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Lesson 9: Atoms 3: The nucleus

Introduction

In this lesson we’ll learn how radioactivity was discovered and how radiation was classified into alpha, beta and gamma.  We’ll see how this led to the extraordinary theory that radioactivity causes the atom of one element to change into the atom of another element.

We’ll also see how the picture of the atom changed from the plum pudding model to one with a tiny nucleus at the centre.

Try Why Do Astronauts Float by Julian Hamm

A strange new ray from a Crookes tube

Last lesson we looked at Crookes tubes.  The green glow is caused by ‘cathode rays’ hitting the glass.

We saw how JJ Thomson suggested that these ‘cathode rays’ were really tiny negative particles called ‘electrons’ which made up all atoms.  But there was another branch of physics that also started off with studying Crookes tubes.

In 1895 the German physicist Wilhelm Roentgen was doing a series of experiments to see if cathode rays could be detected outside a Crookes tube.

He wanted to check no light was escaping so he covered the tube in black card and made sure the room was completely dark.  Roentgen noticed a strange glow on the other side of the room.  It was completely unexpected and he couldn’t see where it came from.

The glow came from a screen that had been painted with a chemical that would glow when ultraviolet light shone on it.  Roentgen was just about to use the screen in his next experiment.

But Roentgen knew that ultraviolet light couldn’t have got through the black card.  And it was known that cathode rays couldn’t travel more than a few centimetres outside the tube.

So it wasn’t ultraviolet and it wasn’t cathode rays.  It seemed that the Crookes tube was producing some completely new ray.  Roentgen gave them the temporary name ‘X’ rays because he didn’t know what they were or where they came from.

Why Roentgen thought everyone might think he'd gone mad

He immediately started doing lots of different experiments to find out as much as he could about these new ‘X’ rays.

As he was setting up one of them his hand passed in front of the screen he was using while the Crookes tube was switched on.  Roentgen saw a shadow on the screen.  The shadow showed the blurred outline of the bones of his hand.

Roentgen was so amazed that he determined to work in secrecy until he knew more in case other scientists thought he was mad.

He found that X-rays were produced in all directions when the cathode rays hit the glass side of the Crookes tube.  He also quickly found that X-rays fogged film.  So if you made X-rays pass through your hand and then onto some film your bones showed up as shadows.

X-rays became like a circus attraction

To the public of the late 1890s, X-rays were THE scientific discovery of the times.

Within months of Roentgen’s discovery the first medical X-rays were being taken to investigate broken bones and find bullets.  And because Crookes tubes were readily available, street entertainers started taking X-ray photographs of the public for a fee.

The dangers of X-rays

But it was soon discovered that X-rays could produce burns and tumours on the hands of the photographers.  It took over a decade before doctors began to understand how to use X-rays safely.

This meant limiting the energy of the X-rays, shielding operators behind lead screens and not taking too many X-rays of the same patient.

Wilhelm Roentgen, a modest man

Roentgen became famous and could have made millions but refused to patent his invention.  This meant anyone could use his invention for free and it could be used for the good of mankind.

Even though it would have been normal to call them Roentgen rays, Roentgen himself preferred his original name of X-rays.

Becquerel wonders if X-rays and fluorescence are related

One of the many scientists who immediately started investigating Roentgen’s X-rays was a Frenchman called Henri Becquerel.

Becquerel was interested in fluorescent minerals.  These glowed brightly in bright sunshine.  Becquerel wondered did the mineral also give off X-rays while it was fluorescing?

First of all he took some unexposed film and put it in a shallow cardboard box.  He put a metal cross on top of the box.  This would cast a shadow if any X-rays shone on it.  Finally he placed a tray of powdered mineral on top of the cross.

Becquerel left this apparatus set up in the bright sunlight for a whole day so that the mineral on top of the cross fluoresced.  Then he removed the mineral and the cross and in the dark developed the film.  He found that the film had been fogged and the shadow of the Maltese cross was clearly visible.

Becquerel thought that only X-rays could have penetrated the box and exposed the film.  He concluded that the fluorescence did indeed cause X-rays to be given off.

Becquerel repeats his experiment and accidentally finds another new ray

Like any good scientist he wanted to repeat his experiment so he set up the apparatus again.  Unfortunately the weather had turned and it was cloudy so the mineral wouldn’t fluoresce.

Becquerel tidied the whole apparatus into a dark drawer and waited for the weather to improve.  After a few days the weather was still no better so he decided to develop the film anyway, expecting to perhaps find a very faint shadow.

To his surprise he found that the film had been exposed just as much as when the mineral had been in the sunlight.  It seemed that sunlight didn’t seem to play any part at all.  Perhaps it wasn’t even X-rays that were fogging the film?

After further experiments Becquerel came to the conclusion that the mineral gave off rays all the time and that they weren’t X-rays but something completely new.

The mineral was a compound of uranium, which had been used for decades to colour glass ornaments.

Becquerel found that any compound of uranium always gave off these rays and that there was no way to change the amount given off.  Becquerel’s 'uranium' rays didn’t merit that much public attention.   They weren’t really fun like X-rays.

The Curies and Ernest Rutherford investigate Becquerel's rays

However scientists were more interested.  In particular a young Polish scientist called Marie Curie and a New Zealander, Ernest Rutherford.

Marie Curie was married to a well-respected French physicist called Pierre and they both worked in Paris.  She persuaded him to stop his current project and work with her investigating uranium rays.

It was Marie Curie who invented the term ‘radioactive’ to describe a substance that gave out these new rays all the time.

Radioactivity was a property of the atoms themselves

Marie and Pierre wanted to see how many different elements were radioactive.  They quickly discovered that thorium was, but much of their time was spent studying uranium compounds.

They found that the amount of radioactivity depended only on the number of uranium atoms present.  It didn’t matter what other atoms the uranium atoms were combined with.  This meant that radioactivity came from the atoms themselves rather than, say, a chemical reaction between uranium and other atoms.

Purifying uranium seemed to reduce its radioactivity!

Uranium is a metal.  It is extracted from an ore called pitchblende.  So pitchblende is also radioactive because it contains uranium.

The Curies expected pure uranium to be more radioactive than pitchblende.  They wanted to know how much more.  They used an electroscope to measure the amount of radioactivity.

But very strangely pitchblende turned out to be more radioactive than pure uranium metal.

The Curies concluded that there must be other radioactive elements in the pitchblende.  And if these other elements hadn’t been discovered before then presumably there must only be very small amounts of them.  This meant they had to be very radioactive.

The struggle to isolate even tiny quantities of other radioactive elements

So if they were going to isolate these other radioactive elements they were going to need a lot of pitchblende.  Over several years they gradually refined tonnes of the stuff, sometimes working in huge vats other times with delicate glassware.

The school of physics at the Sorbonne, the famous Paris university, could only provide them with an old leaky shed to work in.  It was freezing in winter, boiling hot in summer and often filled with the smoke and fumes from their chemical processing.

Eventually the Curies succeeded in isolating invisibly small quantities of two new elements.  The first one Marie called ‘polonium’ after Poland, where she was born, the second ‘radium’ because it was so radioactive.

The discovery of radium helped future research

Radium was important for science because it gave off so much radiation, unlike uranium, which was only weakly radioactive.

This meant you could make the radiation go through narrow slits, giving a ‘beam’ of radiation that scientists could experiment on.

Marie gained two Nobel prizes but lost a husband

In 1903 the Curies shared the Nobel Prize for Physics with Becquerel for their pioneering work on radioactivity.

Pierre was tragically run down by a horse and carriage three years later and Marie was awarded his teaching post at the Sorbonne.  She was the first woman to take up a teaching position there in its 650 year history.

In 1911 she was awarded the Nobel Prize for Chemistry for isolating and studying pure radium metal.  This made her the first person to be awarded two Nobel prizes and the only person ever to hold them in different subjects.

She refused to patent her technique for isolating radium and worked tirelessly to promote the use of radiation for peaceful ends.

She died in 1934, aged 67, from a condition almost certainly caused by her massive exposures to radiation over her life.

The Rutherford jigsaw

And what about the New Zealander, Ernest Rutherford?

Rutherford probably made more of the fundamental discoveries about radioactivity than any other scientist.  He identified alpha and beta radiation, discovered that radioactivity causes one element to change into another and found that radioactivity decreased over time.

But it would be wrong to think that Rutherford was not part of a community of scientists working in different countries.

Think of all the scientists working on one big jigsaw.  The picture they are trying to make shows what all of radioactivity is like.

They cooperate in the sense that they are all trying to complete the same puzzle.  But it doesn’t work by one person picking up a piece and then the whole group discussing where it might go.  Everyone gets to see the whole jigsaw because they all read the same scientific journals.  But they basically work by themselves.  And each scientist sees how his or her piece may fit in to what’s already been put down by the rest of the community.

Just like in a jigsaw you can pick up a piece and not see where it goes, though to someone else it might be obvious.  Or someone can force a piece into the wrong place until another team member sets it right.  There isn’t a jigsaw team captain who decides whether a piece is in the right place.  When a piece fits, it’s up to the whole community to decide whether it’s right.

Some people work obsessively trying to finish off one part.  Others pop in, spot a piece that fits, then go off and work on another puzzle.  Neither does the team gather all the similar pieces together so that the best puzzler can put them in their final position.

Think of Rutherford as just being a strong member of the whole jigsaw team.  Some of the pieces he picked up and placed by himself, others someone else put in roughly the right place and he slotted them in.  Sometime his guesses about where a piece went were good, other times he was wildly wrong.

It would be too confusing to describe exactly how much each person contributed to each piece.  So we’re going to look at some of the pieces that Rutherford helped click into place, even though he relied on what others had done before.

Rutherford, JJ Thomson's star student

Rutherford was one of JJ Thomson’s best students.  He was working for Thomson at Cambridge University in 1897 when Thomson was ‘discovering’ the electron.

In 1898 Rutherford left Cambridge for McGill University in Montreal, Canada, where he could use one of the best laboratories in the world.

Rutherford concludes there must be two types of radioactivity

He did some very careful experiments to see how well the uranium rays were absorbed, which led to some very unexpected results.

A single sheet of paper reduced the amount of radiation detected by a large amount.  You’d expect that another sheet of paper should reduce the amount of radiation detected to almost nothing.

Having two sheets of paper rather than only one made no difference at all!  How could Rutheford explain this?  Rutherford reasoned that there must be two types of radiation, which he called alpha and beta (so HE identified and named them).

One sheet of paper cuts out all the alpha but hardly any of the beta.  When the second sheet of paper is added, the alpha has already been cut out and it hardly affects the beta at all.  That’s why the amount of radiation detected stays almost the same.

In fact Rutherford made lots and lots of measurements and used very thin aluminium foil but the principle is the same.

Rutherford has problems with electric fields

Rutherford tried to bend the paths of the alpha and beta particles using electric and magnetic fields.

Beta particles can be bent quite easily.  But alpha particles have a much bigger mass so it is much harder to change their direction.  In fact you need such big fields to see alpha particles curving that it took Rutherford three years before he managed it.

The fact that the radiation could be deflected by magnets showed that alpha and beta radiation were made up of charged particles.

You can tell what kind of electric charge alpha and beta particles have by seeing what kind of charged plate they are attracted to.

Beta particles are attracted to the positive plate.  So beta particles have a negative charge.

Alpha particles are attracted to the negative plate.  So alpha particles have a positive charge.

In an electric field alpha and beta particles follow a shape called a parabola.

In a magnetic field alpha and beta partices follow part of a circle.  You can use Fleming's left-hand rule to show which sign of charge alpha and beta particles have.

Rutherford uses charge to mass ratio to identify alpha and beta

Rutherford used electric and magnetic fields to calculate the charge to mass ratio of alpha and beta particles.

He found that the charge to mass ratio for a beta particle was exactly the same as for an electron, so presumably that's what they were, since nothing else had anything like such a tiny mass relative to its charge.

The charge to mass ratio for an alpha particle seemed to be the same as the charge to mass ratio of a helium atom (with its electrons removed to give it a positive charge).  There didn't seem to be any particular reason why this should be the case so Rutherford wanted more proof that it wasn't something else that just happened to have the same charge to mass ratio.

Alpha particles just end up as helium gas

William Ramsay and Frederick Soddy published good evidence in 1903 but we’ll look at Rutherford’s experiment of 1909.

He used a radioactive gas in a very narrow tube with paper-thin glass walls.  The gas gave off alpha particles.  The thin tube was kept inside a larger glass tube.  The larger tube had had all of the air removed.  The alpha particles could pass through the very thin wall of the inner tube into the outer one.

There’s no air in the outer tube so the alpha particles speed on until they hit the wall and get stuck between the glass particles.  While they’re there they each steal two nearby electrons and become neutral helium atoms.  Eventually the random vibration of the helium atoms causes them to break free from the wall.

They fly around inside the tube just like any other gas particles.

Rutherford left the gas to collect for several days.  He still only had a tiny amount but it was enough to check that it was helium.  He put a high voltage across the gas so it glowed like a Crookes tube.  The exact colour of the glow would tell him whether the gas was helium.

First of all the light from the glow was made to pass through a narrow slit.  Then the light was split up into its different colours by a screen with thousands of narrow gaps engraved on it, called a diffraction grating.  It was known that each element makes a unique pattern of lines.  The pattern was the pattern for helium.  This technique is called ‘spectroscopy’ because it involves looking at the spectrum of a hot gas.

This confirmed that an alpha particle was just a helium atom with its two electrons removed.

So the first of Rutherford’s great triumphs was to discover the properties of alpha and beta radiation.

Paul Villard finds gamma radiation

A French physicist, Paul Villard, working in Paris at the same time as the Curies, ‘discovered’ gamma radiation in 1900.  Villard called his new rays ‘gamma rays’ to follow Rutherford’s naming system but had no idea what they were.

In 1914 Rutherford showed that gamma rays were just a very high frequency form of electromagnetic radiation.  They are the same type of ‘stuff ‘as radio-waves, microwaves, infrared, light, ultraviolet and X-rays.

A drafty lab gives Rutherford a clue that radioactivity decreases with time

But let’s go back to 1900 just after the ‘discovery’ of alpha and beta radiation.

Scientists had found that the radioactivity of a sample of uranium didn’t seem to change with time. (It does, but too slow to detect).  But thorium, the second element found to be radioactive by Marie and Pierre Curie, seemed to be different.

Rutherford’s methods for measuring radioactivity were quite complex so let’s just imagine he had a special radioactivity meter.

Rutherford found that the radioactivity of a sample of thorium powder suddenly changed when someone opened the laboratory door.  It dropped off suddenly and then slowly increased again until it reaches its original level.  What was going on?

It must have been something to do with the draft created by the moving door.  It couldn’t be that the draft was blowing the alpha or beta particles themselves.  They moved far too quickly to be disturbed by a gentle gust of wind.

Rutherford reckoned that the thorium was giving off a radioactive gas (which we now know is a type of radon).  So there were two sources of radioactivity: the thorium and the gas.

When the door was opened the radioactive gas was blown away and spread out in the lab so the radioactivity reading dropped.

Rutherford wanted to investigate this gas and designed a way to collect a sample in a glass tube.  He found that the radioactivity of the gas decreased with time.  The radioactivity dropped by half about every minute.

Remember that it's the RADIOACTIVITY of the gas that decreases.  The gas itself doesn’t disappear into thin air because it's in a glass tube.

Rutherford was the first person to report that radioactivity decreased and to use the idea of half-life.

He still had no idea where the gas came from.  Was it just the thorium evaporating somehow, or perhaps a very fine dust?

Atoms could actually change from one element to another!

Over the next couple of years he and his colleague Frederick Soddy did lots of experiments to see how radioactivity changed with time.  They came to a remarkable conclusion.

Radioactivity caused the atoms of one element to change into the atoms of another.  Often this atom was also radioactive and so changed into yet another element.

The gas that Rutherford had found came from atoms of thorium eventually turning into atoms of radon (which is a gas).  Rutherford was detecting the extra radioactivity when the atoms of radon turned into the atoms of yet another element.

This was a revolutionary idea.  Since the time of the ancient Greeks the whole point with the atomic theory was that atoms couldn’t change.  And yet here was Rutherford saying that quite spontaneously they could turn into a completely different element.

No one had any idea how this actually happened.

What’s more the Curies had found that radioactivity involved the release of energy.  Often quite a lot of energy.  This energy seemed to be coming from the radioactive atoms themselves but how?

Isotope means 'same place' in the periodic table

There was a great flurry of activity between 1903 and 1910 of scientists discovering new radioactive elements.  They discovered that there were three major natural ‘decay chains’.  One atom decayed into another, which decayed into another and so on, finally ending up with an atom of lead.

Strangely the same element appeared in lots of different places.  But in each place the half-life was different.

In 1910 Frederick Soddy suggested that the atom of an element could come in slightly different forms that he called ‘isotopes’.  Isotope means ‘same place’ of the periodic table.

What made one isotope, of lead for example, different from another was the mass of its atoms.  No one had any idea why the mass was different.  For that matter no one really had any idea where atoms hid their mass at all since the mass of an electron was so small.

This was all about to change and, like so many other discoveries in radioactivity, it was almost by accident.

Rutherford investigates the plum pudding model of the atom

Rutherford had moved from Canada back to England in 1907 and was a Professor of Physics at Manchester University.  He started working with a German physicist called Hans Geiger.

Geiger proved his worth almost immediately by inventing a way of counting individual radioactive particles.  This was the famous Geiger counter.  Some years later the design was refined by his research student, Walther Mueller.

Sometimes Rutherford wanted a record of exactly where alpha particles went.  To do this he used a screen of photographic film.  The film was exposed where the alpha particles hit.

Sometimes an experiment meant that the alpha particles had to pass through a very thin barrier made of a mineral called ‘mica’.  This tended to produce a slight blurring of the image on the film.  Rutherford assumed that the alpha particles must be being scattered a little as they passed through the mica.

At this time everyone accepted JJ Thomson’s plum pudding model of the atom.  But no one knew much about the positive charge.  Rutherford thought that the scattering of alpha particles might be able to tell him something about how the positive charge was arranged.  He set Geiger the task of finding out as much as he could.

Like an artillery shell bouncing off tissue paper

Geiger had to use very thin foils for the alpha particles to make it through.

He detected the alpha particles using a screen which flashed whenever they hit it.  It was much more difficult than it sounds.  The flashes were so small and faint that they could only be seen by attaching the screen to the end of a microscope.  Even then Geiger had to work in a very dark room and wait for his eyes to adjust before he could start.

Geiger found that the alphas could be deflected quite a lot when they passed through the thin metal foils.

Working with Geiger was a 20 year-old undergraduate called Ernest Marsden.  To give young Marsden something to do, Geiger suggested to Rutherford that he see if any of the alpha particles actually bounced back.

Both Rutherford and Geiger knew that it was almost impossible but it would give Marsden some useful experience with the apparatus.  Two days later Geiger came back to Rutherford very excited.  Very, very occasionally Marsden had detected alpha particles bouncing back off the gold foil.

This was extraordinary.  Rutherford compared it to firing artillery shells at tissue paper and finding one come straight back at you.

But what on earth was causing it?  Alpha particles went straight through very thin gold foil.  But not, apparently, all the time.

Rutherford invents the atomic nucleus

Rutherford’s old boss JJ Thomson was the world expert on the plum pudding model of the atom and Rutherford accepted it too.  But there was no way that the plum pudding model of the atom could explain these rebounding alpha particles.

The result puzzled Rutherford so much that over the next year he took over the investigation from Geiger.  Then one day in 1911 Rutherford came into the lab very pleased with himself and said to Geiger that he knew what the atom looked like.

Rutherford’s conjecture was that an atom must have a tiny nucleus with electrons whizzing round it.  The nucleus had a positive charge to balance the negative charge of the electrons.

And almost the whole mass of an atom was concentrated there so the nucleus must be incredibly dense.  The alpha particle and the nucleus both have a positive charge.  Every so often the alpha is aimed at the nucleus and is repelled back.

Rutherford did lots of experiments with different materials and different alpha sources and came up with a pretty good mathematical theory that explained alpha scattering.

Rutherford's atom was almost all empty space

In 1903 the Japanese physicist Hantaro Nagaoka had already suggested that atoms were made up of electrons orbiting a dense centre.  What made Rutherford’s model different was the amount of empty space.

The nucleus must be miniscule and the electrons had no discernible size at all.  So perhaps 99.999% of an atom was just void.  But the electrons were held around the nucleus by very strong electric fields.  So atoms ‘feel’ as big as the outermost electrons not just as big as the nucleus.  And even though atoms are mostly empty space they can’t just pass through each other like ghosts.  This is why the world seems to be full of solid objects like tables, chairs and people.

The nucleus of an atom: an orange in a sports stadium

Rutherford knew that the nucleus was tiny but how small was it?  He had done some rough calculations but didn’t have an experiment that would really confirm them so he put the problem to one side.

Over the course of the next few months he worked out a rule for predicting how many alphas he should see scattered at a given angle.  This rule worked well for gold foil.  But later he found that aluminium didn’t always follow it.  Whenever alpha particles were close to rebounding straight back his scattering rule broke down.

Rutherford reckoned that this was because the alpha particles were actually colliding with the aluminium nuclei.  Rutherford knew the angles where his rule worked.  And he knew the angles where his rule didn’t work.  He managed to use this information to calculate the size of the nucleus of an atom of aluminium.

He found that the diameter of the nucleus was less than 1/10 000th of the diameter of the atom.  If the nucleus were the size of an orange then the whole atom would be the size of a sports stadium.

The nucleus is incredibly dense

The other extraordinary idea is how incredibly dense the nucleus is.

Imagine a block of aluminium the size of a large swimming pool.  The aluminium would have a mass of several million kilograms.  But almost all of this mass is contained in the nuclei of the aluminium’s atoms.  (‘Nuclei’ is the plural of ‘nucleus’: one nucleus, two nuclei).  The nucleus is so dense that all the nuclei would fit into the ball at the tip of a ballpoint pen.

Was the nucleus itself made up of smaller parts?

Rutherford had shown that atoms consist of a tiny positive nucleus surrounded by negative electrons.  But is the nucleus itself made up of smaller parts?  If it is, can the parts help to explain radioactivity?

Alpha and beta radiation made an atom move round the periodic table

We’ve already mentioned Frederick Soddy.  He was a young Englishman who had worked for Rutherford in Canada.  In 1904 he had taken a position as lecturer in chemistry at the University of Glasgow.

Over the next ten years Soddy extended Rutherford’s conclusion that radioactive decay caused atoms to change from one element to another.  He found that when an element emitted alpha particles it moved two places back in the periodic table.  When an element emitted a beta particle it moved a place forward.  But it wasn’t at all clear why this happened.

Protons: Rutherford concludes that all nuclei contain hydrogen nuclei

From 1914 to 1918 Rutherford’s research was interrupted by the First World War, where he worked on submarine detection.

When he returned to Manchester University he quickly discovered strange results firing alpha particles into nitrogen gas.  He found that he was detecting what appeared to be nuclei of hydrogen atoms.  It was later found that oxygen was also produced.  He came to two important conclusions.

First, that it’s occasionally possible to change the nucleus of an atom using an alpha particle, creating a nucleus of a different element.

Second, that all nuclei contain hydrogen nuclei.  So that the hydrogen nucleus must be a fundamental particle.  Rutherford named the hydrogen nucleus the ‘proton’ from the Greek word for ‘first’.

Chemistry and physics meet in the nucleus

The Dutch amateur physicist Antonius van den Broek had suggested that an element is defined by the electric charge of its nucleus.

You can combine this idea with Rutherford’s proton so that an aluminium nucleus always has 13 proton charges.  A radium nucleus always has 88 proton charges and a hydrogen nucleus always has 1 proton charge.

Protons alone weren't enough to make a nucleus

But there was a problem.  The protons didn’t account for all the mass of the nucleus.

Rutherford suggested that a (positive) proton could be combined with a (negative) electron to form a (neutral) ‘neutron’ (this isn't what actually happens).

Soddy had already noticed that elements could have atoms of different masses.  We’ve already seen that these were called ‘isotopes’ of the element.  Perhaps different isotopes of the same element would have the same number of protons but different numbers of neutrons in their nucleus.

Neutrons: Another prediction of something that had never been seen

Rutherford predicted the existence of neutrons in 1920.  But he couldn’t work out an experiment to detect them.

He knew that neutrons would be able to penetrate metals easily.  So they would pass through any measuring equipment and be difficult to detect.  This was because the neutrons had no electric charge.  Nuclei have a positive charge.  So a nucleus can deflect a proton but not a neutron.

The Curie family strikes again

Now Pierre and Marie Curie had a daughter called Irene.  She and her husband Frederick were both physicists in Paris.

In 1932 they had been investigating a very penetrating new ray.  The ray was originally discovered by two Germans, Bothe and Becker.  Alpha particles were directed at a sample of metal called beryllium.  The beryllium gave off this new ray.  If the ray hit a sample of paraffin wax then protons were knocked out at great speed.

The Joliot-Curies thought the new ray was possibly a type of very high energy gamma radiation.

The Joliot-Curies see gamma radiation, Chadwick sees neutrons

Rutherford found out about this experiment through his colleague James Chadwick.  They were convinced the ray was a stream of neutrons.

This is what Chadwick thought was happening.

Beryllium is a metal with a very small nucleus.  Every so often an alpha particle hit the nucleus and knocked out a neutron.  The neutron flew into the paraffin wax.  Paraffin wax is a hydrocarbon and has lots of hydrogen atoms.  The nucleus of a hydrogen atom is just a single proton.  If the neutron happened to hit a proton dead on then the neutron transferred almost all its energy to the proton.  So the proton flew out of the paraffin wax at great speed.

Chadwick repeated the experiment with different gases instead of paraffin wax.  He measured the speed of the charged particles produced and calculated a mass for the neutron.  He found that the mass of a neutron is fractionally bigger than a proton.  For his work Chadwick was awarded the Nobel prize in 1921.

Quantum theory saves Rutherford's unstable atom

Remember that the Japanese physicist Nagaoka had suggested that electrons orbited a central nucleus before Rutherford did.  The problem with both models was that orbiting electrons should radiate energy and quickly crash into the nucleus.

The solution to the problem was proposed by the Danish physicist Niels Bohr in 1913.  He said that there were only certain distances where the orbits were stable.  Electrons were only found in those orbits.  This was the beginning of ‘quantum’ theory and is unfortunately beyond the scope of these lessons.

At last we arrive at a school view of the atom

So finally we have arrived at a model of the atom and the nucleus that is at about the level of introductory high school radioactivity.

Each element is made of very small particles called atoms.  The atom has a tiny incredibly dense, positive nucleus at its centre surrounded by whizzing negative electrons.  The nucleus consists of protons and neutrons.

The number of protons defines what kind of element you have.  The isotope of that element depends on the number of neutrons.

In the next lesson we’ll find out more about alpha, beta and gamma radiation and how they affect the nucleus.

Back to Summary of Radioactivity and Atomic Physics Explained