Looking closer at Argon

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

Tags: Argon, Isotopes