1
MidTerm #2b
CHEMISTRY 20A
SOLUTIONS
March. 7, 2001
.
.
Student Name
.Johnny
Bobby
Rebecca.
Circle Name of TA
.
.
Student I.D. No.
PLEASE WRITE
YOUR NAME
&
NAME OF YOUR TA
and READ the DIRECTIONS on THIS PAGE
PLEASE
DO NOT TURN PAGE UNTIL ASKED TO DO SO
.
DIRECTIONS
.
Do all problems.
All work and answers should be written on the exam.
Please write legibly and clearly.
Partial credit will be given; correct statements will count in
your favor, incorrect ones will count against you, unintelligible or illegible ones will not
count at all.
Wherever applicable, show your work, justify approximations, and explain
what you are doing; otherwise no partial credit can be given. In many of the problems no
credfit will be given without supporting explanation.
Allocate your time between problems
judiciously.
This is a closed book exam, but you may make use of a single 8.5" by 11"
sheet of handwritten notes (both sides).
This exam consists of 8 pages
Problem.
Max Grade
Grade
1
28
2
22
3
16
4
35

Total
101
Some possibly useful data:
μ
= Qr
U
electrostatic
∝
Q
1
Q
2
r
12
K.E. = (1/2)mu
2
prob density = 
Ψ

2
λν
= c
λ
mass
= h/mu
E
n
=  R
H
Z
2
eff
1
n
2
r
n
= a
o
n
Z
eff
h = 6.6x10
34
J s
c = 3.0x10
8
m s
1
m
e
= 9.1x10
27
kg
m
proton
= 1.7x10
27
kge = 1.6x10
19
C
N
o
= 6.0x10
23
particles mol
1
1 eV = 1.6x10
19
J
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1.
What counts in this question is the reasoning by which you arrive at your answer
(whatever it may be).
Be sure to lay out your reasoning succinctly, as quantitatively
as
possible, legibly, and clearly
, or else you will not get credit.
The ionization energy of the H atom is about 1,400 kJ mol
1
, that of He about 2,500 kJ mol
1
a)
Making use of these numbers, estimate the effective nuclear charge, Z
eff
,
on a 1s
electron in He? Why is this reasonable (or unreasonable)?
One can model the electronic energy of He in terms of shielding.
The charge on
the nucleus is Ze with Z = 2, but each of the 1s electrons shields the other so
that Z
eff
lies between 1 and 2.
Thus the ionization energy can be expressed as
Z
eff
2
R
H
/1
2
= 1,400 kJ mol
1
(Z
eff
2
) = 2,500 kJ mol
1
Z
eff
= (25/14)
1/2
1.33.
We expect each 1s electron to be quite effective in shielding the other
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 Spring '11
 baugh
 Chemistry, Atom, Electron, Atomic orbital, Zeff, outer electrons

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