Science Questions

How old is an Atom (say hydrogen)?

Mon, 5th Sep 2016

Listen Now    Download as mp3 from the show How Old is the Average Atom?


David asked:

How old is an Atom (say hydrogen)?


We asked Caroline to give us an atom's age...Scheme of an atom

Caroline - So, that is a very, very tricky question. So, as kind of said in the question, it depends on what kind of atom we’re talking about. It depends on which atom we’re talking about. So, most hydrogen atoms are 13.7 billion years old as they were formed in the Big Bang. If you have a large atom, it’s formed from the fusion of small atoms so it’s likely to be younger. But any sort of random atom you pick could be anything from 13.7 billion years old to made the other day in a fusion reaction.

Kat - So things like iron and stuff like that and the heavier elements, they are made in stars, supernovas?

Caroline - I think they can be made in supernovas, it requires dense, really packed material and a lot of heat to make these sort of bigger elements.

Adam - All we had after Big Bang was hydrogen and helium. Heavier elements are formed in stars and then distributed through the cosmos when the stars throw off material, most famously in supernova explosions.

Kat - Isn't it that Carl Sagan phrase that, “We are all made of stars.”

Adam - We’re star dusts.

Caroline - Which is completely true.

Kat - First time I heard that, I was like, “Wooh!”


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This depends on how you define an atom. If you include both nucleus and the particular electron that is currently paired with said nucleus (for a neutral hydrogen atom 1 electron and one proton) then most of the "hydrogen atoms" in the universe are only tiny fractions of a second old because the electrons are constantly moving from one atom to the next and so on at a blindingly fast rate.

If, instead, one defines the atom as only the nucleus, then most of the hydrogen atoms in the universe are as old as the universe. Sure, there are some protons that have come into being more recently, either by pair production from high energy photons, or from the decomposition of free neutrons (released either by fission or fusion processes). But a vast majority of the protons that exist now were formed within the first few minutes after the big bang, over a dozen billion years ago.

Nuclei of helium and lithium were also formed shortly after the big bang, but essentially all other nuclei were created by the fusion of these light elements in the stars. I'm not really sure what sort of age this translates to, but I think it is reasonable to say that on average, the heavier the nucleus of an atom is, the more recently it formed (for stable nuclei -- in the case of radioactive nuclei, it is probably better to consider the half-life and estimate that, for instance, there is a 99% chance that a given atom of that nucleus is younger than 7 half lives) chiralSPO, Fri, 26th Aug 2016

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