CH101 SPRING 2013 MID TERM REVIEW QUESTIONS (CHAPTERS 1-3) SOLUTIONS
1. Determine whether the following statements are true or false. Write T or F in the boxes
adjacent to the questions. (2 points for each correct answer, -1 point for each incorrect answer
and 0 for no answer)
(i) Xenon difluoride is a V-shape polar ( > 0) molecule.
F
(ii) 1,2,3-butatriene (C4H4) is a planar molecule with sp and sp2 hybridized carbon atoms. T
(iii) The diagrams below represent the VB boundary surfaces of the  bonds in CO2. F
O
C
O
O
C
O
(iv) The ion HHe- has a bond order of zero, according to the LCAO-MO theory and hence should
be unstable. T
(v) Addition of an electron to C2 leads to increased stability (higher bond energy), according to the
LCAO-MO theory. T
(vi) For the lithium atom, the principal quantum number (n) completely defines the energy of a
given electron. F
(vii) Elements with very high first ionization energies tend to have negative electron affinities
(often assigned <0). T
(viii) The ground state electronic configuration of Cu is [Ar]4s 23d9. F
(ix) The ground state electronic configuration of As3+ is paramagnetic. F
(x) The transition elements of the 6th period (Hf-Hg) have somewhat higher first ionization
energies than those of the 5th period, due to prior filling of the 4f orbitals, which are less effective
at screening the outer electrons from the nuclear charge than d orbitals. T
2. Determine the excited electronic state of Li2+ that has the same energy as the first excited state
of hydrogen. [RH (Rydberg constant) = 3.29 x 1015 /s; c (speed of light) = 3.00 x 108 m/s; h
(Planck’s constant) = 6.626 x 10-34 Js]
For H, Z = 1 and for Li2+, Z = 3.
The energy of electronic levels is given in rydbergs by
En = -Z2
n2
For the first excited state of H, n = 2.
Hence
2
_ Z
n22
or
2
_ Z
n?2
=
H
_ 1
4
n?2
=
=
_
Li2+
9
n?2
36
(6 points)
n = 6 ( 5th excited state)
3. Determine the maximum wavelength of light that is needed to eject electrons from the surface
of sodium, if the work function of sodium is 4.41 x 10-19 J.
[Planck’s constant h = 6.626 x 10-34 Js; speed of light c = 2.998 x 108 m/s]
The maximum wavelength of light here is the one that supplies just enough
energy to overcome the work function ( ) at the surface of Na. The electron
kinetic energy here is zero and hence, from the Einstein equation, E = 
6.626 x 10-34 (Js) x 2.998 x 108 (m/s)
E =  = hc Hence,  =

4.41 x 10-19 (J)
= 4.50 x 10-7 m or 450 nm (visible region)
4. Identify and sketch the boundary surface for the orbital with the following wave function:

1
=
81 6 
a0
_ 3
2
2
e
_ 
3
(3cos2  - 1)
 = r/a0, where a0 is the Bohr radius.
5. Sketch the plot of radial distribution function (r 2[R(r)]2) versus r for 1s, 2p, 3d and 4f
orbitals, relating the value of r for the maximum value of the function with a0, the Bohr radius.
6. Give n, l, number of orbitals, number of radial nodes and number of angular nodes for the
orbitals, 3s, 2p, 6d, 5f and 5p.
Orbital
3s
2p
6d
5f
5p
n
l
3
2
6
5
5
0
1
2
3
1
No. orbitals
1
3
5
7
3
No. radial nodes
2
0
3
1
3
No. angular nodes
0
1
2
3
1
7. Predict which of the following pairs of ions have the greatest coulombic attraction in a solid
compound
(a) K+, O2- : Ga3+, O2- ; Ca2+, O2(b) Mg2+, S2- ; Mg2+, Se2- ; Mg2+ O2(a) K, Ca and Ga are all in the 4th period. Ga3+ is smallest and most highly charged cation
and so forms the strongest coulombic attraction with O2-.
(b) O, S and Se are all in group VIA, with O2- being the smallest anion. Hence coulombic
attaction is greatest for Mg2+ O2-.
8. Write the best Lewis diagram for the sulfite anion SO32-.
S is central atom (group VIA)
0
:O :
_ ..
: ..O
S
..
0
_ ..
:O :
_
..
O:
..
Rather than
_ ..
: ..O
+
..
S
_
..
with more
O:
..
formal charges
9. Write down Lewis diagrams, with formal charges on all atoms, for the molecules cyanic
acid (HOCN) and fulminic acid (HCNO). State which molecule is likely to be the more stable.
Lewis diagrams (with all formal charges):
0
0
0
0
0
+
0
0
..
..
..
..
H
O
C
N
H
C
N
O
or
H
C
:
..
..
HOCN can be written with no formal charges and hence is more stable
+
N
0
..
O:
10. Write resonance structures for the carbonate ion (CO 32-) and comment on the C-O bond
length in real carbonate salts.
..
.. _
.... _
_
O:
_
O:
O
..
.. :
..
..
..
:O
C
:O
C
:O
C
..
.. _
..
.. _
O
:
O:
O
..
.. :
..
O
O
C
O
2-
Resonance implies
equal C-O bond
lengths, between
true C=O and C-O
bond lengths (agrees with
experiment)
11. Arrange the following in order of bond strength.
(a) HCl; HF; HBr; HI
(b) O2; F2; N2
(a) The order of atomic size is F<Cl<Br<I, hence order of bond strength is HF>HCl>HBr>HI.
(b) Bond orders are O2 = 2; F2 = 1; N2 = 3, hence order of bond strength is F2<O2<N2.
12. Use the VSEPR theory to determine the shape of the xenon tetrachloride (XeCl 4)
molecule. State whether XeCl4 is polar or nonpolar.
F
xx
F
Lewis diagram
Xe
xx
F
F
electron pairs = 6: octahedral arrangement
F
F
F
Xe
F
Square planar shape, with lone pairs as far apart as possible: nonpolar
13. Use the Valence Bond theory to sketch the molecule allene (CH 2=C=CH2), showing
hybridization modes of C, types of sigma bonds, and pi bond formation.
(The central C atom forms one -bond on each side. These -bonds are orthogonal to bond. The central C atom must be sp-hybridized. The terminal C atoms are sp2 hybridized.
These sp2 orbitals are -bonded with 1s orbitals of H atoms and the sp hybrid orbital of the
central C atom.)
14. Sketch molecular orbital diagrams for the following species, and arrange them in order of
increasing bond order. BC, OF-, N2-, NO+, BN.
15. Sketch the molecular orbital energy diagram for NH and predict its magnetic properties.
16. Use MO theory to predict which of the following has the strongest bond: NO +, N2+, O2+.
Identify and sketch the MO boundary surface of the NO+ highest occupied MO (HOMO).