ATOMS
Ancient Greece to Now
DEFINED

Atom (n.)- A unit of matter, the smallest
unit of an element, consisting of a dense,
central, positively charged nucleus
surrounded by a system of electrons,
equal in number to the number of nuclear
protons, and characteristically remaining
undivided in chemical reactions except for
limited removal, transfer, or exchange of
certain electrons.
THE GREEK ATOMISTS

The word "atom" comes from the Greek
"atomos" and signifies "indivisible" or
“Not to be cut”.
THE GREEK ATOMISTS

Leucippus of Milet invented this notion in
420 B.C. His disciple, Democritus of
Abdere (around 460-370 BC.), asked this
question: If you break a piece of matter
in half, and then break it in half again,
how many breaks will you have to make
before you can break it no further?
THE GREEK ATOMISTS

Democritus thought that it ended at some
point, a smallest possible bit of matter. He
explained that matter was made up of
particles in perpetual motion and endowed
with ideal qualities:
Democritus’ Atomic Theory



invisible because of their extremely
small size
indivisible as their name indicates
solid (no void inside)
Democritus’ Atomic Theory



eternal because they are perfect
surrounded by an empty space (to
explain their movement and changes in
density)
having an infinite number of shapes (to
explain the diversity observed in nature)
End of Atoms???


Unfortunately, the atomic ideas of
Democritus had no lasting effects on other
Greek philosophers, including Aristotle.
Aristotle dismissed the atomic idea as
worthless.
Aristotle

Aristotle believed that matter was made
up of only four elements (the Four
Element Theory) fire, air, earth, and
water. People considered Aristotle’s
opinions very important and if Aristotle
thought the atomic idea had no merit,
then most other people thought the same
also
Aristotle

For more 2000 years nobody did anything
to continue the explorations that the
Greeks had started into the nature of
matter.
THE ALCHEMY OF THE MIDDLE
AGES

Originating in the Middle Ages, alchemy
was born from the progress of metallurgy
and from the inadequacy of the theory of
the 4 elements for representing the
diversity of matter.
THE ALCHEMY OF THE MIDDLE
AGES

The "grand plan" of alchemy was to achieve
the transmutation of lowly metals (like copper)
into "noble" metals such as gold. Without doubt
because the success of such "Grand Works"
(transmutation) opened up prospects of wealth
and power, the activities of alchemists were
surrounded by secrecy and were performed
using extremely ancient processes of the
esoteric and of the occult.
THE ALCHEMY OF THE MIDDLE
AGES

In spite of their esoteric beliefs, alchemists
developed the observation,
experimentation, measurement and
classification of the elements: alchemy is
therefore a respectable precursor of
chemistry.
THE ALCHEMY OF THE MIDDLE
AGES

Anyway, don't forget that Newton was
adept at alchemy and that today's physics
has turned the old dream of transmutation
into reality by transforming certain atoms
into other atoms
1803 John Dalton - first modern
atomic theory


1799 Joseph Louis Proust - Law of Definite
Proportions (matter combines in a definite
proportion consistently)
Dalton - Law of Multiple Proportions
(matter combines in small, fixed, whole
number ratios)
1803 John Dalton - first modern
atomic theory



Dalton applied these ideas to form his
"atomic theory"
*Atoms are small, indivisible spheres
*Atoms of a given element are identical
1803 John Dalton - first modern
atomic theory



*Atoms cannot be created, destroyed or
transformed
*Compounds are the results of small
whole number ratios of atoms
*Relative numbers and kinds of atoms in a
compound are constant
DISCOVERY OF NATURAL
RADIATION

On November 8, 1895, at the University of
Würzburg, Wilhelm Roentgen’s
attention was drawn to a glowing
fluorescent screen on a nearby table.
DISCOVERY OF NATURAL
RADIATION

Roentgen immediately determined that
the fluorescence was caused by invisible
rays originating from the partially
evacuated glass Hittorf-Crookes tube he
was using to study cathode rays (i.e.,
electrons).
DISCOVERY OF NATURAL
RADIATION

Surprisingly, these mysterious rays
penetrated the opaque black paper
wrapped around the tube. Roentgen had
discovered X- rays, a momentous event
that instantly revolutionized the field of
physics and medicine.
DISCOVERY OF NATURAL
RADIATION

However, prior to his first formal
correspondence to the University PhysicalMedical Society, Roentgen spent two
months thoroughly investigating the
properties of X rays. For his discovery,
Roentgen received the first Nobel Prize in
physics in 1901.
DISCOVERY OF NATURAL
RADIATION

He rejected a title (i.e., von Roentgen)
that would have provided entry into the
German nobility, and donated the money
he received from the Nobel Prize to his
University.
DISCOVERY OF NATURAL
RADIATION


Roentgen did accept the honorary degree
of Doctor of Medicine offered to him by
the medical faculty of his own University
of Würzburg.
However, he refused to take out any
patents in order that the world could
freely benefit from his work. At the time of
his death, Roentgen was nearly bankrupt
from the inflation that followed WW I.
DISCOVERY OF NATURAL
RADIATION

One day in 1896, Henri Becquerel (by
chance according to legend?) arranged in
his cupboard, a packet of uranium salt
beside an unexposed photographic plate.
Several days later, he took out the plate
and developed it.
DISCOVERY OF NATURAL
RADIATION

To his surprise, he noticed that the
photographic plate had been exposed
without having been exposed to the light.
Having repeated this experiment, he
concluded that Uranium spontaneously
emits what he called "uranic rays".
DISCOVERY OF NATURAL
RADIATION

Marie and Pierre Curie, together, began
investigating the phenomenon of
radioactivity recently discovered in
uranium ore, pitchblende. In 1898, Marie
Curie discovered after chemical extraction
of uranium from the ore, the residual
material was more "active" than the pure
uranium.
DISCOVERY OF NATURAL
RADIATION

Pitchblende emits more radiation than
uranium itself. She deduced that this ore
contains, in very small quantities, one or
more elements much more active that
uranium.
DISCOVERY OF NATURAL
RADIATION


With the assistance of her husband Pierre Curie
(shortly after his marriage to Marie in 1895,
Pierre subjugated his research to her interests.)
and after two years of effort, she arrived at
isolating two new elements: Polonium (named
thus in tribute to her homeland) and Radium.
It took four more years of processing tons of ore
under oppressive conditions to isolate enough of
each element to determine its chemical
properties.
DISCOVERY OF NATURAL
RADIATION



Although Henri Becquerel discovered the
phenomenon, Marie coined the term
“radioactivity”. For their work on radioactivity,
the Curies were awarded the 1903 Nobel Prize in
physics.
Marie was awarded the 1911 Nobel Prize in
chemistry for her discoveries of radium and
polonium, thus becoming the first person to
receive two Nobel Prizes.
For the remainder of her life she tirelessly
investigated and promoted the use of radium as
a treatment for cancer.
DISCOVERY OF THE ELECTRON

In 1897, J.J. Thompson discovered the
first component part of the atom: the
electron, a particle with a negative
electric charge.
DISCOVERY OF THE ELECTRON

In 1904, he proposed an
initial model of an atom,
since nicknamed "the
plum pudding model". He
imagined the atom as a
sphere full of an
electrically positive
substance mixed with
negative electron "like the
plums in a pudding"
QUANTA

In 1900 Max Planck, a
professor of theoretical
physics in Berlin showed
that when you vibrate
atoms strong enough,
such as when you heat
an object until it glows, it
emits radiation in
separate bursts of energy.
He called these energy
bursts quanta.
QUANTA

Physicists at the time thought that light
consisted of waves. But the quanta
behaved like particles and, in turn, these
particles of light received the name
photons. Atoms not only emit photons,
but they can also absorb them.
QUANTA

Any list of the greatest
thinkers in history
contains the name of the
brilliant physicist Albert
Einstein. His theories of
relativity led to entirely
new ways of thinking
about time, space,
matter, energy, and
gravity. Einstein won his
only Nobel Prize for
physics in 1908 for
discovering that light
absorption can release
electrons from atoms.
QUANTA

This phenomenon has the name
“photoelectric effect”. A heated
controversy occurred for many years on
deciding whether light consisted of waves
or particles. The evidence appeared
strong for both cases.
QUANTA

Later, physicists showed that light appears
as either wave-like or particle-like (but
never both at the same time) depending
on the experimental setup. This is called
the dual nature of light. He also showed
that energy and mass were
interconvertible. His famous equation
E=mc2 related energy and mass. E=
energy, m=mass, c=speed of light.
Oil Drop Experiment

Robert Andrews Millikan, (1868–1953)
was a U.S. physicist who made the first
determination of the charge of the
electron by using his famous “oil drop
experiment”. For these achievements, he
was awarded the 1923 Nobel Prize for
Physics.
DISCOVERY OF THE NUCLEUS


In 1912, Ernest Rutherford (New
Zealand physicist) discovered the atomic
nucleus.
Using his famous “gold foil
experiment”, Rutherford used alpha
particles to bombard atoms.
DISCOVERY OF THE NUCLEUS

He used radium as the source of the alpha
particles and shined them onto the atoms
in gold foil. Behind the screen sat a
fluorescent screen for which he could
observe the alpha particles impact. The
results of the experiment were
unexpected.
DISCOVERY OF THE NUCLEUS

Most of the alpha particles went smoothly
through the foil. Only an occasional alpha
veered sharply from its original path,
sometimes bouncing straight back from
the foil. This surprised Rutherford so
much he said it was like firing a cannon at
a sheet of tissue paper and having the
shell bounce back to you.
DISCOVERY OF THE NUCLEUS

Rutherford reasoned that the alpha
particles must get scattered by tiny bits of
positively charged matter. His new model
of the atom showed that its positive
electric charge and the majority of its
mass were concentrated in an almost
point sized nucleus.
DISCOVERY OF THE NUCLEUS

Rutherford thought most of the space
around the positive center had nothing in
it. It is worth noting that in contrast to the
atom of the Greeks, Rutherford's is neither
indivisible (because it's a composite
structure), nor is it solid as it contains
mostly empty space.
DISCOVERY OF THE NUCLEUS

It was not until 1919 that Rutherford
finally identified the particles in the
nucleus with the discrete positive charges.
He named them protons, from the Greek
for first, because they were the first
identified building blocks of the nuclei. He
found the protons mass was 1,836 times
greater than the mass of the electron.
DISCOVERY OF THE NUCLEUS
DISCOVERY OF NUCLEONS


Rutherford understood that the nucleus is
itself composed of nucleons. These
nucleons are of two types:
Positively charged, it's a proton. Neutrally
charged, it's a neutron
DISCOVERY OF NUCLEONS

James Chadwick effectively discovered the
neutron in 1932.
Moseley, Proton and atomic
number

Henry Gwyn Jeffreys Moseley, (1887–
1915) was a British physicist who first
established the atomic numbers of the
elements by studying their X-ray spectra.
This led to a complete classification of the
elements, and also provided an
experimental basis for an understanding of
the structure of the atom.
Moseley, Proton and atomic
number

All atoms are characterized by their
Atomic Number represented by Z: this is
the number of protons in the atom. For
example, for hydrogen Z = 1, for carbon Z
= 6, for uranium Z = 92 etc.
Moseley, Proton and atomic
number

For a neutral atom, the number of protons
Z is equal to the number of electrons
because the - charge of an electron
cancels out the + charge of a proton. It is
the number of protons Z that defines an
atom.
BOHRS ATOM

In order to take account of atomic
stability, in 1913 Niels Bohr created a
new model of the atom: the Electron
Shell Model, or Solar System Model.
BOHRS ATOM

This model explained why electrons’
negative charge did not spiral toward the
nucleus’ positive charge.
The orbits of the electrons can't be just
anywhere but are "quantified"; only
certain particular orbits are permitted for
the electron. It's not until one jumps from
one orbit to another that it can emit (or
absorb) light.
What is "classical" and
continuous light ?

Not so easy to represent such an
immaterial concept!
At the end of the 19th century, James
Maxwell defined light as being a beam of
electromagnetic waves moving at a
constant speed in the vacuum: the famous
speed c of 300,000 kilometers per second.
Where does light come from?

Quantum physics allows us to better
understand how light is emitted by
matter... The world of the atom according
to Niels Bohr was a model at the frontier
of two ages: the classical age, prequantum and the quantum world.
Where does light come from?

This emission is explained then by the
jump that an electron makes from an orbit
E2 to an orbit E1. During this jump
towards this less energetic orbit E1 (an
inner orbit), the electron will lose part of
its energy in the form of a photon emitted
outwards.
Where does light come from?

Each photon of a radiation (light, radio
waves, X rays...) carries a quantum of
energy characteristic of its frequency
(frequency of visible light = color)
Where does light come from?
Where does light come from?

The higher the frequency of light, the
greater the energy and the more the color
will tend towards blue (and from there
towards ultra-violet, X ray and Gamma
rays).
Where does light come from?
Where does light come from?

An electron, making a "bigger jump" from
one atomic orbit to another, will then emit
a photon correspondingly more energetic
and of a correspondingly higher frequency.
Where does light come from?
Where does light come from?

Conversely, an atom's electron could
absorb a photon of a given energy and
thus jump from a less energetic orbit to a
more energetic orbit: It will thus become
more excited than normal because it is in
a more energetic orbit. It is in becoming
less excited that it would subsequently reemit a photon.
The visible spectrum

Each atom can only emit a precise and
characteristic set of colors: Each color of
light is in fact a particular frequency (and
therefore a level of energy) of a photon.
All of the possible jumps between orbits
that an electron can make within a given
atom translate into the emission (or
absorption) of a characteristic spectrum
of light:
The visible spectrum

Here we have a veritable identity card for
a given type of atom. It's because of this
readily identifiable spectrum that we can
know which atoms exist in stars in the
firmament. Their light is captured by
telescopes, analyzed and compared with
the spectrums of hydrogen, helium etc
The visible spectrum
TOWARDS QUANTUM PHYSICS

What an intellectual pleasure (and what
laziness!) to represent atoms in the form
of little balls turning one around another...
This model is, moreover, still the one that
the general public has in their heads. In
fact this model is false because at the
atomic scale, new laws apply! These laws
are part of a strange physics, very far
from our current concepts: quantum
physics.
TOWARDS QUANTUM PHYSICS


Bohr's model is the last model obedient to
classical physics, that is to say physics that
explains movements and phenomena in terms of
our human scale. These models of atoms are
therefore easy to understand and to represent.
Since the middle of the 1930s, the atom
has become a mathematical description
that is very difficult to transcribe into
images
Wave-particle duality

Louis Victor de Broglie (1892–1987)
was a French physicist who first developed
the principle that an electron or any other
particle can be considered to behave as a
wave as well as a particle. This waveparticle duality is a fundamental
principle governing the structure of the
atom, and for its discovery, de Broglie was
awarded the 1929 Nobel Prize for Physics.
Wave-particle duality

The most important question that
quantum physics has been attempting to
address concerns the manner in which to
represent physical objects and their
properties.
Wave-particle duality

The old physics, known as classical,
distinguished two types of fundamental
entities:


particles, which are sorts of microscopic
balls,
waves, which propagate in space a bit like
the movement of a wave on the sea.
Wave-particle duality

Quantum physics doesn't hold on to this
classification, convenient as it is. The
objects which it considers are neither
particles, nor waves, but "something else".
Wave-particle duality


The following analogy should help us:
Look at a cylinder from two different
angles, a cylinder appears sometimes as a
circle, sometimes as a rectangle. When in
fact it is neither one nor the other.
Wave-particle duality
Wave-particle duality


That’s the way the photon, the electron
and all elementary particles are, thus the
image of a particle is but one facet of a
more complex entity.
De Broglie's discovery of wave-particle
duality enabled physicists to view
Einstein's conviction that matter and
energy are interconvertible as being
fundamental to the structure of matter.
Wave-particle duality

The study of matter waves led not only to
a much deeper understanding of the
nature of the atom but also to
explanations of chemical bonds and the
practical application of electron waves in
electron microscopes
PAULI'S EXCLUSION PRINCIPLE

This fundamental law of quantum physics
was set out for the first time by the
physicist Wolfgang Pauli in 1925.
PAULI'S EXCLUSION PRINCIPLE


All of the particles of the same type have
an identical nature but can have different
properties.
For example, the electrons in an atom
have different energies (associated with
their orbits). Each particle thus possesses
a certain number of its own properties
that form "the state of the particle".
PAULI'S EXCLUSION PRINCIPLE

No two electrons in the same atom
can have the same set of four
quantum numbers (energy level,
sublevel, orbital, or spin).
PAULI'S EXCLUSION PRINCIPLE

In an atom, two electrons can have the
same energy on the condition that their
spins are different. This explains the
progressive filling of the periodic
table of Mendeleev, that is to say the
electronic structure of atoms.
Heisenberg Uncertainty Principle

In 1927 Werner Karl Heisenberg made the
discovery for which he is best known -that of the uncertainty principle. This
states that it is impossible to specify
precisely both the position and the
simultaneous momentum (mass
multiplied by velocity) of a particle.
Heisenberg Uncertainty Principle

Dare we have a new image to illustrate this
principle:
Deep in the woods at night, a nature lover hears
the hooting of an owl. If he would like, at the
same time, see the feathered creature, he would
have to turn a torch on him: But then it's a good
bet that the surprised owl will stop singing. This
gives rise to the insoluble dilemma: We can't
both hear and see the owl at the same time...
Alas!
Heisenberg Uncertainty Principle

Therefore electrons are better thought of
as occurring in an electron cloud
surrounding the nucleus than in orbits
around the nucleus.
Heisenberg Uncertainty Principle

If it is necessary to risk an image to
illustrate this curious phenomena, one
could imagine the electron as a submarine which emerges, long enough for a
measurement, from its probabilistic ocean.
Heisenberg Uncertainty Principle

Later, it submerges and it will be
impossible to an observer from the surface
to localize it with any precision: one could
do no more that define the volume of the
ocean where the submarine could
probably be found.
Orbitals and orbits

When the a planet moves around the sun,
you can plot a definite path for it which is
called an orbit. A simple view of the atom
looks similar and you may have pictured
the electrons as orbiting around the
nucleus. The truth is different, and
electrons in fact inhabit regions of space
known as orbitals.
Orbitals and orbits

Orbits and orbitals sound similar, but they
have quite different meanings. It is
essential that you understand the
difference between them.
The impossibility of drawing
orbits for electrons

To plot a path for something you need to
know exactly where the object is and be
able to work out exactly where it's going
to be an instant later. You can't do this for
electrons.
The impossibility of drawing
orbits for electrons

The Heisenberg Uncertainty Principle
says - loosely - that you can't know with
certainty both where an electron is and
where it's going next. That makes it
impossible to plot an orbit for an electron
around a nucleus. Is this a big problem?
No. If something is impossible, you have
to accept it and find a way around it.
The impossibility of drawing
orbits for electrons

Suppose you had a single hydrogen atom
and at a particular instant plotted the
position of the one electron. Soon
afterwards, you do the same thing, and
find that it is in a new position. You have
no idea how it got from the first place to
the second.
The impossibility of drawing
orbits for electrons

You keep on doing this over and over
again, and gradually build up a sort of 3D
map of the places that the electron is
likely to be found.
The impossibility of drawing
orbits for electrons

95% of the time (or any other percentage
you choose), the electron will be found
within a fairly easily defined region of
space quite close to the nucleus. Such a
region of space is called an orbital. You
can think of an orbital as being the region
of space in which the electron lives.
The impossibility of drawing
orbits for electrons

What is the electron doing in the orbital?
We don't know, we can't know, and so we
just ignore the problem! All you can say is
that if an electron is in a particular orbital
it will have a particular definable energy.
Orbitals


Each orbital has a name.
The orbital occupied by the hydrogen electron is
called a 1s orbital. The "1" represents the fact
that the orbital is in the energy level closest to
the nucleus. The "s" tells you about the shape
of the orbital. s orbitals are spherically
symmetric around the nucleus - in each case,
like a hollow ball made of rather chunky material
with the nucleus at its center.
s orbital

The orbital on the left
is a 2s orbital. This
is similar to a 1s
orbital except that the
region where there is
the greatest chance
of finding the electron
is further from the
nucleus - this is an
orbital at the second
energy level.
s orbital

If you look carefully, you
will notice that there is
another region of slightly
higher electron density
(where the dots are
thicker) nearer the
nucleus. ("Electron
density" is another way of
talking about how likely
you are to find an
electron at a particular
place.)
s orbital

2s (and 3s, 4s, etc)
electrons spend some of
their time closer to the
nucleus than you might
expect. The effect of this
is to slightly reduce the
energy of electrons in s
orbitals. The nearer the
nucleus the electrons
get, the lower their
energy.
p orbitals

Not all electrons inhabit s orbitals (in fact,
very few electrons live in s orbitals). At the
first energy level, the only orbital available
to electrons is the 1s orbital, but at the
second level, as well as a 2s orbital, there
are also orbitals called 2p orbitals.

A p orbital is rather like 2
identical balloons tied
together at the nucleus.
The diagram on the right
is a cross-section through
that 3-dimensional region
of space. Once again, the
orbital shows where there
is a 95% chance of
finding a particular
electron. At any one
energy level it is
possible to have three
absolutely equivalent
p orbitals
d and f orbitals

In addition to s and p orbitals, there are
two other sets of orbitals which become
available for electrons to inhabit at higher
energy levels. At the third level, there is a
set of five d orbitals (with complicated
shapes and names) as well as the 3s and
3p orbitals . At the third level there are a
total of nine orbitals altogether
d and f orbitals

At the fourth level, as well the 4s and 4p
and 4d orbitals there are an additional
seven f orbitals - 16 orbitals in all. s, p,
d and f orbitals are then available at all
higher energy levels as well
d and f orbitals

You have to be aware that there are sets
of five d orbitals at levels from the third
level upwards, but you will not be
expected name them. Apart from a
passing reference, you won't come across
f orbitals at all.
Fitting electrons into orbitals

You can think of an atom as a very bizarre
house (like an inverted pyramid!) - with
the nucleus living on the ground floor, and
then various rooms (orbitals) on the
higher floors occupied by the electrons.
Fitting electrons into orbitals

On the first floor there is only 1 room (the
1s orbital); on the second floor there are 4
rooms (the 2s, and three 2p orbitals); on
the third floor there are 9 rooms (one 3s
orbital, three 3p orbitals and five 3d
orbitals); and so on. But the rooms aren't
very big . . . Each orbital can only hold
2 electrons.
Fitting electrons into orbitals

A convenient way of showing the orbitals
that the electrons live in is to draw
"electrons-in-boxes".
"Electrons-in-boxes"

Orbitals can be represented as boxes with
the electrons in them shown as arrows.
Often an up-arrow and a down-arrow are
used to show that the electrons are in
some way different.
"Electrons-in-boxes"

A 1s orbital holding 2
electrons would be
drawn as shown on
the right, but it can
be written even more
quickly as 1s2. This is
read as "one s two" not as "one s
squared".
"Electrons-in-boxes"

You mustn't confuse
the two numbers in
this notation:
The order of filling orbitals

Electrons fill low energy orbitals (closer to
the nucleus) before they fill higher energy
ones. Where there is a choice between
orbitals of equal energy, they fill the
orbitals singly as far as possible.
The order of filling orbitals

The diagram (not to
scale) summarizes the
energies of the
orbitals up to the 4p
level.
s- and p-block elements

The elements in group 1 of the Periodic
Table all have an outer electronic structure
of ns1 (where n is a number between 2
and 7). All group 2 elements have an
outer electronic structure of ns2. Elements
in groups 1 and 2 are described as s-block
elements.
s- and p-block elements

Elements from group 3 across to the noble
gases all have their outer electrons in p
orbitals. These are then described as pblock elements.
s- and p-block elements
d-block elements


d-block elements are elements in which
the last electron to be added to the atom
is in a d orbital.
Remember that the 4s orbital has a lower
energy than the 3d orbitals and so fills
first. Once the 3d orbitals have filled up,
the next electrons go into the 4p orbitals
as you would expect.
d-block elements
QUARKS

Up until 1964, it was believed that there
only existed three elementary particles
making up the atom: the electron, the
proton and the neutron. However,
numerous unstable particles (with a
lifetime of the order of 10-23
seconds) have been detected, either in
cosmic rays, or in the high-energy impacts
created in particle accelerators
constructed after the 1939-1945 war.
QUARKS


Set out for the first time in 1964 by Murray
Gell-Mann and independently by George
Zweig, the theory of quarks progressively
established its pedigree and won acclaim as
advances corroborated the theory by
experiment; it was not until 1975 that quarks
were detected experimentally.
The strange name of Quark comes from a
Roman phrase of James Joyce in "Finnegan’s
Wake": Three Quarks for Muster Mark!
QUARKS


Quarks have a unique property: they
are incapable of existing alone,
unaccompanied! It is absolutely
impossible to observe a quark in isolation.
Quarks are the constituents of nucleons.
There exist two types in ordinary matter :


up quark
(symbol = u)
down quark (symbol = d)
Antimatter



There exists a mirror universe where
matter is transformed into anti-matter.
Antimatter is composed of antiparticles:
antiquarks, anti-electrons, and
antineutrinos.
An antiparticle is simply a particle
with opposing quantum numbers
How many atoms are there?

In nature, about 90 different atoms
exist which combine to form an infinite
variety of compounds. It is necessary to
add to this natural radioactive atoms (and
therefore unstable) and those created
artificially by man by nuclear reactions.
How many atoms are there?


At the moment, about 115 atoms exist which
have been discovered or created, but the list
could grow. The last atom created is element
114, baptized with a provisional name of
ununquadium (symbol Uuq): It was created in
1998 at the Nuclear Institute of Dubna in Russia.
Late breaking news: Elements 116 and
118 have been synthesized in 1999 in California
at the Lawrence Berkeley National Laboratory
The 3 radiations

There exist three varieties of radioactivity
characterized by the emission of different
rays emitted by the nucleus of the atom:
The 3 radiations
 α (alpha) rays are stopped by 6 cm of
air. They are composed of alpha particles
made up of two protons and two neutrons
(in fact a helium nucleus). The particle is
therefore positively charged. These alpha
particles are nothing more than fragments
of unstable heavy nuclei that reorganize
themselves to become lighter and more
stable nuclei (thus non radioactive!).
The 3 radiations
 β (beta) rays are stopped by an
aluminum screen. They are notably
composed of electrons and are therefore
negatively charged. Beta radiation is
identical to the cathode radiation in your
TV!
The 3 radiations

γ (gamma) rays are extremely
penetrating and can pass straight through
a safe. They are composed of high-energy
photons (particles of light). They are
nothing but pure energy without any
mass.
The 3 radiations

These three varieties of radioactivity are
not emitted simultaneously. Each nuclear
reaction of an atom emits only one
single type of radiation at a time! For
example, radioactive Uranium-238 emits
an alpha ray and thus loses 4 nucleons (2
protons + 2 neutrons): U 238 thereby
transforms itself into Thorium-234
(because 2 protons less - that changes an
atom!).
Nuclear fission

Whenever the nucleus of a heavy atom (like
uranium 235) fissions (fragments) into two
smaller nuclei, it produces a remarkable event:
the sum of the masses of these two remaining
nuclei is less than the mass of the original large
nucleus. Where has the missing mass gone?
It has transformed itself into pure energy
(Einstein's mass-energy equivalence), an
enormous quantity of energy.
Nuclear fusion


In broad terms it's the inverse of fission.
Two light atomic nuclei (like hydrogen)
crash into each other and fuse together
into a single bigger nucleus. Now the final
mass of this big nucleus is smaller than
the sum of the masses of the two initial
nuclei, which is where we get an
enormous release of energy produced by
the annihilation of this difference of mass.
Nuclear fusion

In order to be able to provoke such a fusion
reaction, it is necessary to force the nuclei, all
positively charged, to move together and to
overcome their mutual repulsion : This is not
possible except at very high temperatures (the
temperature corresponding to the intensity
necessary to get the particles to crash into each
other). This is why the nuclear fusion reaction is
also called a thermonuclear reaction (thermo
= heat).
Nuclear fusion

This uncontrolled reaction is used in the
hydrogen bomb or H-bomb. This reaction
is also seen in the heart of our Sun where
temperatures reach hundreds of millions
of degrees.
Natural or artificial radioactivity?

Man has not invented radioactivity. It has
existed since the beginning of the
universe: We speak of natural
radioactivity when it is due to the
durable radio-elements formed in the stars
which have not yet found their stable
state: they will end up transforming
themselves into stable atoms.
Natural or artificial radioactivity?

We speak of artificial radioactivity when
referring to elements fabricated by man. In
this case, these atoms are very heavy (with a
high atomic number Z), very unstable and
therefore have a very short half-life. Physicists
create these artificial radioelements by
bombarding natural atoms with protons or alpha
particles: the nuclei of these atoms acquire
additional protons that transform them into new
heavier atoms.