CHEM 1301 SECOND TEST REVIEW Lewis Structures

CHEM 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 H 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 H)

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)

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) H2O (B) NH3 (C) CH4 (D) Cl2

2. How many electrons should be shown in the Lewis dot structure for carbon monoxide?

(A) 8 (B) 14 (C) 10 (D) 28

3. Which molecule contains only two unshared pairs of electrons?

(A) H2O (B) NH3 (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) :H..:C:::N..: .. .. (C) O::O
(B) :N..::.:.N:
(D) :O::C::O: (E) :H..:H..:

6. Select the correct Lewis structure for TeBr2.

(A) (B) (C) (D)

7. Select the correct Lewis structure for nitrogen trifluoride, NF3.

(A) (B) (C) (D) N
:F N F F F
FNF :F N F : : F:

F :F: :F:

8. Select the best Lewis structure for Oxygen difluoride.

(A) (B) (C) (D)
FOF
FOF FOF :F O F:

9. Select the best Lewis structure for P2I4.

(A) I I (B) :I: :I: I:
I PP I
:I PP

: : : :(C) I I (D) :I: :I:
: :I P P I
: :I P P I

10. Which one of the following Lewis structures is definitely incorrect?

(A) NO (B) HCN (C) NO2- (D) C2H4 (E) PF5
HCH
.. . .. : : .. HCH :..F..:..F.. :.F. : ..F....F:..:

N N O P

.O. : C :N

H :O..:

11. Which one of the following Lewis structures is definitely incorrect?

(A) NO2 (B).O.C:O32- (C) CH4 (D) BH3 :(E.O). SO2
H H
:O: C .. : S
N. .O. HCH HB
H H .O. :
:.O. : :.O. :

12. The Lewis structure of NO2+ is best drawn as

(A) :..O.. .. ..O..: + (B) :..O.. .. ..N..: + (C) :.O. N .. +
(D) :..O.. .N. (E) :.N. .O.
O :

.. + O .. : +

N .O.: O

13. The Lewis structure of which of the following molecules cannot obey the “octet rule?”

(A) N2 (B) O2 (C) CO (D) NO (E) HCN

14. Which of the following molecules does not have an ideal Lewis structure (that is, all atoms happy)?

(A) CH4 (B) H2O (C) BH3 (D) NH3 (E) HF

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) NH3 (B) NH4+ (C) I2 (D) BH4- (E) SF6

16. In which one of the following species is the central atom (the first atom in the formula) likely to
violate the octet rule?

(A) BF4- (B) XeO3 (C) SiCl4 (D) NH3 (E) CH2Cl2

17. 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) NH3 (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 same name as atom
If anion is from an atom take atom name, remove end, add ”IDE”

Polyatomic ions: Table 5.5- LEARN THESE!!!!

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) Al2(SO4)3 / aluminum sulfate. (B) K2PO3 / potassium phosphate.
(C) C2H3O– / acetate. (D) PCl3 / phosphorus chlorite.

22. A compound with the formula CaH2 is called

(A) cadmium hydride. (C) calcium hydride.
(B) hydrogen carbide. (D) calcium hydrate.

23. The correct name of N2O3 is

(A) nitrogen oxide (B) nitrogen(II) oxide (C) nitrous oxide. (D) dinitrogen trioxide.

24. The formula of calcium nitrite is

(A) Ca3N2 (B) Ca(NO2)2 (C) Ca(NO)2 (D) Ca(NO3)2

25. Tetrasulfur dinitride decomposes explosively when heated. What is its formula?

(A) S4N4 (B) S2N4 (C) S4N2 (D) 4SN2 (E) SN8

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-Oδ- (B) δ+Cl-Brδ- (C) δ+N-Bδ- (D) δ+Cl-Clδ-

30. Which of the following compounds contains the LEAST polar bonds?

Atoms H S P As Cl Si Sb
Electronegativity 2.1 2.5 2.1 2.1 3.0 1.8 1.9

(A) PH3 (B) AsCl3 (C) SiH4 (D) SbCl3 (E) H2S

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

32. The BrCl molecule may be represented by the formula Br-Cl. The polarity is best represented as:

(A) Brδ+-Clδ+ (B) Brδ−-Clδ+ (C) Brδ−-Clδ− (D) Brδ+-Clδ−

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 180
3 Trigonal planar 120
4 Tetrahedral 109.5
5 Trigonal bipyramid 90 and 120
6 Octahedral 90

When looking at a molecule, the central atom “thinks” it has the geometry defined as above. However, we
cannot see lone-pair groups, only the atoms. So:

Number of groups Number of bond Number of lone pairs Structure
3 3 0 trigonal planar
3 2 1 bent
4 4 0 tetrahedral
4 3 1 trigonal pyramid
4 2 2 bent

33. What is the molecular shape of N2O?

(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 BeH2?

(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

38. What is the molecular shape of BCl3?

(A) linear (B) trigonal planar (C) bent (D) tetrahedral

39. What is the molecular shape of NO2-?

(A) linear (B) trigonal planar (C) bent (D) tetrahedral

40. What is the molecular symmetry around the carbons in CCl2CH2?

(A) linear (B) trigonal planar (C) V-shaped (D) tetrahedral

41. What is the molecular shape of ClO3F?

(A) trigonal pyramidal (B) square planar (C) square pyramidal (D) tetrahedral

42. What is the molecular shape of HOF?

(A) trigonal pyramidal (B) trigonal (C) tetrahedral (D) bent

43. What is the molecular shape of NH2Cl?

(A) trigonal pyramidal (B) tetrahedral (C) bent (D) trigonal planar

44. What is the molecular shape of SiF62-?

(A) trigonal bipyramidal (B) hexagonal (C) tetrahedral (D) octahedral
45. Which one of the following will have a trigonal pyramidal shape?
(A) PCl3 (B) BF3 (C) SO3 (D) SO2 (E) CO32-

46. Which one of the following molecules and ions will definitely have at least one 90° bond angle in it?
(A) AlCl4- (B) NH3 (C) PCl5 (D) CO2 (E) H2O
47. Predict the ideal bond angles in AsCl3?
(A) 90° (B) 109.5° (C) 120° (D) 180°
48. Predict the ideal bond angles in FNO.

(A) 90° (B) 109° (C) 120° (D) 180°
49. Predict the ideal bond angles around nitrogen in N2F2.
(A) 90° (B) 109° (C) 120° (D) 180°
50. Predict the ideal bond angles around carbon in C2H2.
(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) N2O (B) NF3 (C) H2Se (D) TeO3

52. Which one of the following molecules does not have a dipole moment?

(A) CS2 (B) H2S (C) CH2Cl2 (D) PH3 (E) CH2O

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 |
DDAAA|AC BCB|DD ADA|CBCCD|DA