Shedding Light on Atoms
Episode 5:
Protons, Neutrons, and Electrons
The Shedding Light on Atoms series gives students the perfect introduction to the world of atoms. Using
amazing demonstrations and animations we take students on a journey of discovery to explain not just
what we know about atoms, but also how we know what we know about atoms!
In Episode 5, Protons, Neutrons and Electrons, we take a look at the three types of particles that make up
atoms and describe how they are arranged within atoms. Using live demonstrations and outstanding
animations, we then look back at the experiments that led to the discovery of these three particles.
Contents:
Part A: Introduction: We look back briefly at how the Periodic Table was developed and introduce the
fact that atoms are made of three types of particles called protons, neutrons and electrons.
Part B: Atomic Structure: We show that atoms are made of a central nucleus (which is composed of
positively charged protons and neutral neutrons), which is surrounded by negatively charged electrons.
We explain that it is the number of protons in an atom’s nucleus which uniquely defines each atom. We
also take a look at what isotopes are.
Part C: The Discovery of Electrons: In the late 1890s, J.J. Thomson discovered electrons. We explain
how he did this by demonstrating some of the equipment he used in his lab. We also take a close look at
static electricity.
Part D: Alpha Particles: The discovery of radioactivity in the 1890s opened up a new window into the
study of atoms and their structure.
Part E: Rutherford’s Gold-Foil Experiment: In 1908, Ernest Rutherford used high-speed alpha
particles (produced by radioactive radium) to probe into the structure of the atom and his experimental
results changed our whole idea of what atoms were like. He discovered that atoms had a positively
charged nucleus which was surrounded by orbiting electrons.
Part F: The Discovery of Protons and Neutrons: We take a look at how, after the discovery of the
nucleus, protons and neutrons were discovered.
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Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons.
The images shown below are screen grabs from the program and the text is more or less the program’s
script. We don’t think it’s as good as watching the actual program, but at least you’ll get an idea.
Part A: Introduction
This is Dimitri Mendeleev. In the late 1860s and early 1870s,
Mendeleev devised the periodic table of the elements.
His Table organised the then known elements into groups, according to
how they chemically react. When the elements in Group 1 reacted with
oxygen for example they would produce oxides with a ratio of two
atoms for every one oxygen atom.
The metals also reacted with water in a very similar way.
Using the patterns in the way that the elements reacted with oxygen… and with
hydrogen, Mendeleev left gaps in his table, here and here for example, and
predicted that these gaps would eventually be filled with elements that up until
then had not been discovered. Gallium was discovered 4 years later and
Germanium 15 years later.
He was also able to predict some of the properties of the unknown elements, and
his predictions were quite accurate.
However, the table wasn’t perfect. For example, a Group 8 had to be included for
the leftover elements that didn’t quite seem to follow regular patterns. But that doesn’t mean it was bad
science.
Science is all about trying to make sense of nature and every scientific advance gets us closer to the truth,
and Mendeleev’s Periodic Table provided a brilliant platform from which further discoveries could be
made.
Our modern Periodic Table has sorted out the problems with the original Periodic Table, and each
element now has a specific place within it. However, this arrangement of all the elements was only made
possible with the discovery that atoms themselves were made up of even smaller particles called protons,
neutrons and electrons and it’s these smaller particles that we’ll be looking at in this episode.
Part B: Atomic Structure
So, before we look at how protons, neutrons and electrons were discovered, let’s have a look at what they
are and how they’re arranged inside atoms.
I should warn you though, that when it comes to atomic structure, you really need to know about 10 or so
different things before any of it really makes sense, but you can only learn one thing at a time. Once you
do learn the basics though, everything else is just examples and finer details.
You’ll also need a good imagination: atoms are too small to be
seen so they have to be imagined. Protons, neutrons and
electrons are even smaller!
So let’s begin.
We now know that all atoms are made of what’s called a
nucleus, made up of two types of particles called protons and
neutrons, which is surrounded by much smaller particles called
electrons that move around the nucleus at enormous speeds.
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Protons and neutrons are often collectively called nucleons since they make up the nucleus.
The plural of the word nucleus is either nucleuses or nuclei.
Even though there are more than a hundred elements, the atoms of every single one of them are made up
of just three types of particles, which are often called subatomic particles. Different atoms have different
numbers of these three subatomic particles.
This atom has 2 protons and 2 neutrons in its nucleus which
means it has 4 nucleons all together, and it has 2 electrons moving
around the nucleus. It is in fact a Helium atom or rather a
representation of a Helium atom. We can’t actually see any of
these particles and in this animation they’re not drawn to scale.
The protons have a positive electrical charge and the electrons
have a negative electrical charge. The neutrons are neutral.
But what is electrical charge? Well, it’s a little like magnetism.
Magnets have a North pole and a South Pole. Two North Poles repel one
another, Two South Poles repel one another, while a north pole and a south pole
attract one another. Some magnets can be very strong and provide a large force,
while others are fairly weak.
Electrical charge is similar but is described as being either positive or negative.
The positively charged protons attract the negatively charged electrons and stop them from flying away.
In that sense, it’s a little like the gravitational force of attraction between the sun and the Earth which
keeps us orbiting the sun. The electrons don’t fall into the nucleus because they’ve got energy and they’re
moving around the nucleus.
Gravity and the Strong Nuclear Force
Though the strength of magnets
can vary, the size of the charge on are two of the four “fundamental forces
of nature”. The third, Electromagnetism,
all protons is exactly the same,
is made up of magnetism and
and the size of the charge on all
electrostatic forces, which appear to be
different, but are in fact the same force
electrons is exactly the same.
when you get down to the atomic level.
Also, the size of the positive charge on each proton is equal to
In everyday situations we talk about
the size of the negative charge on each electron, despite the
magnetic forces and electrostatic forces.
fact that protons are much, much bigger than electrons.
The “weak interaction” is the fourth
force of nature and, like the strong
In fact protons and neutrons are approximately equal in mass,
nuclear force, acts only at the atomic
but they’re nearly 2000 times more massive than electrons.
scale.
(One proton has a mass equal to 1836 electrons, while one
neutron has a mass equal to 1839 electrons.)
So why don’t the protons in the nucleus, which are all positively
charged, repel each other and fly apart? Well, while most of us are
familiar with the force of gravity, and with the forces associated with
magnets and electrostatic charges, there is another force of nature with
the somewhat unusual name of the “strong nuclear force”. This force
acts between protons and neutrons within the nucleus and keeps
them bound within the nucleus. We don’t really see it in everyday
life, but without it, there wouldn’t be any atoms.
In a pure sample of any given element, the number of positively
charged protons, in this case three, equals the number of
negatively charged electrons, so that, overall, the charge on the
atom is zero since the positive charges on the protons exactly
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balance the negative charges on the electrons.
However, atoms can often lose or gain electrons when they chemically react. We’ll have a look at how
and when this happens in a later episode.
In previous episodes, we were illustrating atoms as solid spheres,
but it turns out that they’re not really solid at all. They’re mostly
empty space. However, though atoms don’t have a clear outer
boundary, the electrons are spinning around the nucleus so fast,
that the atom effectively becomes, more or less, like a sphere.
It’s a bit like the blades of a fan.
When the fan blades are still, they act like 3 distinct blades. When
they're moving though, it’s almost as if they become one solid disc-like unit.
Even hydrogen atoms, which have only one electron are spherical.
However, when they chemically join onto each other, there’s often a little bit of overlap.
Now atoms are defined by how many protons they have in their
nucleus. By definition all hydrogen atoms have 1 proton in their
nucleus, all Helium atoms have two protons in their nucleus, all
lithium atoms have three, all beryllium atoms four and so on.
The number of protons in the nucleus is referred to as the
atomic number and is given the symbol Z (zed, or zee, if you
prefer). Z stands for zahl which is the German word for number.
Mendeleev based his Periodic Table on the relative atomic
weights of the elements, that is, how heavy each atom is compared to hydrogen.
However, our modern Periodic table organises the elements according to atomic number, which I’ll say
again, is the number of protons in each atom. This produces a much better set of patterns in the physical
and chemical properties of the elements.
Carbon has an atomic number of 6, so every carbon atom, in the
universe, has 6 protons in its nucleus.
Here we’ve drawn all the electrons the same distance from the
nucleus. In fact electrons occupy only certain energy levels,
which are called electron shells, and only a specific number of
electrons can occupy each shell. A carbon atoms has 2 electrons
in its first, or inner, shell and 4 electrons in its second, or outer,
shell. It’s much better to represent atoms with shells because
the way the electrons are arranged in these shells plays a major
part in the way that atoms chemically react. We’ll be taking a
closer look at electron shells in our next episode.
If you give it another proton and another electron, you don’t
have a carbon atom with an extra proton and an extra electron,
you actually have a nitrogen atom, with an atomic number of 7.
Now, getting back to the nucleus, though all carbon atoms have
the same number of protons, 6, they don’t all have the same number of neutrons.
Most, 99% in fact, have 6 protons and 6 neutrons in their nucleus, resulting in a total number of 12
nucleons. About 1% though, are made of 6 protons and 7 neutrons, in other words 13 nucleons. A tiny
number, 1 in every trillion or so, have 6 protons and 8 neutrons in their nucleus, a total of 14 nucleons.
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The total number of protons
and neutrons, or nucleons, in
an atom is called the atom’s
mass number, and, somewhat
confusingly in English, is
given the symbol A.
All these different forms of
the same type of atom are
called isotopes. Here we’re
showing three isotopes of
carbon. They all chemically
react the same way, because
they’re all carbon atoms
(with 6 protons and 6 electrons). The number of neutrons does not affect the way that they chemically
react.
To express each isotope, scientists often write the symbol for the element and next to it place the atom’s
mass number as a superscript and the atom’s atomic number as a subscript: 𝐴𝑍𝑋
This is called atomic notation.
The three isotopes of carbon can therefore be written as 126𝐶 , 136𝐶 and 146𝐶 .
However, since all carbon atoms have 6 protons, we can also refer to each isotope of carbon as either
carbon-12, carbon-13 and carbon-14, or, even more simply as C-12, C-13 and C-14.
Another common practise, especially on web pages, is writing the atomic notation without the atomic
number, as in 12C, 13C and 14C. The numbers are simply superscripts.
Now I should just mention, even though it’s probably already obvious, that the number of neutrons in a
nucleus is equal to the atom’s mass number minus the atom’s atomic number.
Let’s look at one more example of using
atomic notation where the atomic number,
mass number, and the element’s atomic
symbol are shown. 49𝐵𝑒 is a particular
isotope of beryllium. Its atomic number is
4, which means it has 4 protons, which
we’ve coloured blue. It also has 4 electrons.
Its mass number is 9, which means that it
has 9 protons and neutrons in total. Four of
them as I said are protons and five of them
(9-4) are neutrons. Since all beryllium
atoms by definition have 4 protons, this isotope can also be written as Beryllium-9 or Be-9. It’s also
sometimes written as 9Be (on webpages and elsewhere because it’s a little easier to format with just the
one superscript).
As I said, isotopes of any given element all chemically react the same way. A typical sample of Lithium is
made of 92.5% Lithium-7 atoms (each with 3 protons and 4 neutrons in their nucleus) and 7.5% Lithium6 atoms, but both Lithium-7 and Lithium-6 react with water in the same way, because it’s the protons and
electrons in the atom that determine the way something chemically reacts. However, the number of
neutrons can affect whether a nucleus is stable or radioactive. Both isotopes of Lithium which occur
naturally are stable. C-12 and C-13 are stable, but C-14 is unstable, that is, it’s radioactive. We’ll take a
brief look at radioactivity a little later in the program.
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Many Periodic tables include what’s called the element’s
atomic weight.
This number, for example 6.94 for Lithium, reflects the fact
that most elements are made of different isotopes. It’s
basically an average (of the relative atomic weights of all the
different isotopes).
Rounding the atomic weight up or down to the nearest
whole number, will usually tell you the mass number of the
element’s most common isotope.
So how were protons, neutrons and electrons discovered?
Well, as was the case with the discovery of atoms, it wasn’t
with some machine that could suddenly see them.
Just as the discovery of atoms occurred after many many
experiments and observations, so too the realization that
atoms were made of protons, electrons, and neutrons came
only after scientists had conducted many many experiments
and had made lots and lots of observations.
Based on their experiments, scientists slowly built up a
picture of what atoms were like. So let’s have a look at some
of the experiments.
Part C: The Discovery of Electrons
If I rub this balloon on my shirt, the balloon becomes electrically charged and it can
attract small pieces of paper.
I can also change the direction of this stream of water. Ancient Greek philosopher
Thales first reported these mysterious effects and forces in about 600BC. In his
experiments, he used a type of fossilized tree resin called amber, which in Greek is
called electron. (ηλέκτρον)
More than 2000 years later, in about 1600AD, English scientist William Gilbert started using the word
“electric” to describe these forces.
In the 1700s French scientist Charles du Fay discovered that there were two types of electric charges
which we now call positive and negative.
Two objects that have the same charge repel one another (both of
these balloons are negatively charged), but two objects that have
opposite charges attract one another. The balloon has a negative
charge but my jeans have a positive charge.
By the 1890s, batteries, generators, light globes, electric
motors, the telegraph, the telephone and lots of other things
that used electricity had been invented.
However, no-one knew what electricity actually was until
1897, when English scientist J.J. Thomson discovered
electrons.
He was experimenting with the flow of electricity in glass
tubes which had had all their air sucked out.
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He used an induction coil to generate a voltage of many thousands of
volts. When this really high voltage was applied between what was
called the cathode and what was called the anode, rays of …
something, we now know that they were electrons, were emitted
from the cathode and moved towards the anode. The rays were
therefore originally called cathode rays. Though most of the cathode
rays slammed into the anode, a small slit allowed some of them to
travel through it towards the angled fluorescent screen which lit up when the cathode rays hit it.
Any part of the screen that is struck by an electron gives off a little light in a process called scintillation,
which is a type of fluorescence.
Now it didn’t matter what metal he used for his cathode, the rays were always the same. Thomson then
suggested that these rays were made of tiny particles which were coming from inside the atom and which
were in fact part of every atom.
Thomson’s discoveries would
Thomson named these particles electrons. Atoms, which were thought
not have been possible if highto be the smallest units of matter, were themselves made up of even
quality pumps had not been
invented. To study electrons
smaller particles.
(and, as we’ll see, other
Placing a magnet near the moving electrons deflected their path, as did
subatomic particles) properly,
electrically charged plates, and Thomson worked out that electrons are
you need a vacuum tube.
negatively charged. By measuring how much the electrons deflected,
Every little invention and
discovery leads to more
Thomson was able to show that the mass of an electron is tiny
inventions and discoveries.
compared even to the mass of a hydrogen atom, which is the lightest of
all atoms.
Further experimentation eventually revealed that the mass of an electron is about 1/2000 of the mass of a
proton.
1
(the mass of an electron is about
of the mass of a proton)
1836
Now since electrons were negatively charged, Thomson reasoned that the rest of the atom had to have a
positive charge, so that, overall, the atom would be neutral.
He proposed what was later called Thomson’s Plum Pudding Model of the Atom. A “model” in this sense
is an idea or a representation of something.
Atoms were composed, Thomson said, of a stationary positive
blob or something, he didn’t know what, which contained
negative electrons scattered throughout it, a little like a
traditional English pudding with raisins mixed in, the raisins
being the electrons. It could just as easily have been called the
fruit cake model of the atom, that’s what I’m actually holding,
it’s similar to a pudding, or the date scone model of the atom,
or the blueberry muffin model of the atom.
The positive charge of the blob was exactly equal to the overall negative charge of the electrons, so atoms
are normally neutral, Thomson said.
However the positive blob was stationary while the electrons moved freely around the atom.
Thomson’s discovery of electrons helped explain current electricity; electricity was the flow of these
electrons from atom to atom in a conductor when a voltage is applied. The positive part of the atom
always stayed where it was but the electrons, being lighter, could move from atom to atom (although as
we’ve seen, if you can generate tens of thousands of volts, electrons can be forced to jump off a metal
plate and move towards another one even if there’s nothing between the two plates).
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It also helped explain static electricity. Normally the amount
of positive charge on something is exactly equal to the
amount of negative charge on it. When some things are
rubbed together they can sometimes either gain electrons
which gives them an overall negative charge, or they can
lose electrons which gives them an overall positive charge.
This produces a force of attraction between them.
However, Thomson’s Plum Pudding model of the atom did
not really accurately represent what atoms are like. More discoveries would soon follow.
Part D: Alpha Particles
In 1895, French scientist Henri Becquerel discovered that certain elements gave off a mysterious form of
radiation that could expose photographic film. Before the invention of digital cameras, film was used to
record images.
When film was exposed to ordinary light, typically in a camera, it darkened and formed an image.
However, though Becquerel’s newly discovered rays were invisible to the eye, they could still expose
film, even if the film was covered in opaque black paper, which we’re not showing here. The rays, which
were produced by uranium, could obviously pass through the paper.
Of course the rays radiated out in every direction, but we’re only
showing the ones moving towards the film.
Becquerel then placed a metallic cross between the uranium and
some covered film, remember, the rays couldn’t be seen, and a
few days later developed this image, where the outline of the
cross was visible. The rays could penetrate the black paper but not
the metal of the cross. So what were these rays?
We now know that many elements with large atoms, like uranium, thorium
and radium emit what are called alpha particles from their nucleuses.
A Uranium-238 atom for example
(here we’re not showing any of its
electrons) is one of the heaviest
types of naturally occurring atoms
on earth. It has 92 protons and 146 neutrons in its nucleus, so a
combined total of 238 nucleons. But this many protons and
neutrons is unstable and so the nucleus ejects, at really high
speeds, what is called an alpha particle,
a group of 2 protons and 2 neutrons.
It’s never 2 protons and 3 neutrons, or
1 proton and 2 neutrons, or any other
combination; it’s always 2 protons and
2 neutrons.
After emitting the alpha particle, our
original U-238 atom is no longer a U238 atom. Instead of having 92
protons, it now has only 90, and since
it has lost 2 neutrons as well, it now
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has only 144 neutrons, resulting in a total of 234 nucleons.
So, what kind of atom has 90 protons in its nucleus. Looking at the periodic table, we can see that it’s a
thorium atom. Our Uranium-238 atom has turned into a thorium atom, specifically, a thorium-234 atom
by emitting the alpha particle. Since the alpha particle has 2 protons, it is in fact identical to the nucleus of
a helium atom, a really fast-moving nucleus of a helium atom.
Since alpha particles come from the nucleus of the uranium atom, alpha particles are a form of nuclear
radiation and uranium is said to be radioactive.
Now we’ve already spent lots of time in previous episodes writing chemical equations. We can now also
write a nuclear equation to express what happens when a U-238 atom gives of an alpha particle:
238
234
4
92𝑈  90𝑇ℎ + 2𝑎𝑙𝑝ℎ𝑎
The alpha particle is given the symbol alpha (α), which is the Greek letter “a”, pretty much the same as an
English “a” but drawn with curvy lines to distinguish it.
The atomic number of the original atom has dropped by two since two protons have been ejected, and the
mass number has dropped by 4, since 4 nucleons overall were ejected (2 protons and 2 neutrons).
In the early 1900s, New Zealand-born Ernest Rutherford set out to investigate the rays that Becquerel had
discovered.
He found that there were in fact three different types of nuclear radiation, which he named alpha, beta and
gamma radiation (after the first three letters of the Greek alphabet), and that they had all played a part in
exposing the photographic film in Becquerel’s experiments.
[Not in the Program: It turns out that the Uranium atoms were giving off alpha-particle radiation and
gamma rays, but the thorium atoms that that the uranium atoms were turning into were giving off betaparticle radiation and gamma rays. The thorium atoms were also slowly turning into different atoms
which were also radioactive!
It further turns out that some isotopes of some elements are alpha emitters, while some isotopes of the
same element are beta emitters. Some elements only have radioactive isotopes, some have both stable and
radioactive isotopes, and some only have stable isotopes (not including artificial isotopes produced in
laboratories or nuclear reactors, most of which are
radioactive).
Radioactivity is a really big topic!]
Right now though we’re only focussing on alpha particles.
In 1898, Polish scientist Marie Curie discovered radium.
Radium was much much more radioactive than uranium,
producing far more alpha particles per second than
uranium did. Research into alpha particles and atomic
structure was therefore made a lot easier.
Rutherford placed the newly discovered element into a metal container that had a small hole in it.
The alpha particles that the radium was producing came streaming out of the hole in a kind of beam,
which Rutherford directed towards the space between two metal plates that were connected to a strong
battery. The alpha particles deflected towards the plate attached to the negative side of the battery, so it
was realized that they were positively charged.
Alpha particles also did not deflect anywhere near as much as electrons did when electrons were fired
past charged metallic plates, and it was calculated that they must have been thousands of times heavier
than electrons. In fact alpha particles were about as heavy as helium atoms. This technique of firstly
measuring the amount of deflection of charged particles as they flew past charged metallic plates or
magnets and then, from those measurements calculating the particle’s mass, had been developed by JJ
Thomson a few years earlier. It’s called mass spectrometry, and, over time, it improved to the point where
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we can now measure the mass of atoms and molecules really accurately. It works on the basic principle
that heavier objects don’t deflect as much as lighter objects if a sideways force acts on them. So, what
were alpha particles?
In 1907, Rutherford placed radium inside a thin glass tube, which itself was
placed inside another glass tube which had no air in it. He allowed the alpha
particles emitted by the radium to collect inside the larger glass tube, and, after a
few days, was able to demonstrate that helium gas had accumulated inside the
tube. Through collisions with the atoms of the glass, the alpha particles had
picked up some electrons to become helium atoms.
We now know, as I said earlier, that alpha particle are in fact
the same as the nucleus of helium atoms, having two protons
and two neutrons just like helium atoms. Alpha particles are
ejected from the nucleuses of certain radioactive elements.
Alpha particles can’t penetrate our skin, but they can be very
harmful to us if they’re emitted by an alpha emitter inside our
bodies, because they can crash into the various molecules in
our cells and cause them to stop chemically reacting properly,
which can lead to the cells dying or turning cancerous. Once they slow down and pick up a few electrons
though, they turn into Helium atoms and they’re harmless.
(having sucked in some helium and talking with a high-pitched voice) Most of the
helium atoms on Earth in fact were originally alpha particles which acquired
electrons to become Helium atoms.
The discovery of protons and neutrons though and the fact that atoms had a nucleus,
only came after Rutherford’s next series of experiments, which were by far his most famous.
[A quick note about mass spectrometry, which is not included in the video. It’s a difficult concept until students
learn about ionization, which is the process by which atoms lose or gain electrons and therefore become charged.
In 1912, Thomson injected neon gas into a small chamber, and, with a high voltage electron gun, blasted the gas
with electrons. The electrons from the electron gun struck the electrons of the neon atoms and knocked one or two
of them out, so the neon atoms ended up being positively charged (since they now had more protons than electrons,
although at that stage, protons had not been discovered). These charged neon atoms could then be accelerated by
charged metallic plates into a tube which was surrounded by electromagnets. This caused the neon atoms to deflect
from their original path. However, Thomson found that the deflection produced two different streams of atoms.
Clearly, there were two types of neon atoms! The heavier type deflected less than the lighter type, and by doing
some fairly complicated mathematics, Thomson showed that they had a relative atomic weight of 20 and 22. These
two isotopes are neon-20 (neon-20 atoms make up about 90% of all neon atoms) and neon-22 (neon-22 atoms make
up about 10% of all neon atoms). We now know neon-21 atoms also exist but make up only about 0.3% of all neon
atoms. Mass spectrometry is now fairly easy to perform and it can accurately determine the exact mass of atoms
and molecules!]
Part E: Rutherford’s Gold-Foil Experiment
In terms of working out what atoms were like, alpha particles proved to be invaluable.
Rutherford realized that because alpha particles could pass through very thin sheets of metal, he could use
them as a kind of high speed bullet to probe into atoms.
J.J. Thomson had proposed that atoms consisted of a positive sphere of something which had electrons
embedded in it.
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Hoping to discover how the
electrons and the
positive…something were
arranged within atoms,
Rutherford began a series of
experiments in 1908 where
he directed a stream of alpha
particles towards a very very
thin layer of gold foil that
was only a few hundred
atoms thick.
To detect the path of the
alpha particles he used a
screen which would light up whenever an alpha particle hit it, very similar to the screen I showed you
earlier that lit up when electrons hit it.
Each flash of light is called a “scintillation”. His equipment was about the size of a pot.
If Thomson’s Plum Pudding model of the atom was accurate and there was a positive sphere with
negative electrons spread evenly throughout it, then the alpha particles
Why did Rutherford use gold?
He actually tried different metals
wouldn’t deflect much.
but found that gold was best
However, Rutherford found that while the vast majority of alpha particles
because it could be flattened into
the thinnest sheets. Gold is the
did pass straight through the gold foil with very little deflection, a small
most malleable of all metals.
proportion were deflected by large amounts, and some even bounced back
in the direction from which they had come.
Rutherford was very surprised and said that it was and I quote “almost as
incredible as if you fired a 15-inch shell at a piece of tissue paper and it came
back and hit you.”
In 1911, Rutherford used the results of his experiments to calculate that a tiny
positively charged nucleus exists at the centre of the atom and that the even tinier
negatively charged electrons swarm around it. Most of the mass of the atom was concentrated in the
nucleus.
The alpha particles could pass through the thin gold foil because of the fact that atoms are mostly empty
space and electrons are very light. However, when the positively charged alpha particles occasionally
passed close to the positively charged nucleus, they were
deflected.
This arrangement of the atom was called Rutherford’s Planetary
Model, because it looked a little like our Solar System, where the
sun is at the centre and the planets orbit around it.
Since only a tiny fraction of alpha particles were deflected,
Rutherford proposed that the nucleus must be tiny compared to
the overall size of the
atom. We now know,
based on similar
experiments but with a
higher accuracy, that the
nucleus has a diameter of
only about 1 100,000th of
www.liacoseducationalmedia.com Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons Page 11 of 16
the diameter of the atom itself.
If this sticker, which has a diameter of 1cm, was the size of the nucleus, the diameter of the whole atom
would be about 1 kilometre (100,000cm).
If I place the 1cm wide sticker, representing the nucleus, onto this flag and then hold the flag up, the
camera has to be a long, long, long long long way away if we want to fit the whole atom in the shot.
And once we’ve zoomed right out, you can’t even see the flag let alone the nucleus.
The diagrams we draw of atoms, are totally not to scale, not even close, but there is no way around it. If
the atom was drawn to scale, the nucleus and electrons would be too small to see.
So, the existence of a positively charged nucleus surrounded by negatively charged electrons was
established, but what was the nucleus made of? How were the nucleuses of, for example, tin atoms
different to the nucleuses of zinc atoms.
Part F: The Discovery of Protons and Neutrons
In 1913, British scientist Henry Moseley, using X-ray tubes
like these ones here, discovered a way of finding the exact
charge in an atom’s nucleus and how many electrons the
atom had.
We’ve already seen how high voltages can produce streams
of electrons. It had been discovered that when high-speed
electrons crash into atoms, X-rays are produced.
Moseley fired electrons at Aluminium (or aluminum if you
prefer) and measured the frequencies of the X-rays that were produced. He then replaced the aluminium
with silicon (which came next on the periodic table), and found that the set of X-rays produced by the
silicon had different frequencies.
He repeated the experiment with most of the elements up to gold. He found that every element produces
X-rays with a unique set of frequencies, and that there was a pattern in the way that the frequencies
changed from element to element.
This allowed him to determine how much
charge each nucleus had. He was able to state
that, “We have here a proof that there is in the
atom a quantity which increases by regular steps
as one passes from one element to the next. This
quantity can only be the charge on the central
positive nucleus.” Henry Moseley, Philosophical
Magazine, Vol. 26, 1913, p1030.
He found that, for example, if the nucleuses of
the lightest atoms, that is hydrogen atoms, have a positive
charge of exactly 1 unit, then the nucleuses of helium atoms
have a positive charge of exactly 2, the nucleuses of lithium
atoms have a charge of exactly 3, and so on.
It then became clear that hydrogen atoms had to have one
electron, which has a charge of negative one, helium atoms
had to have two electrons, lithium atoms three electrons, and
so on.
www.liacoseducationalmedia.com Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons Page 12 of 16
Gold atoms, the heaviest atoms he tested had a nucleus with a positive charge of exactly 79 and they had
79 electrons.
Moseley found that none of the known elements had a charge of 43, 61, 72 or 75 and he predicted that
these elements would eventually be discovered. Hafnium was discovered 10 years later, Rhenium 12
years later, Technetium 23 years later and Promethium 32 years later.
Once again, we can see how the discovery of patterns in nature, can lead to even more discoveries.
As a result of Moseley’s work, the Periodic Table, which had been organised on the basis of each
element’s atomic weight, was modified and the elements were placed in order of atomic number.
We now know that the charges in the nucleus relate to how many protons the nucleus has. A hydrogen
atom has one proton in its nucleus, a helium atom has two protons and so on, but how were protons
discovered?
Well, it wasn’t easy, but it kind of involved playing pool, atomic pool,
using alpha particles as the cue ball.
In 1917, Rutherford fired alpha particles into nitrogen atoms and found that
oxygen and hydrogen atoms were produced. He had managed to split the
nitrogen atoms into two new atoms in the world’s first controlled nuclear
reaction. He reasoned that hydrogen atoms, being the lightest atoms, must have in their nucleus a single
positively charged particle, which he named the proton.
The proton we’re showing here was originally part of the nucleus of the
nitrogen atom but was broken off when the alpha particle struck it. Protons,
Rutherford said, must be the building blocks of the nucleuses of all atoms.
Since the nucleus of every oxygen atom has a charge of positive 8, then it must
have 8 protons in its nucleus.
We now know (although they didn’t at the time) that in
Rutherford’s experiment, the nitrogen atom’s nucleus, made of 7
protons and 7 neutrons, here we’ve drawn them in two neat rows so
that you can see them more easily, was struck by an alpha particle,
made of two protons and two neutrons, and a single proton was
knocked off it leaving behind an oxygen atom that had 8 protons
and 9 neutrons in its nucleus.
We can write a nuclear equation to express what happened:
14
4
17
1
7𝑁 + 2𝑎𝑙𝑝ℎ𝑎  8𝑂 + 1𝐻
However, though the proton had been discovered, the neutron had not been, so there was still a major part
of the atom that was unaccounted for (in 1917 when Rutherford
performed his experiment).
Helium atoms, for example, with two positively charged protons,
were actually four times heavier than hydrogen atoms and Lithium
atoms, with three protons, were seven times heavier than hydrogen
atoms.
Rutherford therefore suggested that there was probably some other
particle inside the nucleus that hadn’t been detected yet, and that it
probably had a mass similar to the mass of the proton.
Remember, most of the mass of the atom is in the nucleus.
The mass of electrons is practically zero compared to the mass of
protons, though they have an equally sized but opposite charge, and
www.liacoseducationalmedia.com Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons Page 13 of 16
neutrons, we now know are uncharged but have a similar mass to protons.
Neutrons, it turns out, were really hard to detect because they’re uncharged. Their lack of charge means
that they don’t get deflected by electric or magnetic fields and they don’t produce flashes of light when
they hit fluorescent screens like electrons and alpha particles do.
Neutrons were finally discovered by English scientist James Chadwick in
1932, more than a decade after protons were discovered, in quite a complicated
experiment that was also a little like atomic pool (or snooker or billiards). It
had been discovered that polonium emits particularly fast alpha particles,
which produced, when they crashed into beryllium atoms, a form of radiation that had a really high
penetrating ability.
We now know that the radiation was actually made of fast-moving neutrons that were being knocked out
of the beryllium nuclei when the alpha particles coming from the polonium crashed into them.
Though the radiation could not be detected directly with any scientific instruments, they knew that it was
being produced because when it was aimed at, for example, hydrogen atoms, whatever it was made of
would very occasionally strike the nuclei of the hydrogen atoms and send electrons and protons flying off
at high speeds, and these could be detected relatively easily.
No-one knew what the radiation was initially, but Chadwick provided an answer. He lined up polonium, a
beryllium target, some paraffin wax which is what most candles are made of (it’s a compound made of
carbon and hydrogen atoms), and finally an electrical device called a valve counter, which could detect
electrically charged particles.
The valve counter was very similar to a Geiger counter, which here is detecting alpha particles coming
from a tiny speck of americium-241, an alpha-emitter found in most types of smoke detectors.
Chadwick basically bombarded the nuclei of the beryllium atoms with alpha particles which knocked out
neutrons that then went crashing into the paraffin wax. Some of the neutrons scored a direct hit on the
nuclei of the hydrogen atoms in the paraffin wax, which remember are just protons. The struck protons
then went flying into the valve counter.
So why did alpha particles knock protons out of
Chadwick first measured the energy of the protons
nitrogen atoms and neutrons out of beryllium atoms?
Why didn’t neutrons get knocked out of nitrogen
and was then able to show that they had accelerated
atoms? Well, I’m not really sure! But Rutherford and
the way they did because they had been struck by
Chadwick (and others) were bombarding lots of
neutral particles that had a similar mass to protons.
different atoms with alpha particles to see what would
In other words, he had established the existence of
come up. It just so happens that the particular
experiments we mention in the video were the ones
neutrons, and the value he found for their mass was
that produced information about protons and neutrons.
out by only about 0.1% of the currently accepted
value.
I did say it was complicated!
Chadwick’s calculations were a little similar to calculating the
mass of a cue ball by studying its effect on the other balls on the
pool table. (Chadwick died in 1974, 6 years after I was born. One
of science’s greatest discoveries was made by a person who lived
at the same time as me!)
So, all three basic components of the atom were now known, and over
time their mass has been measured pretty accurately.
How brilliant were Chadwick, Moseley, Thomson and especially
Rutherford among many others?! They could conceive of, and then
design and build all this technical equipment and they had the
mathematical ability to interpret the results of their experiments and
www.liacoseducationalmedia.com Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons Page 14 of 16
they had the imagination to picture all these subatomic particles that make up atoms. Imagination is just
as important as knowledge.
However, as I said earlier, electrons don’t just circle the nucleus in any old position.
In the 1910s and 1920s scientists figured out that electrons could
occupy only certain fixed energy levels called electron shells, which
are usually represented as circles and only a certain number of
electrons could fit in each shell.
This discovery was the real key to understanding how atoms bond
together to form compounds like, for example, water and salt, so it’s
electron shells that we’ll be looking at in our next episode. See you then.
CREDITS:
 Photos of the elements (gold, germanium, gallium, radium, beryllium,
hafnium and rhenium) © Heinrich Pniok. Used with permission. Visit
http://pse-mendelejew.de/en/ to see originals. Heinrich’s photos of the
elements are arguably the best collection on the internet. See also
http://commons.wikimedia.org/wiki/User:Alchemist-hp?uselang=fr.
 The Clip Art used in this program was created by Microsoft.
 Amber hg.jpg (http://commons.wikimedia.org/wiki/File:Amber_hg.jpg) by Hannes
Grobe is licensed under CC-BY-SA-2.5.
 Djurgårdsbrunnskanalen 1905a.jpg
(http://commons.wikimedia.org/wiki/File:Djurg%C3%A5rdsbrunnskanalen_1905a.jpg)







by Holger.Ellgaard is licensed under CC BY-SA 3.0.
Old field camera.jpg (http://commons.wikimedia.org/wiki/File:Old_field_camera.jpg)
by Arne Nordmann (norro), Germany, is licensed under CC BY-SA 3.0.
Potassium water 20.theora.ogv
(http://commons.wikimedia.org/wiki/File:Potassium_water_20.theora.ogv) by Ozone
aurora / Philip Evans is licensed under CC BY-SA 3.0.
3D anamation of the Rutherford atom.ogv
(http://en.wikipedia.org/wiki/File:3D_anamation_of_the_Rutherford_atom.ogv) by
Damek (David Marin) is licenced under CC BY-SA 3.0.
Balloons and Static Electricity (simulation)
(http://phet.colorado.edu/en/simulation/balloons) by PhET, University of Colorado is
licensed under a Creative Commons Attribution 3.0 United States License.
Moseley x-ray tube.jpg
(http://upload.wikimedia.org/wikipedia/commons/f/f7/Moseley_x-ray_tube.jpg) by
Anders Sandberg is licensed under Creative Commons Attribution 2.0 Generic.
Promethium and Polonium images by http://images-of-elements.com/ is licensed under
Creative Commons Attribution 3.0 Unported License.
Special Thanks to the University of Melbourne School of Physics.
Written, directed, and presented by Spiro Liacos
(who isn’t really much of a surfer or a pool player)
Produced by Liacos Educational Media
www.liacoseducationalmedia.com Shedding Light on Atoms Episode 5: Protons, Neutrons and Electrons Page 15 of 16
SUMMARY: The Structure of the Atom: Protons, Neutrons and Electrons
So how many facts have we covered so far.
1. Protons and neutrons make up the nucleuses (or nuclei) of atoms
2. Electrons orbit around the nucleus.
3. The atom is therefore mostly empty space.
4. The protons are positively charged and the electrons are negatively charged, so there is a force of
attraction between them. This force is called electromagnetism.
5. The electrons don’t spiral into the nucleus because they’re moving.
6. The electrons spin around so fast that an atom is effectively more-or-less spherical.
7. The size (or the strength if you like) of the positive charge on each proton is exactly equal to the
size of the negative charge on each electron even though…
8. Protons are about 1840 times more massive than electrons are.
9. Neutrons, which are neutral, have about the same mass as protons.
10. Most of the mass of the atom is concentrated in the nucleus.
11. Even though the protons repel one another, they’re held in place by a force that’s called the Strong
Nuclear Force.
12. The number of protons that are in an atom’s nucleus defines the type of atom that it is. ALL
carbon atoms have 6 protons in their nucleus.
13. The neutron number can vary. There are three naturally occurring isotopes of carbon. About 99%
have 6 protons and 6 neutrons, about 1% have 6 protons and 7 neutrons, and about 1 in a trillion
have 6 protons and 8 neutrons.
14. Some nuclei are unstable and give off nuclear radiation. Uranium-235 atoms for example emit
alpha particles and turn into Thorium atoms.
Particle
Neutron
Proton
Relative Mass
1
0.9986
(about the same as
a neutron)
Charge
0 (neutral)
+1
Mass (kg)
1.6749×10−27
1.6726×10−27
Diameter (m)
about 1.75 x 10-15
about 1.75 x 10-15
Electron
0.00054
1/1839 mass of
neutron,
1/1836 mass of
proton
-1
9.10938291(40)×10−31
UNKNOWN!!!
(you might be the
one to discover it
one day!
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