When Rutherford split the atom

28 March 2017

Interview with

Malcolm Longair, Cambridge Cavendish Laboratory

100 years ago when our idea of what atoms were was very different. Our understanding changed radically and rapidly, thanks to a Nobel prize-winning scientist called Ernest Rutherford. His feats were so impressive that element number 104 on the periodic table, Rutherfordium, was named after him. But what did Rutherford do? To find out Ricky Nathvani went to the Cavendish Laboratory in Cambridge where Rutherford first announced his landmark breakthrough...

Malcolm - I’m Malcolm Longair. I’m the Jacksonian Professor Emeritus of Natural Philosophy. I was Head of the Cavendish Laboratory for eight years and I’ve written a history of the whole of the laboratory from its founding in 1874 until the present day.

Ricky - And, of course, today we’re here to discuss the Head of the Cavendish from 98 years ago now.

Malcolm - Yes. Rutherford came to Cambridge from Manchester in 1919 following the retirement of J.J. Thompson. Rutherford was clearly the outstanding nuclear physicist of the time. He discovered the nucleus and carried on doing brilliant experiments with very simple apparatus to establish the properties in nature of the nucleus.

Ricky - In the early twentieth century, scientists had established the existence of atoms - the tiny building blocks of the world. At the time, scientists thought that atoms were a blob of positive electric charge with negatively charged electrons stuck in it, like chocolate chips in a ball of cookie dough. The electrons, these negatively charged chocolate chips were already known about, but nothing was known about the dough.

Now if you fire a bullet through cookie dough… you’d expect the bullet to go straight through and that’s essentially what Rutherford’s team did. Instead of bullets, they fired tiny objects called alpha particles from a radioactive source into a sheet of gold foil. They could then measure these particles (these bullets), as they went into the foil and collided with gold atoms - a process called scattering. But something remarkable happened: some of these alpha particles came flying straight back the way they came…

Malcolm - This is the famous statement by Rutherford says “it was the most remarkable event of his life. It’s as if you’ve fired a large, heavy shell at a piece of tissue paper and it came back and hit you.”

Ricky - So there must have been something very small and concentrated to repel these alpha particles. This wasn’t behaving like cookie dough at all. So Rutherford’s team investigated the scatterings and built up a new picture of the atom…

Malcolm - Rather than the positive charge in atoms being all distributed in the same sort of ball, there was an extremely tiny nucleus with the atoms, only about a hundredth of the size of the atom itself which had all the positive charge.

Ricky - So atoms are mostly empty. In our new picture, the electrons orbit around this tiny positively charged nucleus. If an atom was the size of a football pitch, the nucleus, where all the positive charge and most of the mass is concentrated, would be the size of a garden pea in the middle of the pitch. The electrons would be whizzing around the stands with nothing between them and the nucleus.

But what actually was inside this nucleus? To find out Rutherford’s team bombarded nitrogen atoms in the air with those same alpha particles to investigate further and this time they found something new being emitted...

Malcolm - Rutherford carried on doing these experiments, and by 1917 he had discovered that these particles really had to be very light, fast particles coming out of the nucleus and made a suggestion that these were fast protons and that was the nuclei of hydrogen atoms.

Ricky - In other words, what Rutherford found was that there was a small charged particle ejected from the nucleus of the nitrogen atom. This particle turned out to be a fundamental building block of all atoms, known as the proton.

Now we have negative electrons and positive protons, but our atom isn’t complete because protons alone couldn’t account for the mass of the nucleus. There had to be something more hiding in there. This mystery prompted another one of Rutherford’s brilliant insights…

Malcolm - Now, in 1920, in his pencurian lecture Rutherford proposed well maybe there is a new particle, which he called the neutron, inside the nucleus. So that the nucleus consists of protons, hydrogen nuclei, and neutrons, rather than be lots and lots of protons being neutralised by electrons. So that was the suggestion he made, and he and Chadwick carried out a very large number of experiments for the next ten years. Some of them extremely wild, trying to find evidence for this particle and it didn’t turn up.

It was only in 1932 that Chadwick carried out the crucial set of experiments in a matter of three weeks which absolutely identified there was a neutral particle with high energy coming out of the nucleus.

Ricky - And so the picture of the atom fell into place. This nucleus in the middle of atoms was made up of positively charged protons and the aptly named “neutral neutrons.” Add negatively charged electrons whizzing around the nucleus and “viola,” an atom. And by adding up different amounts of these building blocks of protons and neutrons to make up a nucleus, you could make up all the different elements of the periodic table that make up the world around us.

So, from the beginning of the twentieth century without a clue what atoms were, we arrived at the picture of the atoms we still use today. And it’s no understatement to say that it was Ernest Rutherford who got us there…

Malcolm - He really is one of the greatest experimentalists of all time - if not the greatest! And the thing about Rutherford is that he would leave no stone unturned.

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