CHEM 1301 SECOND TEST REVIEW. Covalent bonds are sharing of electrons (ALWAYS valence electrons). Use Lewis structures to show this sharing.

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1 CEM 1301 SECOND TEST REVIEW Lewis Structures Covalent bonds are sharing of electrons (ALWAYS valence electrons). Use Lewis structures to show this sharing. Rules OCTET RULE an atom would like to have 8 valence electrons [except which wants 2] COMMON SENSE an atom forms the number of bonds equivalent to the number of electrons wanted Most molecules obey both octet and common sense. Can use Single bonds [2 electrons, one from each atom] Double bonds [4 electrons, two from each atom] Triple bonds [6 electrons, three from each atom] OCTET and NO COMMON SENSE Atoms obey octet rule but form fewer or more bonds than expected a) use DATIVE bonds [one atom puts both electrons into a bond] b) POLYATOMIC IONS extra electron(s) reduce number of bonds wanted fewer electron(s) increase number of bonds wanted Simplest approach is to use The Bott Method! 1. Determine number of valence electrons (group no. of each atom; include charge;cation take away electrons, anions add electrons). 2. Set up connectivity (a) hydrogen only ever has one bond (b) apply common sense (c) try to put atom furthest from F in middle (d) however, no rings (unless carbon); no atoms bonded to same type (except carbon, N2O or when absolutely necessary) (e) can ONLY have more than four bonds to atoms bigger than Ne 3. Add in dots 2 for each bond; then add more IN PAIRS around each atom until it has 8 (except ) 4. Compare numbers from 1 (electrons available) to 3 (number of dots) (a) 1 = 3 done (b) 3 > 1 reduce number of dots by (i) multiple bonds (two atoms with lone pairs that are already bonded; remove lone pair from each and add extra bond pair) (ii) if have to, erase lone pair(s) from central atom - ELECTRON DEFICIENT (c) 1 > 3 increase number of dots by putting extra lone pair(s) on central atom (if bigger than Ne)

2 NO OCTET and NO COMMON SENSE [screwed-up structures] 1. Odd number of electrons 2. not enough electrons 3. too many electrons or atoms atoms higher than NEON can have more than 8 Example questions are 1. Which molecule contains one unshared pair of valence electrons? (A) 2O (B) N3 (C) C4 (D) Cl 2 2. ow many electrons should be shown in the Lewis dot structure for carbon monoxide? (A) 8 (B) 14 (C) 10 (D) Which molecule contains only two unshared pairs of electrons? (A) 2O (B) N3 (C) SCl2 (D) CO2 4. Select the best Lewis structure for ClCN (assume enough lone pairs to make every atom happy) (A) Cl-C N (B) Cl C-N (C) Cl-C-N (D) Cl=C-N 5. Which of the following Lewis dot structures is correct? (A) CN (B) NN (C) OO (D) OCO (E) 6. Select the correct Lewis structure for TeBr 2. (A) (B) (C) (D) 7. Select the correct Lewis structure for nitrogen trifluoride, NF 3. (A) F N F (B) (C) F N F F N F (D) F N F F F F F 8. Select the best Lewis structure for Oxygen difluoride. (A) F O F (B) F O F (C) F O F (D) F O F

3 9. Select the best Lewis structure for P 2 I 4. (A) I I I P P (B) I I P I I P I (C) I I I P P (D) I I I I P P I 10. Which one of the following Lewis structures is definitely incorrect? (A) NO (B) CN (C) NO 2 - N. O N C (D) C 2 4 (E) PF 5 C O F C F N P F F F O 11. Which one of the following Lewis structures is definitely incorrect? (A) NO 2 (B) CO 3 2- (C) C 4 (D) B 3 (E) SO 2 O N. O C O O O C 12. The Lewis structure of NO2 + is best drawn as B S + O N O (E) + N O O (A) O N O (B) O O N (C) O N O (D) O O The Lewis structure of which of the following molecules cannot obey the octet rule? (A) N 2 (B) O 2 (C) CO (D) NO (E) CN

4 14. Which of the following molecules does not have an ideal Lewis structure (that is, all atoms happy)? (A) C 4 (B) 2 O (C) B 3 (D) N 3 (E) F 15. In which one of the following species is the central atom (the first atom in the formula) an exception to the octet rule? (A) N 3 (B) N 4 + (C) I 2 (D) B 4 - (E) SF In which one of the following species is the central atom (the first atom in the formula) likely to violate the octet rule? (A) BF 4 - (B) XeO 3 (C) SiCl 4 (D) N 3 (E) C 2 Cl Which of the following atoms can have more than 8 valence electrons when bonding? (A) N (B) C (C) O (D) P 18. Which has a Lewis dot structure with the greatest number of unshared pairs on the central atom? (A) N3 (B) IF3 (C) SeCl2 (D) ICl2 - Naming Compounds Ionic cation first then anion Formula use subscripts to show how many of each type of ion In cation is from an atom If anion is from an atom same name as atom take atom name, remove end, add IDE Polyatomic ions Table LEARN TESE!!!! Covalent Binary compounds less electronegative first (one to left and lower) More non-metallic second; change end to IDE Use prefixes (Table 5.3) to describe how many Formula use subscripts to show how many of each type of atom 19. All of the following have a 2- charge except (A) sulfate (B) sulfide (C) carbonate (D) nitrate (E) sulfite 20. NaBr is the correct formula of (A) sodium bromate. (B) sodium bromide (C) sodium bromite (D) sodium bromine 21. Which is the correct formula/name combination? (A) Al 2 (SO 4 ) 3 / aluminum sulfate. (B) K 2 PO 3 / potassium phosphate. (C) C 2 3 O / acetate. (D) PCl 3 / phosphorus chlorite.

5 22. A compound with the formula Ca 2 is called (A) cadmium hydride. (C) calcium hydride. (B) hydrogen carbide. (D) calcium hydrate. 23. The correct name of N 2 O 3 is (A) nitrogen oxide (B) nitrogen(ii) oxide (C) nitrous oxide. (D) dinitrogen trioxide. 24. The formula of calcium nitrite is (A) Ca 3 N 2 (B) Ca(NO 2 ) 2 (C) Ca(NO) 2 (D) Ca(NO 3 ) Tetrasulfur dinitride decomposes explosively when heated. What is its formula? (A) S 4 N 4 (B) S 2 N 4 (C) S 4 N 2 (D) 4SN 2 (E) SN 8 Electronegativity and Polar Bonds Closer an atom to F the more electronegative it is (likes electrons) Causes POLAR BONDS. More electronegative is a bit negative (δ-)as it gets bigger share of electrons. 26. Which would be expected to be the most electronegative? (A) P (B) As (C) Si (D) Al 27. The element with the greatest tendency to gain electrons is (A) F (B) At (C) O (D) N (E) Bi 28. Which atom has the highest electronegativity? (A) Br (B) Mg (C) C (D) O 29. In which bond are the partial charges on the atoms correct? (A) δ+ Si-O δ- (B) δ+ Cl-Br δ- (C) δ+ N-B δ- (D) δ+ Cl-Cl δ- 30. Which of the following compounds contains the LEAST polar bonds? Atoms S P As Cl Si Sb Electronegativity (A) P 3 (B) AsCl 3 (C) Si 4 (D) SbCl 3 (E) 2 S 31. Which set of bonds is arranged in order of increasing polarity? (A) Si-S < Si-O < Si-P < Si-F (B) Si-O < Si-F < Si-S < Si-P (C) Si-F < Si-S < Si-O < Si-P (D) Si-P < Si-S < Si-O < Si-F

6 32. The BrCl molecule may be represented by the formula Br-Cl. The polarity is best represented as (A) Br δ+ -Cl δ+ (B) Br δ -Cl δ+ (C) Br δ -Cl δ (D) Br δ+ -Cl δ Structures (shapes) of molecules Draw the Lewis structure. Look at the CENTRAL atom and count how many electron groups it has. These try to get as far apart from each other as possible Number of groups Geometry Angle 2 Linear Trigonal planar Tetrahedral Trigonal bipyramid 90 and Octahedral 90 When looking at a molecule, the central atom thinks it has the geometry defined as above. owever, we cannot see lone-pair groups, only the atoms. So Number of groups Number of bond Number of lone pairs Structure trigonal planar bent tetrahedral trigonal pyramid bent 33. What is the molecular shape of N 2 O? (A) linear (B) bent (C) angular (D) trigonal 34. What is the molecular shape of the thiocyanate anion, SCN -? (A) linear (B) bent (C) angular (D) trigonal 35. What is the molecular shape of ClCN? (A) linear (B) bent (C) angular (D) trigonal 36. What is the molecular shape of Be 2? (A) linear (B) bent (C) angular (D) trigonal 37. What is the molecular shape of NOCl? (A) linear (B) trigonal planar (C) bent (D) tetrahedral

7 38. What is the molecular shape of BCl 3? (A) linear (B) trigonal planar (C) bent (D) tetrahedral 39. What is the molecular shape of NO - 2? (A) linear (B) trigonal planar (C) bent (D) tetrahedral 40. What is the molecular symmetry around the carbons in CCl 2 C 2? (A) linear (B) trigonal planar (C) V-shaped (D) tetrahedral 41. What is the molecular shape of ClO 3 F? (A) trigonal pyramidal (B) square planar (C) square pyramidal (D) tetrahedral 42. What is the molecular shape of OF? (A) trigonal pyramidal (B) trigonal (C) tetrahedral (D) bent 43. What is the molecular shape of N 2 Cl? (A) trigonal pyramidal (B) tetrahedral (C) bent (D) trigonal planar 44. What is the molecular shape of SiF 2-6? (A) trigonal bipyramidal (B) hexagonal (C) tetrahedral (D) octahedral 45. Which one of the following will have a trigonal pyramidal shape? 2- (A) PCl 3 (B) BF 3 (C) SO 3 (D) SO 2 (E) CO Which one of the following molecules and ions will definitely have at least one 90 bond angle in it? (A) AlCl 4 - (B) N 3 (C) PCl 5 (D) CO 2 (E) 2 O 47. Predict the ideal bond angles in AsCl 3? (A) 90 (B) (C) 120 (D) Predict the ideal bond angles in FNO.

8 (A) 90 (B) 109 (C) 120 (D) Predict the ideal bond angles around nitrogen in N 2 F 2. (A) 90 (B) 109 (C) 120 (D) Predict the ideal bond angles around carbon in C 2 2. (A) 90 (B) 109 (C) 120 (D) 180 Dipole Moments Add polar bonds up molecular DIPOLE MOMENT. Can cancel a) Are there polar bonds NO, no dipole moment b) Are all the atoms attached to a central atom the same? NO dipole moment c) If YES are there lone pairs on the central atom? NO no dipole moment 51. Which of the following has no net dipole moment? (A) N 2 O (B) NF 3 (C) 2 Se (D) TeO Which one of the following molecules does not have a dipole moment? (A) CS 2 (B) 2 S (C) C 2 Cl 2 (D) P 3 (E) C 2 O ANSWERS B C A A D D C D B B E C D C E B D D D B A C D B C A A D A A D D A A A A C B C B D D A D A C B C C D D A

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