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Chemistry for Engineers
Homework 2
Electron arrangements &
BONDING
Answer Key
Q1: Fill in every blank box regarding the atoms or ions of the
following isotopes:
Species
3
no. of
neutrons
He+
no. of
protons
26
[Ne]
Mn3+
73
tungsten-186
indium-113
Species
3
electron
configuration
2
13
53
no. of
nucleons
He+
26
Al3+
53
Mn3+
125
Te2160
Dy
tungsten-186
indium-113
52
66
160
186
[Xe]
[Xe]6s24f10
64
no. of
neutrons
1
13
28
73
94
112
64
no. of
protons
2
13
25
52
66
74
49
no. of
nucleons
3
26
53
125
160
186
113
electron
configuration
1s1
[Ne]
[Ar]3d4
[Xe]
[Xe]6s24f10
[Xe]6s24f145d4
[Kr]5s24d105p1
Q2: The Lewis structures for the following ions are almost complete.
They show the bonding pairs and lone pairs but charges on atoms are
not shown. Redraw each and add charges to atoms that need them.
a) H3O+
H
O
H
+
H
_
b) NCS-
c)
HCO+
N
C
C
H
_
S
+
H
_O
d) OH-
O
e) N3-
N
+
N
_
N
Q3: For each molecule:
ClF
SF2
PH2Br
HCOOH
a) How many valence electrons are there?
b) Draw out the Lewis structure
c) Deduce the oxidation state of the central atom
d) Indicate bond polarity using d+ and d- symbols and overall molecule
polarity with
ClF
SF2
PH2Br
HCOOH
ClF
Count up valence electrons
Count up valence electron pairs
= 14
=7
Determine which atoms are bond to which (struc. formula)
= ClF
Form single bonds between neighbouring atoms:
(6 electron pairs remain unused)
F
Cl
Use remaining electron pairs to complete octets of each atom:
(three lone pairs on each halogen atom)
c) Cl oxidation state is +1
Cl
d+
F
d-
ClF
SF2
PH2Br
HCOOH
SF2
Count up valence electrons
Count up valence electron pairs
= 20
= 10
Determine structural formula:
Form single bonds between neighbouring atoms:
(8 electron pairs remain unused)
S
F
F
Use remaining electron pairs to complete octets of each atom:
(F octet must be completed first)
leaves two lone pairs on S
c) S oxidation state is +2
S
F
d-
d+
F
d-
ClF
SF2
PH2Br
HCOOH
PBrH2
Count up valence electrons
Count up valence electron pairs
= 14
= 7
Determine structural formula:
Form single bonds between neighbouring atoms:
(4 electron pairs remain unused)
Br
H
P
H
Use remaining electron pairs to complete octets of each atom:
d+
(Br octet must be completed first)
leaves one lone pair on P
H
c) P oxidation state is +1
d-
P
H
Br
ClF
SF2
PH2Br
HCOOH
HCOOH
Count up valence electrons
Count up valence electron pairs
= 18
= 9
O
Determine structural formula:
C
Place C at centre and form single bonds between
neighbouring atoms: (5 electron pairs remain unused) H
Use remaining electron pairs to complete
octets of each electronegative atom:
H
O
C
H
H
O
C has incomplete
octet!
O
BUT - not enough electrons to complete
carbon’s octet so C=O double bond
must form by donating O lone pair:
d+ C
c) C oxidation state is +2
H
O
dH
O
d-
Q4: Consider the sulphite anion (SO32-).
a) How many valence electron pairs are there?
Draw the Lewis resonance structures of the anion.
valence electrons = 26 (13 pairs)
place S as central atom and use 3 pairs – single bonds
_
_
O + O
S
(S has 6 valence electrons
so retains a lone pair)
_
_O
O
S can expand valency so double bond can be drawn:
_O
now move p bond to illustrate resonance structures:
_
_
O
_ O
S
O
S
O
_
O
S
O
O
_
O
S
O _
O
b) What is the charge on each oxygen atom?
- 0.667 (two thirds of a negative charge)
c) Given that the strength of a typical S-O single bond is
360 kJ/mol, estimate the S-O bond strength in the sulphite
ion.
bond order = 1.33
bond strength expected = 480 kJ/mol.
Q5:
For the following species:
I2
IF4+
IF3
PI3
a) Deduce the oxidation state of iodine
b) How many valence electron pairs are there?
c) Draw the Lewis structure
d) What is the geometry of the electron pairs
(e.g. linear, trigonal bipyramidal etc)?
What are the bond angles and molecule shape?
e) What related anion has I in the +1 oxidation state?
IF2-
Q5:
I2
IF4+
iodine oxidation state
valence electrons
valence electron pairs
0
14
7
+5
34
17
+3
28
14
F
F
I
F
I
F
+ F
I
F
F-I-F = 90º
& 120º
IF3
I
F
F-I-F = 90º
trigonal
trigonal
bipyramid
bipyramid
(molecule is
(atoms arranged T-shaped)
as sawhorse)
PI3
-1
26
13
I
I
P
I
I-P-I ~102º
distorted
tetrahedral
(molecule is
pyramidal)