count bonding and nonbonding e- groups separately. Bond angles Step 4 Molecular shape (AXmEn)

28. PROBLEM: Draw the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) PF3 and (b)COCl2. SOLUTION: (a) For PF3 - there are 26 valence electrons, 1 nonbonding pair SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three, or Four Electron Groups The shape is based upon the tetrahedral arrangement. The F-P-F bond angles should be <109.50 due to the repulsion of the nonbonding electron pair. The final shape is trigonal pyramidal. <109.50 The type of shape is AX3E

29. 124.50 1110 SAMPLE PROBLEM 10.6 Predicting Molecular Shapes with Two, Three, or Four Electron Groups continued (b) For COCl2, C has the lowest EN and will be the center atom. There are 24 valence e-, 3 atoms attached to the center atom. C does not have an octet; a pair of nonbonding electrons will move in from the O to make a double bond. Type AX3 The shape for an atom with three atom attachments and no nonbonding pairs on the central atom is trigonal planar. The Cl-C-Cl bond angle will be less than 1200 due to the electron density of the C=O.

30. PROBLEM: Determine the molecular shape and predict the bond angles (relative to the ideal bond angles) of (a) SbF5 and (b) BrF5. SOLUTION: (a) SbF5 - 40 valence e-; all electrons around central atom will be in bonding pairs; shape is AX5 - trigonal bipyramidal. SAMPLE PROBLEM 10.7 Predicting Molecular Shapes with Five or Six Electron Groups (b) BrF5 - 42 valence e-; 5 bonding pairs and 1 nonbonding pair on central atom. Shape is AX5E, square pyramidal.