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|