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Lesson 8: Atoms 2: Electrons

Introduction

Are atoms as small as you can go or are atoms themselves made up of smaller parts?

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In this lesson we’ll see how scientists during the late 19th and early 20th century developed ideas about electrons being part of an atom.  And in lesson 9 we’ll look at how scientists came to the conclusion that atoms must be mostly empty space with a tiny nucleus at their centre.

Science was only a small part of the Enlightenment

Remember we saw how science really took off in the ‘Enlightenment’ of the mid 1700s, where Aristotle’s ideas were increasingly abandoned.  Also being abandoned were the ideas that a small number of very rich families should always be in power.

People wanted to control their own lives.  So in 1776 Thomas Jefferson drafted the United States Declaration of Independence.  And in 1789 The French Revolution saw ordinary citizens violently seizing power from the French King, Louis XVI and his nobles.

You needed to be rich to be a scientists so many scientists were also nobles, like Antoine Lavoisier, the father of chemistry.  He was tried as a traitor by a revolutionary court, convicted and executed by guillotine all on the same day.  The judge sentencing him said ‘The Republic [meaning France] has no need of geniuses’.

The industrial revolution meant science could also produce wealth

At about this time, the 1780s, the ‘Industrial Revolution’ was beginning to gather pace in Britain.  Wealth stopped being measured by how much land you had but more by what you could make and how much you could sell.

Looms to weave cloth started being driven by water wheels rather than by hand.  Eventually James Watt’s improvements to the coal-powered steam engine replaced water power.  Food, clothes, pots, pans and everything else people wanted to buy could be made cheaper and quicker by steam machines.

There was plenty of money around to invest.  A few people got very rich.  Many got better off.  But most worked long and hard for little pay.

Scientific progress captured the public imagination

The industrial revolution went hand in hand with great public enthusiasm for science and technology.  So changes in science happened against a background of rapid change in the way people lived, worked and thought.

Science was still mostly done my wealthy individuals

The way science was done was also very different from today.  Many scientists were wealthy amateurs with a wide range of interests.

Scientists would discuss their research at places like the Royal Institution and there were a number of learned journals.  But research grants, narrow specialism and peer-reviewed work did not become widespread until the 1950s.

The invention of the battery made it easier to study electricity

In 1800 the Italian scientist Alessandro Volta had invented the first electric battery.  This meant scientists could see the effect of electric currents without having to use static electricity from rubbing two materials together.  No one really had a clue what electricity was.  Was it a bit like light? Or a particle? Or something completely different?

There was no really good core theory to guide research so at first scientists spent a lot of time just trying out new stuff.

Could you still get sparks if there was no air?

For example you could get sparks by winding two coils of wire over each other and then making and breaking a circuit in the shorter coil.

Since coils act as electromagnets you can use one of the coils to automatically make and break the circuit very rapidly.  The best models of this kind of coil were designed by Heinrich Ruhmkorrf, a German instrument maker who set up in Paris in the 1850s.

The important thing is that scientists had ready access to supplies of several thousand volts.

‘Electricity’ seemed to be able to pass through air as sparks, but did you need the air?  No one really knew because they didn’t have the skills to blow glass into the right shape or create a really good vacuum.  The German glassblower, Heinrich Geissler, created the first suitable tube and hand pump around 1857.

He found that some liquids glowed in his tubes and by using different gases he could get different colours.  By the 1880s a huge variety of Geissler tubes were mass produced and people bought them because they were amusing and pretty.

William Crookes gets a better vacuum

Let’s just recall what we saw when most of the air was sucked out by Geissler.  There is a glow which fills most of the tube.  This seemed fair enough.  There must have been some air left in and for some reason it glowed with a high voltage across it.

One of the many scientists interested in Geissler tubes was the Englishman, William Crookes.  He invented an even better vacuum pump than Geissler and managed to reduce the pressure even more.

This time there was no glow throughout the gas.  The inside of the tube was just dark.  But the glass itself at one end of the tube glowed green.  There seemed to be an invisible ray that could travel through the empty space between the few air atoms that remained.  When it hit the end of the tube then for some reason it made the glass glow.

But did this strange ray come from the negative end (the ‘cathode’) or the positive end (the ‘anode’)?

If the anode was moved ‘round the corner’ then it was clear that the green glow was opposite the negative electrode.  Since whatever was causing the green glow seemed to be coming from the cathode they were eventually named ‘cathode rays’.

What were these cathode rays?

But what were cathode rays?  A kind of light?  Or maybe normal atoms that something had happened to? Or something else?

Crookes placed a metal shape called a Maltese cross in the path of the cathode rays.  The edges of the shadow were sharp, which suggested that the cathode rays travel in straight lines.  Light casts shadows with sharp edges.  Perhaps cathode rays were a type of light?

Crookes made a very lightweight wheel to see if the cathode rays caused it to turn.  This setup is called the Crookes Railway.  The wheel moved away from the cathode.  Now ordinary light won’t make a wheel turn in this setup so it looked like cathode rays might be some sort of tiny particle.  Perhaps when these particles collided with the wheel they caused it to turn?

The German physicist Julius Plucker had already shown that cathode rays could be deflected by a magnet.  The magnet didn't really ‘attract’ or ‘repel’ the cathode rays but seems to move them at right-angles to it.

You can also bend cathode rays using an electromagnet.  If the electromagnet is set up as a pair of Helmholtz coils then you can use a fine-beam tube to calculate the charge to mass ratio of the cathode ray particles.  This method coudn't be used at the time.

Back in 1820 Andre Ampere had greatly added to ideas about how magnets affected electricity.  Ampere’s work led Crookes to conclude that cathode rays were indeed negatively charged particles. Light can’t be deflected by a magnet.

His working assumption was that the particles were normal atoms which had become charged by the high voltage.

A big setback: cathode rays didn't bend with an electric field

If cathode rays were negatively charged particles then they should be repelled from negative charges and attracted to positive ones.

Many attempts were made by the English physicist Joseph John ‘JJ’ Thomson but he couldn't get the cathode rays to behave like this.  This was a big problem for the scientists who thought that cathode rays were negatively charged particles.

In fact, to start with there was intense rivalry between British and German scientists about what cathode rays were.  British scientists saw them as particles. German scientists saw them as a type of ultraviolet light.

To try and understand the importance of this setback it's worthwhile knowing something about the philosophy of science.

Perrin catches cathode rays

Cathode rays made a wheel turn and were deflected by a magnet, which pointed to their being fast moving charged particles.  The lack of deflection by charged plates was a real problem but maybe that could be put to one side and another experiment found.

In 1895 the French scientist Jean Perrin presented a paper at the Paris Academy of Sciences showing a vital piece of new evidence.  The key idea is to ‘catch’ the cathode rays and show that they have an electric charge.

Perrin used a ‘gold-leaf electroscope’.  When the gold leaves have the same electric charge they repel and drift apart.  The electroscope deflected showing that it was charged.  Perrin found that the charge was negative.

So cathode rays were negatively charged particles travelling through empty space.  The green glow presumably came from these negatively charged particles doing something to the air or glass atoms.  But what exactly were these negatively charged particles?  Were they normal atoms with an electric charge or something else?

Thomson uses an even better vacuum

After Perrin’s experiment Thomson was even more determined to show that cathode rays could be deflected by electric charges.

He thought carefully about his apparatus and thought that perhaps stray air atoms were reducing the effective charge on the plates.  Lower charge would mean less deflection.

He needed an even better vacuum, in other words even lower air pressure.  He managed to reduce the pressure further than anyone had managed before and this time he did see the cathode rays bending as they passed between two charged plates.

Thomson finds the charge to mass ratio of a cathode ray particle

Thomson wanted to know two things.  1: the mass of the particles.  And 2: how big their electric charge was.

Unfortunately there was no obvious experiment that he could do to find either the mass or the charge by itself.  So he devised an experiment to measure the ratio of electric charge to mass.

He made a magnetic field bend the cathode rays in one direction and at the same time used an electric field to bend them in the other.  By adjusting the strength of the two fields so they exactly cancelled out and the cathode rays went in a straight line he was able to calculate the ratio of charge to mass for one of the cathode ray particles.

Thomson's bold idea about Stoney's electrons

Thomson concluded that his cathode ray particles either had a huge electric charge or a very tiny mass.  Thomson reckoned that the weight of evidence was for a particle of unusually tiny mass and a normal-sized charge.

He repeated his charge/mass experiments using cathodes made from all sorts of different metals.  He found that the answer was always the same so it obviously didn't depend on what type of atoms the cathode was made from.

His bold conjecture was this: These particles were a completely new sort of matter (stuff).  Not only that but they were PART of all atoms.  Thomson called them corpuscles, which is an old word for anything very tiny.

Atoms themselves were made of smaller parts!

In 1874 the Irish scientist George Johnston Stoney had suggested that electricity came in tiny quantities that he called ‘electrons’.  Stoney’s conclusion came from studying how electricity causes chemical reactions in liquids.  This is called ‘electrolysis’.

It wasn’t really clear what electricity was and he certainly didn’t suggest that electrons were actual ‘things’.

It was Thomson who really pushed the idea that there were these new particles that were actually part of atoms.  Thomson didn’t really ‘discover’ the electron as you might ‘discover’ a new planet.  He ‘discovered’ it in the sense that he used his creativity to imagine the existence of something completely new and then stuck to his guns.

Everything that involved electricity, whether it was cathode rays or chemical reactions, could be explained by the movement of electrons.

Thomson's student Townsend shows electrons have a normal-sized charge

But there was still a problem.  Thomson had only found the RATIO of electric charge to mass.  He hadn’t measured either quantity by itself.  He really wanted to show that electrons had a tiny mass because that would be compelling evidence that they were smaller than atoms.

He couldn't think of any workable experiment to measure the mass directly but if he could find the charge and he knew the ratio of charge to mass (which is what he'd just worked out) then that would give him the mass indirectly.

Thomson set one of his research students, John Townsend, the task of trying to find the electric charge carried by an electron.

Townsend used a method involving very fine clouds of water and came up with a value of about half what is accepted today.  The problem was that the water droplets kept evaporating so Thomson knew his results were probably inaccurate.

However the result confirmed that the electron had a ‘normal’ sized charge and a very tiny mass.  So from then on it became clear that electrons were actually much smaller than atoms.

Robert Millikan's oil drop experiment

The American physicist Robert Millikan finally pinned down a more precise value for the charge on the electron using tiny droplets of oil.  Much of the work was done by his assistant Harvey Fletcher but it was Millikan who was awarded a Nobel Prize in 1923.

The advantage of using oil was that the droplets didn’t evaporate as much as water.  They had to be lit with a very bright light and then observed through a microscope to be seen.

Millikan had to pick a single drop to observe.  As the oil travelled through the nozzle each droplet could collect a few extra electrons from nearby atoms.  In relative size, an electron is to an oil drop as a grain of sand is to the whole world.  This gave the droplet a very, very tiny negative charge.

Millikan used a radioactive source to randomly change the number of electrons during the experiment.

The single oil drop chosen drifts down very, very slowly at a constant speed.  The drop is so tiny that it doesn’t need to be going very fast before air resistance balances its weight.

Millikan used a voltage source to make the top plate of the chamber positive.  The oil drop is negative so it was attracted to the top plate.

The drop almost immediately reaches a constant speed upwards through the air.  Millikan could turn the voltage on and off and watch the oil drop drift up and down.

Millikan timed the drop as it fell.  Then he timed it as it rose.  He then did some complicated calculations to find the electric charge caused by the extra electrons on the oil drop.

Then he used the radioactive source to change the number of electrons and measure the charge again.  Remember Millkan wanted to find the charge on a single electron.

You can't have a drop with three and a half electrons on it.

For each reading he knew the charge on the oil drop.  But he didn’t know how many electrons caused that charge.

Say an electron had a charge of 2 units.  2 electrons would give the oil drop a charge of 4 units.  3 electrons would give it a charge of 6 units and so on.  It would be impossible to have a charge of 7 units because that would mean having half an electron.

The data from Millikan’s experiment consisted of lots of charges.  He had to work out what number would give a whole number of electrons for each charge.

The charges that Millikan had to work with were really messy numbers.

Most of Millikan's drops weren't right for the experiment

He studied 175 drops but most were too big or too small for the measurements to be reliable.  His final 1913 paper includes measurements on only 58 drops.

Millikan had to make up some ‘fiddle factors’ in his calculations because no one really knew how drops that small really fell through air.

The fiddle factors were fair enough but some people say he should have published the figures for all 175 drops.  He could then have explained why 117 should have been discarded.

In any case it was an extraordinary achievement.  The technique is incredibly fiddly and it took two years to get the results.

Thomson's plum pudding model of the atom

So we’ve seen how it took about half a century for scientists to really understand that atoms were made up of smaller parts.  During this time there was no real ‘core’ theory.  Scientists had to create one by playing around with differerent ideas and keeping an open mind.

Atoms were made up of electrons.  Electrons are negative but atoms are neutral.  So atoms must have a lot of positive charge as well.

Thomson imagined that the negative electrons whizzed around in a cloud of positive charge.  This was called the plum pudding model.  In the next lesson we’ll see how the plum pudding model of the atom couldn’t explain some strange results.

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