Growing up I knew that the answer to the quiz question ‘Who first split the atom?’ was ‘Ernest Rutherford’, even if I didn’t necessarily know exactly what the question meant.  It still occurs in quiz circles – check out question 20 in the Daily Telegraph’s science and nature quiz from 2010.

The answer is Ernest Rutherford. It must be ‘Ernest Rutherford’ – in Manchester there is even one of those blue plaques we have on historic sites in the UK…

Creative Commons Nick Harrison on Flickr

Now, years later, I’m less sure what ‘splitting the atom’ means, yet it’s one of those rare scientific phrases that passes into popular currency, like ‘survival of the fittest’. So what does it mean? And will our answer to that question influence our answer to the question of who first split it?

Did JJ Thomson split the atom?

The word ‘splitting’ seems to refer to the days of the supposed indivisibility of the atom – we are told that ‘a tomos’ in ancient Greek means ‘un-cuttable’. After all, if an atom isn’t indivisible, why would splitting it be a big deal? Until 1897, it was assumed that atoms, if they really existed – and although scientific opinion leaned towards their reality, there was a level of dissention on this point – were indeed indivisible, the smallest building blocks out of which all substances were made.

Atoms of a given element were taken to be all the same as each other, yet different from those of other elements. This is still the view today, although – spoiler alert – the more modern view is that atoms of all elements are made of the same types of constituent parts, but in differing numbers. This is a more elegant solution (and more true) because you can make 92 naturally occurring elements by just having different numbers of protons, neutrons and electrons in their atoms, rather than needing 92 qualitatively different types of atom.

The discovery/invention of the periodic table is basically a story of studying complex patterns of behaviour and explaining them with a model using a very small number of simple constituent parts. A similar thing happened in the field of particle physics, when the standard model was created by postulating that ‘strong-force-feeling’ particles are made of six different types of ‘quark’. We may get round to doing a post on that some day…

Still, we are getting ahead of ourselves; we should be in 1897 when protons and neutrons were not yet discovered, and JJ Thomson in Cambridge was performing experiments that discovered the electron, a particle that is approximately one two-thousandth the mass of the lightest atom, hydrogen. If atoms are the smallest things, how can something exist which is lighter than the smallest atom? The world of subatomic particles was born, and the indivisibility of the atom was in doubt. By 1904 Thomson had developed a model of the atom in which electrons (known to be negatively charged because of the paths they took in electric and magnetic fields) were embedded in the atom, over which was ‘smeared’ a uniform positive charge, to balance the negative charge of the electrons. This picture of the atom became known as the plum pudding model (although I’m not sure you couldn’t find more appropriate desserts to conjure the same image).

Electrons (negative, and represented as green) embedded in a positive atom in the ‘plum pudding model’

If we rip an electron off an atom, which can be done, for example using strong enough electric fields, we create an ion, and the process is called ionisation. In 1899, Thomson announced that “electrification [ionisation] essentially involves the splitting of the atom [our italics], that is part of the mass of the atom getting free and becoming detached from the original atom”. Despite the reasonableness of Thomson’s definition, nobody nowadays thinks of ionisation as splitting the atom, even if he did! And the answer to the quiz question is never JJ Thomson…

The nuclear atom

So if Ernest Rutherford was the one to first split the atom, what was it he actually did to earn his place in quiz folklore? Well, you’ll see from his blue plaque that he also ‘discovered the nuclear atom’. We won’t discuss the alpha-scattering experiment conducted by him and Marsden in 1911 that led to this conclusion; suffice it to say, it was an enormous contribution to science (and the only mention of Rutherford in most UK school science curriculums), replacing the plum pudding model with a picture of the atom that had electrons orbiting a comparatively tiny nucleus that contained almost all the mass. Not unreasonably, people started to think of the atom using the analogy of the solar system, with the Sun being the analogue of the nucleus, and the electrons being represented by ‘all the stuff that orbits the Sun’. Overwhelmingly the majority of the volume of the atom (and therefore of collections of atoms, and therefore of you) in this model is empty space.

We are going to do a post on the use of analogy in science when we get round to it, too.

It would be plausible to think that discovering the nucleus might count as ‘splitting the atom’. After all, you could interpret ‘split’ to mean ‘show that it’s mainly empty space with some stuff in the middle’, even if Thomson’s definition might seem preferable. But the blue plaque mentions discovering the nuclear atom, AND splitting the atom. Which suggests that whatever ‘splitting the atom’ means, it is separate from the discovery of the nucleus.

Nuclear reactions

Instead, the term ‘splitting the atom’, as applied to Rutherford, is referring to his work in 1917 in which he fired ‘alpha’ particles at nitrogen atoms. This wasn’t as random a procedure as it might sound. Alpha particles were comparatively massive and doubly charged (at least, compared to the other known particles of radioactivity – beta particles) and that made them ideal projectiles for blasting into known materials to see what would happen. Rutherford found that an isotope of oxygen was created and a proton was ejected. This experiment is often described in just such bland terms, perhaps with the inclusion of the equation 𝑁+𝛼→𝑂+𝑝, as though it is obvious or mundane. In fact, it was revolutionary for at least two reasons:

• It is alchemy. Alchemists sought to transform base metals into gold (hence the need for a philosopher’s stone, Harry Potter fans…), in other words to turn one element into another. Rutherford’s experiment was the first known example of such ‘transmutation’ of elements. It’s true that radioactivity, discovered in 1896, changes one element into another, but it happens spontaneously. Rutherford made nitrogen turn into oxygen, using the alpha particles
• This was the discovery of the proton (Rutherford named the proton after its discovery in this nuclear reaction)

This was the experiment that people are referring to when they say he ‘split the atom’. It is sometimes said that ‘splitting the atom’ is a bit of a misnomer that would be better phrased as ‘splitting the nucleus’ (which then rules out JJ Thomson from quiz fame). But look at this process again. Here’s a diagram…

Would you choose the word ‘split’ here? We are lobbing 4 things into 14 things. By and large they ‘stick’ but one falls off [gross simplification of particle interactions]. Is that splitting? Well, according to received wisdom, and all the quizzes, yes!

Cockcroft and Walton

But this is not the end of the story! Certainly nuclear-physics-wise, and also not quiz-wise! Because if you are ever in a quiz and are told that your answer of ‘Rutherford’ is wrong, it is likely that the desired answer was ‘Cockcroft and Walton’. In some texts they are described as being the first to split the atom – for example the CERN courier in 2007, who probably know a thing or two about physics…

What did Cockcroft and Walton do differently from Rutherford to deserve the distinction? The answer is really rather subtle.

In 1932 C&W used their invention, an ingenious particle accelerator, to accelerate protons and fire them at lithium atoms. The nuclear equation to describe what happened is 𝐿𝑖+𝑝→𝐻𝑒+𝐻𝑒

The difference from Rutherford is that theirs was a human-accelerated battering ram (the proton), whereas Rutherford used naturally occurring alpha-particles. Cockcroft and Walton are sometimes therefore described as being the first to ‘split the atom by artificial means’, a subtle distinction that has been known to elude quiz compilers. Also, the diagram corresponding to 𝐿𝑖+𝑝 shows a more intuitive use of the word ‘split’, with two equally sized things emerging at the end…

So does Rutherford or Cockcroft & Walton deserve the accolade of being the first to split the atom? Well, maybe the Cockcroft and Walton reaction matches better the common use of the word ‘split’, but authors seem to decide not on this criterion, but on whether they want to restrict the reaction to being caused by artificially accelerated particles.

Perhaps we should leave this with Jim Al-Khalili, who, in a BBC documentary on Sellafield, described Otto Hahn as being the first to split the atom, via the process of nuclear fission.

Now, fission really looks like ‘splitting’, just like the Cockcroft and Walton reaction, but from experience I can give you this advice: never answer Otto Hahn in the quiz – it will get you nowhere…