1
00:00:16,466 --> 00:00:17,634
Now okay.

2
00:00:17,634 --> 00:00:22,906
So now I'm going to get
to the why this matters.

3
00:00:22,906 --> 00:00:26,810
So there's two different
sources, two different targets.

4
00:00:26,810 --> 00:00:29,846
Remember, we can call
them sources or targets.

5
00:00:29,846 --> 00:00:36,820
And notice they both have
this continuous radiation.

6
00:00:36,820 --> 00:00:38,054
And I will talk about that.

7
00:00:38,054 --> 00:00:40,357
The [INAUDIBLE] will
end with a discussion

8
00:00:40,357 --> 00:00:43,259
a little bit on what
you can do with that.

9
00:00:43,259 --> 00:00:46,329
But for now, let's keep
focused on these lines,

10
00:00:46,329 --> 00:00:48,031
these discrete
characteristic lines.

11
00:00:48,031 --> 00:00:49,632
So there's the k
alpha from molybdenum.

12
00:00:49,632 --> 00:00:51,501
There's the k alpha for copper.

13
00:00:51,501 --> 00:01:00,477
OK, now Moseley, Henry Moseley,
was a brilliant scientist.

14
00:01:00,477 --> 00:01:04,781
And he was working with
Rutherford and others.

15
00:01:04,781 --> 00:01:10,520
And he was really interested
in looking at the trends.

16
00:01:10,520 --> 00:01:12,655
So he took all of
these elements.

17
00:01:12,655 --> 00:01:15,358
So from calcium all
the way to zinc.

18
00:01:15,358 --> 00:01:16,826
20 to 30.

19
00:01:16,826 --> 00:01:18,862
And he said I want
to look at the k

20
00:01:18,862 --> 00:01:21,464
alpha lines of all of these.

21
00:01:21,464 --> 00:01:24,134
I want to look at the K
alpha lines of all of these.

22
00:01:24,134 --> 00:01:28,271
And what he found
was extraordinary.

23
00:01:28,271 --> 00:01:29,305
He did more.

24
00:01:29,305 --> 00:01:30,640
38 in total.

25
00:01:30,640 --> 00:01:33,410
But I'm just going to show
you his data for these.

26
00:01:33,410 --> 00:01:34,144
OK?

27
00:01:34,144 --> 00:01:39,416
And what he did was absolutely
profound because what

28
00:01:39,416 --> 00:01:40,350
he noticed--

29
00:01:40,350 --> 00:01:41,217
there are the lines.

30
00:01:41,217 --> 00:01:42,685
These are actually
his measurements.

31
00:01:42,685 --> 00:01:43,186
OK?

32
00:01:43,186 --> 00:01:46,156
These are his k alpha lines.

33
00:01:46,156 --> 00:01:50,026
And what he noticed in
going from calcium down to--

34
00:01:50,026 --> 00:01:51,795
ohh-- brass?

35
00:01:51,795 --> 00:01:54,097
OK, we'll talk about
that in a minute.

36
00:01:54,097 --> 00:01:56,032
It's supposed to
be zinc, isn't it?

37
00:01:56,032 --> 00:01:57,100
Why is it zinc?

38
00:01:57,100 --> 00:01:58,201
Well, why is it not zinc?

39
00:01:58,201 --> 00:02:00,336
Let's think about that later.

40
00:02:00,336 --> 00:02:05,375
For now he noticed that this
has a square root relationship

41
00:02:05,375 --> 00:02:06,609
to the energy.

42
00:02:06,609 --> 00:02:09,512
So Moseley came up with--

43
00:02:09,512 --> 00:02:12,382
he was working on this in 1912.

44
00:02:12,382 --> 00:02:14,217
OK?

45
00:02:14,217 --> 00:02:17,887
You got to remember 1913.

46
00:02:17,887 --> 00:02:23,159
1912 was when Rutherford did
the gold foil experiments.

47
00:02:23,159 --> 00:02:26,663
It had been 44 years
since Mendeleev

48
00:02:26,663 --> 00:02:30,100
put his periodic table to
paper and published that.

49
00:02:30,100 --> 00:02:33,670
So for 44 years, we
had a periodic table.

50
00:02:33,670 --> 00:02:36,305
But see, the thing is,
there was a huge problem

51
00:02:36,305 --> 00:02:41,911
with the periodic
table because Mendeleev

52
00:02:41,911 --> 00:02:46,950
had this, sort of, brilliant
realization that periodicity--

53
00:02:46,950 --> 00:02:53,456
periodicity-- was related
to, both, the atomic mass--

54
00:02:53,456 --> 00:02:55,391
and remember, we
talked about this--

55
00:02:55,391 --> 00:02:59,629
and the properties, the
chemical properties.

56
00:02:59,629 --> 00:03:02,699
That chemical properties.

57
00:03:05,201 --> 00:03:10,440
That allowed him to create
in ordering of the elements.

58
00:03:10,440 --> 00:03:12,108
In ordering of the elements.

59
00:03:12,108 --> 00:03:13,943
That is still the
ordering, essentially,

60
00:03:13,943 --> 00:03:16,646
that we have today.

61
00:03:16,646 --> 00:03:21,484
But the problem is why did
they have that ordering?

62
00:03:21,484 --> 00:03:25,288
I didn't really tell you why,
sometimes, he was like, well,

63
00:03:25,288 --> 00:03:27,257
the properties win.

64
00:03:27,257 --> 00:03:28,791
OK, maybe the mass-- no, no.

65
00:03:28,791 --> 00:03:29,526
Properties win.

66
00:03:29,526 --> 00:03:31,794
Properties need to be
aligned in this column,

67
00:03:31,794 --> 00:03:33,296
so I'm I'm going
to move those over.

68
00:03:33,296 --> 00:03:35,865
Like, that's what he did.

69
00:03:35,865 --> 00:03:39,802
But he didn't know why, except
that it made sense to him.

70
00:03:39,802 --> 00:03:42,238
Moseley's experiments told us.

71
00:03:42,238 --> 00:03:43,473
They gave us the why.

72
00:03:43,473 --> 00:03:45,508
Aw, it was so important.

73
00:03:45,508 --> 00:03:48,144
And Moseley's law--

74
00:03:48,144 --> 00:03:52,281
Moseley's law--
was, essentially,

75
00:03:52,281 --> 00:03:54,784
him thinking about
the Bohr model

76
00:03:54,784 --> 00:03:57,120
for these characteristic x-rays.

77
00:03:57,120 --> 00:04:01,024
So he said that h nu--

78
00:04:01,024 --> 00:04:05,094
so that's the frequency
for some k alpha line--

79
00:04:05,094 --> 00:04:08,164
is equal to 13.6 ev.

80
00:04:08,164 --> 00:04:09,599
All that looks familiar.

81
00:04:09,599 --> 00:04:13,203
Times z minus 1 squared.

82
00:04:13,203 --> 00:04:16,873
And then, he did the
difference in energy,

83
00:04:16,873 --> 00:04:21,010
just like we've done now a
number of times in this class.

84
00:04:21,010 --> 00:04:22,278
That's what he did.

85
00:04:22,278 --> 00:04:24,781
And this is 3/4.

86
00:04:24,781 --> 00:04:25,548
Why this?

87
00:04:25,548 --> 00:04:27,417
Well, let's talk about
that in a second.

88
00:04:27,417 --> 00:04:39,862
So 13.6 ev times 3/4
times z minus 1 squared.

89
00:04:39,862 --> 00:04:41,464
Two things about this, right?

90
00:04:41,464 --> 00:04:48,104
One is why is it 1 over 1
squared minus 1 over 2 squared?

91
00:04:48,104 --> 00:04:51,741
Well that's because they
knew, or at least they

92
00:04:51,741 --> 00:04:54,978
were pretty confident, that
you had this positive charge

93
00:04:54,978 --> 00:04:56,145
in the middle here.

94
00:04:56,145 --> 00:04:58,548
That's the protons
and the nucleus.

95
00:04:58,548 --> 00:04:59,949
And then, you had
these electrons.

96
00:04:59,949 --> 00:05:00,450
Right?

97
00:05:00,450 --> 00:05:03,286
So you had, like,
the 1s electrons.

98
00:05:03,286 --> 00:05:06,422
And then, you had
another shell out here.

99
00:05:06,422 --> 00:05:08,725
And so they knew, OK, 2s.

100
00:05:08,725 --> 00:05:13,196
Maybe that's combined
with 2p, so it goes on.

101
00:05:13,196 --> 00:05:16,332
So what happens in
the x-ray experiment?

102
00:05:16,332 --> 00:05:18,901
Well what happens is you
shoot an electron in.

103
00:05:18,901 --> 00:05:21,871
Rank and just crank
the voltage up

104
00:05:21,871 --> 00:05:25,775
so high that an electron
could come and knock that out.

105
00:05:25,775 --> 00:05:26,276
Right?

106
00:05:26,276 --> 00:05:28,978
And so that's what did it.

107
00:05:28,978 --> 00:05:32,982
And so now one of these
can cascade down there

108
00:05:32,982 --> 00:05:38,021
and give off a k alpha photon.

109
00:05:38,021 --> 00:05:38,554
Right?

110
00:05:38,554 --> 00:05:40,123
This is nothing new.

111
00:05:40,123 --> 00:05:41,858
We've talked about this.

112
00:05:41,858 --> 00:05:44,694
But you see it here
in the equation.

113
00:05:44,694 --> 00:05:46,095
You see it in two ways.

114
00:05:46,095 --> 00:05:50,066
First, we're going from 2 to 1.

115
00:05:50,066 --> 00:05:52,034
1 squared minus 1/2 squared.

116
00:05:52,034 --> 00:05:53,469
3/4.

117
00:05:53,469 --> 00:05:54,237
Right?

118
00:05:54,237 --> 00:05:57,774
But second-- and
this was critical--

119
00:05:57,774 --> 00:05:59,142
the z minus.

120
00:05:59,142 --> 00:06:06,683
The z minus told us that
all these positive charges

121
00:06:06,683 --> 00:06:09,085
in here, all these
positive charges,

122
00:06:09,085 --> 00:06:12,388
are screened perfectly by
one electron, this one that

123
00:06:12,388 --> 00:06:13,856
was left.

124
00:06:13,856 --> 00:06:15,692
And it works.

125
00:06:15,692 --> 00:06:18,394
He assumed perfect [INAUDIBLE].

126
00:06:18,394 --> 00:06:20,296
So what do all these see?

127
00:06:20,296 --> 00:06:22,231
They see z minus 1.

128
00:06:22,231 --> 00:06:28,204
They see z minus 1 if z is
related to the atomic number.

129
00:06:28,204 --> 00:06:31,174
Now here's where
this was so powerful

130
00:06:31,174 --> 00:06:33,443
because when you-- and
if you can't read this,

131
00:06:33,443 --> 00:06:34,409
don't worry about that.

132
00:06:34,409 --> 00:06:37,947
I just want to show you that it
is a perfectly straight line.

133
00:06:37,947 --> 00:06:41,150
When you plot the K
alpha transitions,

134
00:06:41,150 --> 00:06:43,386
these are different elements.

135
00:06:43,386 --> 00:06:45,588
These are his
different elements.

136
00:06:45,588 --> 00:06:51,159
And if you plot the square
root of the frequency

137
00:06:51,159 --> 00:06:56,999
versus element, it is a
perfect line that holds.

138
00:06:56,999 --> 00:06:58,901
And so what Moseley
wrote in the paper

139
00:06:58,901 --> 00:07:03,005
is we have your proof in 1930
that there is, in the atom,

140
00:07:03,005 --> 00:07:04,307
a fundamental quantity.

141
00:07:04,307 --> 00:07:07,977
A fundamental quantity, which
increases by regular steps

142
00:07:07,977 --> 00:07:10,480
as one passes from one
element to the next.

143
00:07:10,480 --> 00:07:12,915
This quantity can
only be-- only be--

144
00:07:12,915 --> 00:07:15,752
the charge on the
central positive nucleus,

145
00:07:15,752 --> 00:07:18,121
of the existence of which we
already have definite proof.

146
00:07:18,121 --> 00:07:19,155
Now we know about that.

147
00:07:19,155 --> 00:07:22,792
We know from experiments
that Rutherford

148
00:07:22,792 --> 00:07:25,895
did that that nucleus
had the positive charge.

149
00:07:25,895 --> 00:07:28,431
But they didn't know
that it was connected

150
00:07:28,431 --> 00:07:30,400
to the position in
the periodic table.

151
00:07:33,169 --> 00:07:37,573
In fact, years later,
people talking about those

152
00:07:37,573 --> 00:07:38,107
experiences--

153
00:07:38,107 --> 00:07:41,344
I mean, people didn't even
take Rutherford's experiments

154
00:07:41,344 --> 00:07:46,682
as seriously until
Moseley's work came along.

155
00:07:46,682 --> 00:07:51,220
44 years had come
since Mendeleev.

156
00:07:51,220 --> 00:07:55,091
But Moseley gave it the
foundation that it needed.

157
00:07:55,091 --> 00:08:00,196
Periodicity is because
of atomic number.

158
00:08:00,196 --> 00:08:02,698
Periodicity is atomic number.

159
00:08:02,698 --> 00:08:05,802
That was not known.

160
00:08:05,802 --> 00:08:08,771
That z gives you
the periodicity.

161
00:08:08,771 --> 00:08:09,272
Right?

162
00:08:09,272 --> 00:08:11,974
And that gives you, also,
the number of protons.

163
00:08:14,177 --> 00:08:16,512
This was a time when they
didn't know what was going on.

164
00:08:16,512 --> 00:08:20,049
Why did the mass change so much
the neutron wasn't discovered

165
00:08:20,049 --> 00:08:23,753
till 1932, 20 years later?

166
00:08:23,753 --> 00:08:25,822
But this gave the
grounding that was

167
00:08:25,822 --> 00:08:28,357
needed to the chemistry
of the periodic table.

168
00:08:28,357 --> 00:08:30,660
It was a very
important discovery.

169
00:08:30,660 --> 00:08:31,994
Very, very important discovery.

170
00:08:31,994 --> 00:08:33,462
So that's my why this matters.

171
00:08:33,462 --> 00:08:40,970
And it's really tragic
because he died, tragically,

172
00:08:40,970 --> 00:08:43,806
in World War I at age 27.

173
00:08:43,806 --> 00:08:47,910
And the Nobel Prize
was not given in 1916

174
00:08:47,910 --> 00:08:49,312
for either physics or chemistry.

175
00:08:49,312 --> 00:08:50,780
He died in 1915.

176
00:08:50,780 --> 00:08:52,949
And most people, at the
time, believe that he

177
00:08:52,949 --> 00:08:55,585
would have won it at age 28.

178
00:08:55,585 --> 00:08:58,855
That's how important
that discovery was.