Looking closer at Argon

Graph showing the relative mass of atoms as the number of protons - the atomic number - increases. Notice that Argon is out of sequence - it is heavier than the element with one more proton. Why?
Graph showing the relative mass of atoms as the number of protons – the atomic number – increases. Roughly speaking the relative atomic mass increases linearly with the number of protons – but notice that argon is unusual. It is heavier than the element with one more proton – potassium. Why?

It never fails to amaze me how dumb I can be!

I have just spent three years making all kinds of precision measurements on argon gas, but it took a chance remark from Andrew Marmary in a short movie on the RI Channel to alert me to a simple astonishing fact: argon atoms – with 18 protons in each nucleus are on average heavier than potassium atoms which have one more proton in each nucleus. It’s a simple fact that hides a remarkable story!

The relative mass of atoms of each element is tabulated at the end of this article, and shown as a graph at the top of the page. The Atomic Number is the number of protons in the nucleus of each atom and the relative atomic mass is – very roughly – the combined number of neutrons and protons in the nucleus. So for example, a hydrogen atom has 1 proton and no neutrons has a relative atomic mass of 1. Helium atoms have 2 protons and 2 neutrons a relative atomic mass of 4. The graph shows that atoms with more protons in the nucleus tend to have around one extra neutron for each extra proton – but not exactly one. Notice that the relative atomic masses do not fall exactly on the red dotted line, but ‘wiggle’ a little. And some atoms such as chlorine – with 17 protons – have a relative atomic mass of 35.45 no where near an integer. Does a chlorine nucleus contain a fraction of a neutron? No. But to understand this we need to learn about isotopes

Even pure elements contain atoms with different numbers of neutrons. Naturally occurring chlorine, for example, has two isotopes both with 17 protons, but one has 18 neutrons and a relative mass of approximately 35 and the other has 20 neutrons and a relative mass of approximately 37. The former type outnumbers the latter by approximately 3 to 1 so the average mass of chlorine atoms turns out to be roughly 35.5.

So what about argon? Does that have isotopes too? Yes. Argon in the atmosphere has three isotopes, all with 18 protons – but one type (called 36Ar) has 18 neutrons and a relative mass of approximately 36 ; a second type (called 38Ar) has 20 neutrons and a relative mass of approximately 38, and the final and most common type (called 40Ar) one has 22 neutrons and a relative mass of approximately 40. Measurements made by my colleagues at the Scottish Universities Environmental Research Centre have shown that in normal argon there is roughly 300 times more 40Ar than 36Ar – and that  38Ar is even rarer. That is why the average atomic mass is just a little less than 40.

The astonishing fact is is that if we had made this measurement 4 billion years ago as the Earth formed, or if we made the measurement on argon gas from another planet – we would get a different answer – an answer much closer to 36. That is because the ‘natural’ argon is actually the 36Ar. If we re-plot the experimental data from the head of the page, but with a mass of of 36 for argon instead of the experimental value, then we see that the point fits neatly on the line.

So where did all the 40Ar come from? The answer is that it came from the radioactive decay of potassium-40 (40K). Most potassium on Earth has 20 neutrons (39K) giving potassium a relative mass close to 39. However, there is a small amount of potassium with 22 neutrons (41K) giving of potassium a relative mass slightly greater than 39. Additionally there is an even tinier amount of potassium with 21 neutrons (40K) and this isotope is radioactive, and decays into 40Ar with a half-life of around 1.2 billion years. So over the course of the Earth’s 4 billion year history around 90% of our original gift of  40K has decayed into 40Ar

In the solar system, argon is actually more common than potassium but on Earth potassium if far more abundant than argon. And so even though  (40K) is a tiny fraction of the potassium atoms on Earth – there is so much potassium (its about 1/500th part of the Earth by weight) that 40Ar from the radioactive decay of 40K is now the dominant isotope of argon on Earth.

So the graph at the head of the page seems mute, but if one can read the data and spot the patterns, one finds that the graph speaks volumes. It speaks of the history of the Earth and of the birth of the elements in the death throes of stars (Nucleosynthesis). Wow! And how could I not have noticed?

Data I took the data below from Wikipedia, so I know it must be correct 🙂

Atomic Number Symbol Name Relative Mass
1 H Hydrogen 1.01
2 He Helium 4.00
3 Li Lithium 6.94
4 Be Beryllium 9.01
5 B Boron 10.81
6 C Carbon 12.01
7 N Nitrogen 14.01
8 O Oxygen 16.00
9 F Fluorine 19.00
10 Ne Neon 20.18
11 Na Sodium 22.99
12 Mg Magnesium 24.31
13 Al Aluminium 26.98
14 Si Silicon 28.09
15 P Phosphorus 30.97
16 S Sulfur 32.07
17 Cl Chlorine 35.45
18 Ar Argon 39.95
19 K Potassium 39.10
20 Ca Calcium 40.08
21 Sc Scandium 44.96
22 Ti Titanium 47.87
23 V Vanadium 50.94
24 Cr Chromium 52.00
25 Mn Manganese 54.94
26 Fe Iron 55.85
27 Co Cobalt 58.93
28 Ni Nickel 58.69
29 Cu Copper 63.55
30 Zn Zinc 65.38
31 Ga Gallium 69.72
32 Ge Germanium 72.64
33 As Arsenic 74.92
34 Se Selenium 78.96
35 Br Bromine 79.90
36 Kr Krypton 83.80
37 Rb Rubidium 85.47
38 Sr Strontium 87.62
39 Y Yttrium 88.91
40 Zr Zirconium 91.22
41 Nb Niobium 92.91
42 Mo Molybdenum 95.96
43 Tc Technetium
44 Ru Ruthenium 101.07
45 Rh Rhodium 102.91
46 Pd Palladium 106.42
47 Ag Silver 107.87
48 Cd Cadmium 112.41
49 In Indium 114.82
50 Sn Tin 118.71
51 Sb Antimony 121.76
52 Te Tellurium 127.60
53 I Iodine 126.90
54 Xe Xenon 131.29
55 Cs Caesium 132.91
56 Ba Barium 137.33
57 La Lanthanum 138.91
58 Ce Cerium 140.12
59 Pr Praseodymium 140.91
60 Nd Neodymium 144.24
61 Pm Promethium
62 Sm Samarium 150.36
63 Eu Europium 151.96
64 Gd Gadolinium 157.25
65 Tb Terbium 158.93
66 Dy Dysprosium 162.50
67 Ho Holmium 164.93
68 Er Erbium 167.26
69 Tm Thulium 168.93
70 Yb Ytterbium 173.05
71 Lu Lutetium 174.97
72 Hf Hafnium 178.49
73 Ta Tantalum 180.95
74 W Tungsten 183.84
75 Re Rhenium 186.21
76 Os Osmium 190.23
77 Ir Iridium 192.22
78 Pt Platinum 195.08
79 Au Gold 196.97
80 Hg Mercury 200.59
81 Tl Thallium 204.38
82 Pb Lead 207.21
83 Bi Bismuth 208.98

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5 Responses to “Looking closer at Argon”

  1. Nick Day Says:

    Very interesting; this is something I’d been wondering about for some time; your blog prompted me to find http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements which you may find interesting, Michael.

  2. Michael Says:

    The abundance of the elements (a) in the Universe large and (b) on Earth are fascinating graphs. I remember now staring at them when I first encountered them and I remember the sense of awe that humans could even get >close< to explaining the relative abundances. It makes me feel small.

    All the best


  3. Camille Franicevich Says:

    Thanks for this post! You might also like to check out our isotope-specific blog, http://www.isotope.info, or our Facebook page: IsotopeDigest.

  4. Dibanos Says:

    That’s awesome, But what about Cobalt (58.9332) and Nickel(58.6934). Can you explain that too.

    • protonsforbreakfast Says:

      Well spotted. I don’t know the explanation for their weights, but I will ask around 🙂

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