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)
b)
use DATIVE bonds
POLYATOMIC IONS:
[one atom puts both electrons into a bond]
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)
(b)
1=3
3>1
(c)
done
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
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.
2.
3.
Odd number of electrons
not enough electrons
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
2.
How many electrons should be shown in the Lewis dot structure for carbon monoxide?
(A)
8
3.
Which molecule contains only two unshared pairs of electrons?
(A)
H2O
4.
Select the best Lewis structure for ClCN (assume enough lone pairs to make every atom happy)
(A)
Cl-C≡N
5.
(D)
Which of the following Lewis dot structures is correct?
.. ..
:H:C:::N:
(B)
:N:::N:
(C)
O::O
..
..
.. ..
:O::C::O:
(E)
:H:H:
.. ..
6.
Select the correct Lewis structure for TeBr2.
(A)
(B)
(B)
(B)
(B)
(A)
7.
NH3
(C)
14
CH4
(C)
NH3
10
(C)
Cl≡C-N
(D)
SCl2
(C)
(B)
(D)
(D)
Cl-C-N
(C)
(D)
Cl2
28
CO2
Cl=C-N
(D)
Select the correct Lewis structure for nitrogen trifluoride, NF3.
F
N
F
:F
N
F
:F
N
F
:
F
N
F
: F:
:F:
: F:
F
8.
(D)
(C)
(B)
(A)
Select the best Lewis structure for Oxygen difluoride.
F
O
F
(D)
(C)
(B)
(A)
F
O
F
F
O
F
:F
O
F
:
9.
Select the best Lewis structure for P2I4.
(A)
I
:I
10.
..
N.
..O :
11.
P
:I
I
P
P
P
:
I
:I: :I:
(D)
I
P
:I
:
P
P
I
:
(B) HCN
(C) NO2-
..
(E) PF5
(D) C2H4
..
:O
H
C
H
C
:N
H
C
H
H
:O..:
N:
..
:F :
..
: ..F
..
: ..F
P
..
F:
..
..
..F :
Which one of the following Lewis structures is definitely incorrect?
(A) NO2
:O:
N.
(B) CO32-
(C) CH4
(D) BH3
O:
H
H
:O
B
S
H
..O :
..
C
..
..O :
H
:..O :
:..O :
(D)
P
:I: :I:
(B)
Which one of the following Lewis structures is definitely incorrect?
(A) NO
(A)
I
:I: :I:
(C)
12.
I
C
H
H
H
(E) SO2
..
The Lewis structure of NO2+ is best drawn as
:....O
:....O
..N ..O : +
.. ..
..N O +
..:
..
(B) : O
..
..O
..
:..N
O
(E)
..N : +
..
..O +
:
(C)
..
:O
N
13.
The Lewis structure of which of the following molecules cannot obey the “octet rule?”
(A)
N2
(B)
O2
(C)
CO
(D)
NO
(E)
..O
+
:
HCN
14.
Which of the following molecules does not have an ideal Lewis structure (that is, all atoms happy)?
(A)
CH4
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
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-
17.
Which of the following atoms can have more than 8 valence electrons when bonding?
(A)
N
18.
Which has a Lewis dot structure with the greatest number of unshared pairs on the central atom?
(A)
NH3
(B)
(B)
(B)
(B)
(B)
H2O
(C)
NH4+
(C)
XeO3
(C)
C
(C)
IF3
(C)
BH3
(D)
I2
(D)
SiCl4
(D)
O
(D)
SeCl2
(D)
NH3
(E)
BH4-
(E)
NH3
(E)
ICl2-
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
Polyatomic ions:
Covalent
same name as atom
take atom name, remove end, add ”IDE”
Table 5.5-
LEARN THESE!!!!
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
20.
NaBr is the correct formula of
(A)
sodium bromate.
21.
Which is the correct formula/name combination?
(A)
(C)
Al2(SO4)3 / aluminum sulfate.
C2H3O– / acetate.
(B)
sulfide (C)
carbonate
(D)
nitrate
(E) sulfite
(B) sodium bromide (C) sodium bromite (D) sodium bromine
(B)
(D)
SF6
CH2Cl2
P
Naming Compounds:
Ionic
HF
K2PO3 / potassium phosphate.
PCl3 / phosphorus chlorite.
22.
A compound with the formula CaH2 is called
(A)
(B)
cadmium hydride.
hydrogen carbide.
23.
The correct name of N2O3 is
(A)
nitrogen oxide
24.
The formula of calcium nitrite is
(A)
Ca3N2
25.
Tetrasulfur dinitride decomposes explosively when heated. What is its formula?
(A)
S4N4
(C)
(D)
calcium hydride.
calcium hydrate.
(B) nitrogen(II) oxide (C) nitrous oxide.
(B)
(B)
(C)
Ca(NO2)2
S2N4
(C)
Ca(NO)2
S4N2
(D)
(D)
dinitrogen trioxide.
(D)
Ca(NO3)2
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
27.
The element with the greatest tendency to gain electrons is
(A)
F
28.
Which atom has the highest electronegativity?
(A)
Br
29.
In which bond are the partial charges on the atoms correct?
(A)
δ+
30.
Which of the following compounds contains the LEAST polar bonds?
(B)
(B)
Si-Oδ-
(B)
(B)
As
(C)
At
(C)
Mg
δ+
(C)
Cl-Brδ-
P
2.1
(C)
Si
O
C
δ+
N-Bδ-
Cl
3.0
(C)
SiH4
Si
1.8
Al
(D)
N
(D)
O
(D)
δ+
Atoms
Electronegativity
H
2.1
S
2.5
(A)
PH3
(B)
AsCl3
31.
Which set of bonds is arranged in order of increasing polarity?
(A)
(C)
Si-S < Si-O < Si-P < Si-F
Si-F < Si-S < Si-O < Si-P
(B)
(D)
As
2.1
(D)
(E)
Bi
(E)
H2S
Cl-Clδ-
Sb
1.9
(D)
Si-O < Si-F < Si-S < Si-P
Si-P < Si-S < Si-O < Si-F
SbCl3
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
2
3
4
5
6
Geometry
Linear
Trigonal planar
Tetrahedral
Trigonal bipyramid
Octahedral
Angle
180
120
109.5
90 and 120
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
3
3
4
4
4
Number of bond
3
2
4
3
2
Number of lone pairs
0
1
0
1
2
Structure
trigonal planar
bent
tetrahedral
trigonal pyramid
bent
33.
What is the molecular shape of N2O?
(A)
linear
34.
What is the molecular shape of the thiocyanate anion, SCN-?
(A)
linear
35.
What is the molecular shape of ClCN?
(A)
linear
36.
What is the molecular shape of BeH2?
(A)
linear
37.
What is the molecular shape of NOCl?
(A)
linear
(B)
(B)
(B)
(B)
(B)
bent
bent
bent
bent
trigonal planar
(C)
(C)
(C)
(C)
angular
(D)
trigonal
angular
(D)
trigonal
angular
(D)
trigonal
angular
(D)
trigonal
(C)
bent
(D)
tetrahedral
38.
What is the molecular shape of BCl3?
(A)
linear
39.
What is the molecular shape of NO2-?
(A)
linear
40.
What is the molecular symmetry around the carbons in CCl2CH2?
(A)
linear
41.
What is the molecular shape of ClO3F?
(A)
trigonal pyramidal
42.
What is the molecular shape of HOF?
(A)
trigonal pyramidal
43.
What is the molecular shape of NH2Cl?
(A)
trigonal pyramidal
44.
What is the molecular shape of SiF62-?
(A)
trigonal bipyramidal (B)
45.
Which one of the following will have a trigonal pyramidal shape?
(A)
PCl3
46.
Which one of the following molecules and ions will definitely have at least one 90° bond angle in it?
(A)
AlCl4- (B)
47.
Predict the ideal bond angles in AsCl3?
(A)
90°
48.
Predict the ideal bond angles in FNO.
(B)
(B)
(B)
(B)
(B)
BF3
NH3
trigonal planar
trigonal planar
trigonal planar
(B)
(B)
(B)
(C)
(C)
109.5° (C)
(C)
bent
(D)
tetrahedral
(C)
bent
(D)
tetrahedral
(D)
tetrahedral
(C)
square planar
trigonal
tetrahedral
hexagonal
SO3
PCl5
120°
(D)
(D)
(D)
V-shaped
(C)
square pyramidal
(C)
tetrahedral
(D)
(C)
bent
trigonal planar
(C)
tetrahedral
SO2
CO2
180°
(E)
(E)
(D)
(D)
(D)
tetrahedral
bent
octahedral
CO32-
H2O
(A)
49.
90°
(B)
109° (C)
120° (D)
180°
Predict the ideal bond angles around nitrogen in N2F2.
(A)
90°
50.
Predict the ideal bond angles around carbon in C2H2.
(A)
90°
(B)
(B)
109°
109°
(C)
(C)
120°
120°
(D)
(D)
180°
180°
Dipole Moments
Add polar bonds up – molecular DIPOLE MOMENT. Can cancel:
a)
b)
c)
Are there polar bonds – NO, no dipole moment
Are all the atoms attached to a central atom the same?
NO – dipole moment
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
52.
Which one of the following molecules does not have a dipole moment?
(A)
CS2
(B)
(B)
NF3
H2S
(C)
(C)
H2Se
CH2Cl2
(D)
(D)
TeO3
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