From infinitely large to infinitely small

1 > The atom

ATOMS: AT THE HEART OF THE MATTER

PHYSICAL-CHEMICAL PROPERTIES
OF MATTER
THE ATOMIC NUCLEUS, A WHOLE NEW
WORLD, A WHOLE NEW PHYSICS
2 > SUMMARY

The atom

Crédit : D.R.
ATOMS: AT THE HEART
OF THE MATTER 4
Origin and fate of atomic
nuclei5
Theoretical chemistry
What is an atom? 6 modelling.

Diameters of atoms THE ATOMIC NUCLEUS,

and nuclei 7 A WHOLE NEW WORLD,
Volume, mass, A WHOLE NEW PHYSICS 17
and density 7 Drilling down to
Electric charge 8 the elementary 18
Chemical elements Stability of atomic nuclei 20
and isotopes 9 Nuclear physics 21
New nuclei to synthesize
and study 21
PHYSICAL-CHEMICAL
PROPERTIES OF MATTER  11 Nuclear matter 22
Chemical properties
of the atom 12
Physical properties
of matter 13
The four fundamental
interactions15

Production: Agence Gimmik - Illustrations: Yuvanoe - In collaboration with F. Saint-Jalm, physics-chemistry teacher
Cover photo: ©F. Charra/CEA - Nanoscopic sieve formed by ordered molecular assembly on an atomically flat graphite surface - 07/2017

From infinitely large to infinitely small 1 > The atom

 > INTRODUCTION 3

© Jacques Boyer/Roger-Viollet

© Roger-Viollet

© Roger-Viollet
From left to right:
John Dalton,
Ernest Rutherford
and James Chadwick.

T
he idea that matter is composed of indi- In the twentieth century, the weight of com-
visible units called “atoms” has traversed pelling left physicists and chemists forced to
the centuries, sometimes refuted, some- accept the microscopic structure underpinning
times embraced. all matter in the Universe. They set about stu-
The advent of quantitative chemistry, in the dying these immensely complex objects called
early nineteenth century, brought the concept atoms.
back to the fore, sparking a century-long battle
between opponents and proponents of the
“atomic hypothesis”.

TIMELINE
• The theory that matter is composed of indi- • In 1908, Jean Perrin definitively demonstrates
visible elements stretches back to the fifth that matter is composed of atoms.
century BC. • In 1911, Ernest Rutherford discovers, by
• From 1600 to 1800, early philosophizing of shooting particles at a gold foil, that most
extremely small entities (molecules or atoms) of the mass of an atom is concentrated in a
to describe matter emerges in treatises by tiny-volume nucleus surrounded by electrons,
renowned thinkers: Galileo in Il Saggiatore (The although how they behave remains a mystery. In
Assayer) or Descartes in Le Monde (The World) 1918, the same Rutherford conceptualizes the
• In 1808, John Dalton borrows the concept of idea that each atomic nucleus is composed of
atoms to explain the laws of chemistry. In his protons-particles that are far more massive than
atomic theory, Dalton posits that the ultimate electrons, and positively-charged. However,
particles of a homogeneous body are perfectly later mass and charge measurements of atomic
alike, but different from body to body. By exten- nuclei demonstrate the existence of neutral
sion, any and every chemical reaction had to protons, which were dubbed neutrons in 1920
be identifiable as a new arrangement of atoms and which James Chadwick finally discovered
which cannot, themselves, be changed. in 1932.
• In 1897, Joseph John Thomson shows that • In 1913, Niels Bohr introduced the first model
cathode rays are composed of massive and describing electron energy levels.
negatively-charged particles, i.e. electrons. • In 1964, Murray Gell-Mann and Georg Zweig
He thus theorizes atoms as composed of a sketched out an early theory of quarks, which
positively-charged matter, as well as being were proven to exist later in 1968.
full of electrons.

From infinitely large to infinitely small 1 > The atom

4

AN ATOM IS MADE UP OF A NUCLEUS OF PROTONS

AND NEUTRONS, AND AN ELECTRON CLOUD.

Atoms: at the heart

of the matter
© F. Bournaud/CEA-Irfu

Simulation of a galaxy formation.

From infinitely large to infinitely small 1 > The atom

 > ATOMS: AT THE HEART OF THE MATTER 5

ORIGIN AND FATE OF ATOMIC

NUCLEI
Matter as we know it constitutes 5% of eve-
rything in the universe. Most of the atoms the
universe that compose it (hydrogen, helium,
and some lithium) were formed in the very first
moments after the Big Bang, in what is called
primordial nucleosynthesis. All its stable atomic
nuclei were formed in the cores of stars by
nuclear fusion reactions causing lighter nuclei
to fuse and form heavier nuclei.
Over the course of their existence, stars will
thus create nuclei that can have up to 26
protons, i.e. nuclei of iron atoms.
When they reach their last evolutionary
stage, the most massive stars explode
into a supernova.
The titanic energy of this explo-
sion serves to synthesize many
even heavier atomic nuclei.
Those that are stable, or that
have a very long half-lives,
get ejected into the clouds

© Photodisc
of gas and cosmic dust from
which new stars are formed.
Earth is thus composed of
32.1% iron, 30.1% oxygen,
15.1% silicon, 13.9%
magnesium, and all the other
chemical elements in far smaller
proportions. It also harbors uns-
table (radioactive) isotopes, whose
steady decay has dictated the thermal
state of Earth’s core: mainly potassium-40,
uranium-238 and thorium-232.

From infinitely large to infinitely small 1 > The atom

6 > ATOMS: AT THE HEART OF THE MATTER

All material bodies are made up

of an assembly of atoms.
From left to right: metal - crystal
or polymer.

© P. Avavian/CEA

© CEA
© MNHN
WHAT IS AN ATOM? the nucleons (from the Latin word nucleus,
meaning kernel).
Although a large proportion of the mass of the
The electrons, which are negatively charged
universe is probably of unknown nature (dark
(and classed as fermions), orbit close around
matter), the ordinary matter that we do know
the nucleus, which is positively charged. Taken
is composed of atoms.
individually, electrons are not really particles as
The myriad modes of atoms assembly yield an such, but together they form an electron cloud
immense diversity of molecules, macromole- where their different levels of energy lend them
cules, polymers, crystals, metals, nanomate- different degrees of excitation.
rials, and more.
It is like they are unbound in space, but we
Atoms are built from three building blocks; can calculate the probability of finding them
protons, neutrons, and electrons. in a given position.
The atomic nucleus is an assembly of protons
and neutrons. The protons and neutrons are

Symbolic representation of the components of an atom

Aluminium (Al) atom Aluminium nucleus

14 neutrons

13 protons

13 electrons

From infinitely large to infinitely small 1 > The atom

 > ATOMS: AT THE HEART OF THE MATTER 7

DIAMETERS OF ATOMS
AND NUCLEI
The diameter of an electron cloud runs from
0.5×10-10 m (hydrogen) up to 4.3×10-10 m
(radium). This is incredibly small—you would
have to ‘stack’ 1,000,000 hydrogen atoms just
to reach the diameter of a hair! Long considered
inexistent as they are impossible to see, the
invention of scanning probe microscopy now
brings tangible proof that they are real. The
atomic nucleus is much smaller still.
The diameter of the nucleus of a hydrogen atom
(a single proton) is 2×10-15 m, and that of a
uranium atom is 2×10-14 m. The nucleus has
REPRESENTATION OF a diameter that is about 100,000 times smaller
THE ELECTRON CLOUD than the atom itself.
OF A LITHIUM ATOM
The lithium atom shown has three protons, VOLUME, MASS, AND DENSITY
four neutrons and three electrons.
The bodies that make up our everyday environ-
We cannot give the exact position of the three ment (metals, crystals, polymers) are made up
electrons in the lithium atom’s electron cloud.
of atoms that are attached to other atoms by
In this representation, the electrons are most
likely to be found in the darker areas. This
bonds that hold them together. The density of
image was produced using these atoms is therefore much the same as the
mathematical formulae. density of these solid bodies.
Atoms are incredibly small, and so their mass
and volume are tiny. For example, take a pin-
Atomic nucleus head of iron of 1 mm³ in volume. It is made
up of 60 million billion iron atoms!
A proton and a neutron share roughly the same
mass, which is 1840 times greater than that
of an electron, which means practically all an
atom’s mass is concentrated in the nucleus. Take
an iron atom. Its nucleus has a diameter of
around 10-14 m, for a mass of 9.3×10-26 kg,
so the density of the nucleus comes to
1.4×1017 kg.m-3, which equates to just over a
hundred billion kilograms per centimeter cubed.
So if the pinhead was made up exclusively of
nuclei of iron atoms, its mass would equal
1.4×108 kg, or 140,000 tons!

Electron cloud

From infinitely large to infinitely small 1 > The atom

8 > ATOMS: AT THE HEART OF THE MATTER

© C. Dupont/CEA
To estimate the mass of a nucleus, you sim-
ply have to know how many nucleons it has.
Given that the mass of a nucleon is approxi-
mately 1.67×10-27 kg, it is easy to calculate
the approximate mass of an atom. However, the
result of the calculation is still just an estimate.
But scientists can measure the mass of an atom
directly, using a mass spectrometer. The atoms Mass spectrometry enables fast isotopic
measurement on a sample.
are introduced in vapor phase into an ionization
chamber, then accelerated through a tunnel by
an electrical field. They then reach a magnetic
ELECTRIC CHARGE
field which causes to curve their trajectories. The positive electric charge of the proton is
The point where they hit the detector characte- exactly the opposite of the negative charge
rizes their mass, which can thus be measured of the electron. The neutron has no electric
with precision. charge and is neutral. Any atom that counts
as many protons in its nucleus as electrons in
its electron cloud therefore has a net neutral
electric charge.
However, in certain conditions (typically che-
mical reactions), the atom can lose or gain one
or more electrons, and thus become positively
or negatively charged, in which case it is called
an ion.
Optical scanning probe microscope.

© P. Avavian/CEA

From infinitely large to infinitely small 1 > The atom

 > ATOMS: AT THE HEART OF THE MATTER 9

CHEMICAL ELEMENTS
AND ISOTOPES
For a given atom, the number of protons Z, (e.g. 1H for hydrogen, which has just one pro-
which is also the number of electrons, is its ton, or 26Fe for iron which has 26). The origi-
atomic number. The number of neutrons is N. nal periodic table devised in 1869 by Dmitri
The simple calculation N + Z = A thus sums its
Mendeleev to classify the atoms according to
number of nucleons, called the mass number.
their mass and chemical properties has pro-
These are the numbers that define the
chemical elements. Each element is de- gressively grown into today’s version.
noted by a symbol and its atomic number On Earth, there are 94 chemical elements.

Periodic table of chemical elements alkali metals

alkaline earth metals
transition metals
other metals
metalloids
other nonmetals
group lanthanides halogens
1 Atomic actinides noble gases 18

Fe
26
1 number 2

1 H Symbol (in white and green : no stable isotope) He

hydrogen Name Iron helium
1,0079 2 55.845 Atomic mass, based on 12C 13 14 15 16 17 4.0026

3 4 5 6 7 8 9 10

2 Li
lithium
Be
beryllium
B
boron
C
carbon
N
nitrogen
O
oxygen
F
fluorine
Ne
neon
6.941 9.0122 10.811 12.0107 14.0067 15.9994 18.9984 20.1797

11 12 13 14 15 16 17 18

3 Na
sodium
Mg
magnésium
Al
aluminium
Si
silicon
P
phosphorus
S
sulfur
Cl
chlorine
Ar
argon
22.9898 24.3050 3 4 5 6 7 8 9 10 11 12 26.9815 28.0855 30.9738 32.065 35.453 39.948

19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36

4 K
potassium
Ca
calcium
Sc
scandium
Ti
titanium
V
vanadium
Cr
chromium
Mn
manganese
Fe
iron
Co
cobalt
Ni
nickel
Cu
copper
Zn
zinc
Ga
gallium
Ge
germanium
As
arsenic
Se
selenium
Br
bromine
Kr
krypton
39.0983 40.078 44.9559 47.867 50.9415 51.9961 54.9380 55.845 58.9332 58.6934 63.546 65.38 69.723 72.64 74.9216 78.96 79.904 83.798

37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54

5 Rb
rubidium
Sr
strontium
Y
yttrium
Zr
zirconium
Nb
niobium
Mo
molybdenum
Tc
technetium
Ru
ruthenium
Rh
rhodium
Pd
palladium
Ag
silver
Cd
cadmium
In
indium
Sn
Tin
Sb
antimony
Te
tellurium
I
iodine
Xe
xenon
85.4678 87.62 88,9058 91,224 92.9064 95.96 [98] 101.07 102,9055 106.42 107.8682 112.411 114.818 118.710 121.760 127.60 126.9045 131.293

55 56 57-71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86

6 Cs Ba La- Hf Ta W Re Os Ir Pt Au Hg Ti Pb Bi Po At Rn
cesium
132.9054
barium
137.327 Lu hafnium
178.49
tantale
180.9479
tungsten
183.84
rhenium
186.207
osmium
190.23
iridium
192,217
platinum
195.084
gold
196.9666
mercury
200.59
thallium
204.3833
lead
207.2
bismuth
208.9804
polonium
[209]
astatine
[210]
radon
[222]

87 88 89-103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118

7 Fr Ra Ac- Rf Db Sg Bh Hs Mt Ds Rg Cn Uut Fl Uup Lv Uus Uuo

francium
[223]
radium
[226] Lr rutherfordium
[267]
dubnium
[268]
seaborgium
[271]
bohrium
[272]
hassium
[277]
meitnerium darmstadtium roentgenium
[276] [281] [280]
copernicium
[285]
ununtrium
[284]
flevorium
[289]
ununpentium
[288]
livermorium
[293]
ununseptium
[291]
ununoctium
[294]

57 58 59 60 61 62 63 64 65 66 67 68 69 70 71

Lanthanides La
lanthanum
Ce
cerium
Pr
praseodymium
Nd
neodyme
Pm Sm
prométhium samarium
Eu
europium
Gd
gadolinium
Tb
terbium
Dy
dysprosium
Ho
holmium
Er
erbium
Tm
thulium
Yb
ytterbium
Lu
lutetium
138.9055 140.116 140.9077 144.242 [145] 150.36 151.964 157.25 158.9253 162.500 164.9303 167.259 168.9342 173.054 174.9668

89 90 91 92 93 94 95 96 97 98 99 100 101 102 103

Actinides Ac
actinium
Th
thorium
Pa
protactinium
U
uranium
Np
neptunium
Pu
plutonium
Am Cm
americium curium
Bk
berkélium
Cf
californium
Es
einsteinium
Fm
fermium
Md
mendelevium
No
nobelium
Lr
lawrencium
[227] 232.0381 231.0359 238.0289 [237] [244] [243] [247] [247] [251] [252] [257] [258] [259] [262]

From infinitely large to infinitely small 1 > The atom

10 > ATOMS: AT THE HEART OF THE MATTER

Symbolic representation of the isotopes of hydrogen

Hydrogen 1H Deuterium 2H or D Tritium 3H or T

1 electron 1 electron 1 electron

Nucleus {1 proton}
{
Nucleus 1 proton
1 neutron } {
Nucleus 1 proton
2 neutrons }

All the atoms of a given chemical element will For example:

have the same number of protons (which, again, • All the isotopes of hydrogen have just one
is a number that defines the chemical element) proton but can have zero, one or two neutrons.
but they may not all have the same number of They are hydrogen (the most ubiquitous form),
neutrons. Two atoms sharing the same number deuterium and tritium.
of protons but a different number of neutrons • All the isotopes of carbon have 6 protons.
are isotopes of that element. The most abundant have 6, 7 or 8 neutrons.
• All uranium atoms have 92 protons. There are
two isotopes in natural uranium: uranium-235
which has 143 neutrons (235 = 92+143)
and uranium-238 which has 146 neutrons
(238 = 92 + 146).

An isotope takes the name of its chemical ele-

ment associated with its total nucleon number,
which, taking carbon as an example, gives: 12C,
13C and 14C.

From infinitely large to infinitely small 1 > The atom

 11
THE ELECTRON CLOUD DICTATES ALL MATERIAL DIVERSITY.

Physical-chemical
properties
of matter

© CEA/LEM

Self-assembled monolayer observed

by scanning tunneling microscopy.
Each “bump” represents a molecule.

From infinitely large to infinitely small 1 > The atom

12 > PHYSICAL-CHEMICAL PROPERTIES OF MATTER
© C. Dupont/CEA

Chemical manipulations on a vacuum gas manifold.

The chemical properties of an atom depend only

on the number and arrangement of the electrons
The structure of the electron cloud that results
in its electron cloud; all isotopes of the same
from the distribution of these properties has two
element thus share the same chemical proper-
consequences. The first is that it dictates how
ties. However, the slight difference in the mass of
chemical symbols are laid out in Mendeleev’s
their nucleus means that their physical proper-
periodic table. The second is that it dictates
ties are also slightly different.
the type of chemical properties of the different
elements.
CHEMICAL PROPERTIES Certain electron cloud configurations are parti-
cularly stable. Atoms configured like this are not
OF THE ATOM chemically reactive - they are inert.They are the
The electrons in an atom’s electron cloud have atoms of noble gases, whose symbols are written
to be orbiting in allowed states. We cannot as- in the column farthest to the right in Mendeleev’s
sign them clearly defined trajectories, but we periodic table.
can describe the state each electron occupies.
Atoms near the noble gases tend to realign their
This is done based on four properties of electrons: electron cloud to make it resemble a noble gas.
their energy level, their angular momentum, They can do this by ionizing, by gaining or lo-
the projection of this A property that describes their sing one or more electrons, or by establishing a
angular momentum on kinetic potential in orbital covalent bond with other atoms. They thus share
a given direction, and motion. the property of certain
Where each of the bonded
their spin. atoms shares an electron in electrons.
one of its outer shells to form
An intrinsic property an electron pair binding the
of the electron, analo- two atoms.
gous to rotation.

From infinitely large to infinitely small 1 > The atom

 > PHYSICAL-CHEMICAL PROPERTIES OF MATTER 13

The aspirin molecule (at left)

and paracetamol molecule
(right) look much the same.
Both are composed of carbon
atoms (in grey), hydrogen
atoms (in white) and oxygen
atoms (in red). A nitrogen
atom is shown in blue.

Matter that we call “organic”, i.e. built around

the covalence of atoms of carbon, oxygen,
nitrogen and hydrogen, offers an inexhaustible
source of molecules.
The other atoms in the periodic table have more
complex electronic structures. They attract
together and coalesce by metallic bonding. The
metal obtained is Where the atoms share one
solid at normal or more free electrons, called
temperature, and delocalized electrons.
conducts electricity.

PHYSICAL PROPERTIES
OF MATTER
All of the physical properties of matter: hard-
ness, malleability, ductility, transparency,
color, phase transition temperatures, electri-
cal conductor or insulator, etc... like all of its
chemical properties: acid or base, oxidizer or
reducer, solvent or solute, etc... are entirely
© P. Dumas/CEA

due to the different behaviors adopted by the

electrons in the electron clouds.
We now know how to organize these atoms to
get new properties, such as high-temperature
The MesoXcope is a photoelectron emission microscope
superconductivity, to get supermagnetic
that serves to study the chemical and electronic structure
properties, to get miniature electrical circuits of surfaces, at simultaneously high spatial (50 nm)
or use them to store data. and spectroscopic (50 meV) resolution.

From infinitely large to infinitely small 1 > The atom

14 > PHYSICAL-CHEMICAL PROPERTIES OF MATTER

SEEING AND PROBING ATOMS

From macroscopic scale down to micrometric scale,
it is possible to form images based on light waves,
using optical microscopy.
To form images of smaller objects, it becomes
necessary to use particles, like electrons, which
have a sub-micrometer wavelength. Electron micros-
copes work as the same principle as optical micros-
copes (such as scanning electron microscopes, or
SEM for short). By pushing their performances to
the extreme, scientists have managed to get down
to atomic scale (0.1 nm).
Scanning probe microscopes broke onto the scene
in the early 1980s. They work to the principle of
examining a relatively flat surface with an extremely
fine probe that interacts with the atoms. Landmark
types since include the scanning tunneling micros-
cope (STM) that uses a weak electric current travel-
ling between the sample and a conducting tip,
Luminescent effect of silver nanoparticles produced by an the atomic force microscope (AFM) that uses the
electrical current locally injected by the tip of a scanning mechanical interaction between the sample and a
tunneling microscope. tip mounted on a flexible
cantilever, and the near-
field scanning optical
microscope that, with an
extremely fine optical fiber,
exploits the properties of
evanescent waves in the
near-field sample surface.
The latest type to emerge
is the scanning tunneling-
induced luminescence
microscope (STL). The
common denominator to
all these microscopes is
that they enable atomic-
scale studies of various
molecules and their
behavior on different
substrates.
It remains impossible to
form images of atomic
nuclei, but it is possible to
generate images by calcu-
lating the distribution of
© CEA

masses and charges inside

the nuclei and mapping
these calculations against measurements of certain
of their properties.

From infinitely large to infinitely small 1 > The atom

 > PHYSICAL-CHEMICAL PROPERTIES OF MATTER 15

THE FOUR FUNDAMENTAL INTE-

RACTIONS
Physicists manage to explain all observable phy-
sical phenomena in the universe with a set of
just four forces, or interactions, considered as
“fundamental”. What are they?
• Gravitation - a classic discovered by Isaac
Newton over three centuries ago;
• Electromagnetic interaction, identified as
such by James Clerk Maxwell in the second
half of the nineteenth century, and which
explains the binding of everyday matter;
• Weak interaction, discovered in the 1930s,
responsible for certain radioactive processes,
including beta decay;
• Strong interaction, discovered around about
the same time as weak interaction, which
very solidly binds together the constituents
of atomic nuclei.

Gravitation governs a vast array of phenomena,

from why objects fall to the movement of the
planets. Gravitation is also what makes stars
form from primordial gaseous matter, which it
Electromagnetic interaction is far stronger than
forces to contract. Gravitation is also why stars, gravitation. Its effects are manifest in everyday
once they have formed, attract and coalesce life, as it is electromagnetic force that makes
together and form galaxies. household appliances work. However, on a more
Gravitation interaction is an attractive force with fundamental level, it is electromagnetic inter-
infinite range (which means the force between action that holds atoms and molecules toge-
two masses is null only if the distance sepa- ther, that governs all chemical reactions, and
rating them is infinite). Gravitation cannot be that explains macroscopic optical phenomena
shielded against, so any attempt to weaken or (since light is formed of electromagnetic waves,
abolish its influence is destined to fail. Howe- which are made of photons). Electromagnetic
ver, it is by far the weakest of the four interac- interaction, like gravitation, has infinite range,
tions, so much so that its effects at particle but since it can be attractive or repulsive (de-
scale can be considered negligible, especially as pending on the sign of the electric charges
involved), its combined cumulative effects get
particles are subjected to far stronger forces...
cancelled out over large distance due to the net
neutrality of matter.

From infinitely large to infinitely small 1 > The atom

16 > PHYSICAL-CHEMICAL PROPERTIES OF MATTER

Weak interaction has an extremely short range, of

the order of a billionth of a billionth of a meter.
This makes it basically, like glue, a contact
interaction: two particles can only interact by
weak force if they are practically touching. It
is weak interaction that is responsible for beta
decay, the process by which a neutron decays
into a proton and an electron. Weak interaction,
as its name suggests, is a very weak force, Strong interaction is by far the strongest of
which is what makes it so difficult to observe. the four fundamental interactions, yet for a
Even so, weak interaction still plays an absolu- long time it remained in the dark. Physicists
tely capital role, crucially in the nuclear reac- deduced its existence in the 1930s, when they
tions that fuse hydrogen to power the sun. If it realized that there was something amazing
were to disappear from the universe, our Earth’s about the stability of atomic nuclei. As they
star would cease to shine... carry same-sign electrical charges, the protons
in the nucleus should repel one another due
to the electrical force that tends to separate
them. And yet, they appear to be very solidly
bound together. So what could be counter-do-
minating their electrical repulsion? As no force
in classical theory could explain this nuclear
binding energy, the hypothesis was postulated,
and has since been experimentally confirmed,
that inside atomic nuclei there must be a hugely
intense force - strong nuclear interaction - with
an extremely short range, of the order of a mil-
lionth of a billionth of a meter...
This force acts like a kind of glue cementing two
nucleons (proton or neutron) that are in contact
but that very quickly loses weakens the instant
that they are even slightly apart. Even so, strong
interaction is still incredibly powerful - it can
stop a proton launched at a hundred thousand
kilometers per second, and stop it within a few
millionths of billionths of meters…

From infinitely large to infinitely small 1 > The atom

 17
IN NUCLEAR PHYSICS, PRACTICALLY EVERYTHING
IS WAITING TO BE DISCOVERED.

The atomic
nucleus,
a whole new
world, a whole
new physics
© P. Dumas/CEA

Joule-Thomson scanning tunneling microscope,

purpose-engineered to the electronic properties of matter
at subatomic scale and super-low temperature.

From infinitely large to infinitely small 1 > The atom

18 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS

DRILLING DOWN
TO THE ELEMENTARY
While the size-scale of atoms and their electron
clouds is the nanometer (10-9), the size-scale
of atomic nuclei and nucleons is the femtome-
ter (10-15). The size of the particles that are
considered with today’s science as elementary Matter
is of the order of 10-18 meters.

The nucleus is an extremely dense and complex

6 Quarks
ultrasmall object. It is like a Russian doll, with
nested layers that get increasingly small. It
was long thought that protons and neutrons Molecule 10-9 m
were elementary particles i.e. ultimate consti-
tuents without any substructure. But observa-
tions in experiments performed in the 1950s
and 1960s with steadily-improving bigger and Electron
more powerful particle accelerators showed that
large numbers of particles are produced after Atom 10-10 m
the collisions.

6 Leptons
This diversity was interpreted by assuming that
they were composed of even smaller consti- Proton
tuents, dubbed quarks. The nucleons contain Neutron
up quarks and down quarks that assemble in Nucleus 10 -14
m
triplets by strong interaction.
Quarks like electrons are fermions. The quark
model today counts six types of quarks, grouped
into three generations. Quarks interact through
u u
attraction by exchanging gluons that are not
d
Gluon
fermions but bosons. As well as an electric
charge equal to -1/3e or⅔2/3e, they also carry Quark 10-18 m
Proton 10-15 m
another charge called color, labeled blue, green
or red nothing to do with real perceived colors,
but a code borrowing the same three-valued Mass: expressed, at this scale, as energy,
logic as strong interaction. and thus in electronvolts, due to the
mass–energy equivalence E=mc2.
Electric charge: positive or negative.
Spin : operates many magnetic properties at
subatomic scale, and differentiates fermions
(1/2-integer spin) from bosons (integer-spin).

From infinitely large to infinitely small 1 > The atom

 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS 19

FERMIONS
1st family 2nd family 3th family

2,4 MeV 2/3 1,27 GeV 2/3 173,2 GeV 2/3

VECTORS
U
up
C
charm
t
top
BOSONS
0 eV 0
1/2 1/2 1/2
1964 1968 1970 1974 1977 1995 Strong interaction g
gluons
4,8 MeV -1/3 104 MeV -1/3 4,2 GeV -1/3
1

d s b
1,25 GeV 0 1965 1979

down
1/2
strange
1/2
bottom
1/2 Higgs
H
1964 1968 1964 1968 - 1977 boson 0
1964 2012
0,511 MeV -1 105,7 MeV -1 1,777 GeV -1 0 eV 0

e
electron
μ
muon
τ
tau
Electromagnetic interaction
photon
1/2 1/2 1/2 1
1874 1897 - 1936 - 1975 1900 1922

2,2 eV 0 0,17 MeV 0 15,5 MeV 0 80,4 GeV 1 91,2 GeV 0

νe
e-neutrino
μ ν
μ-neutrino
ντ
τ-neutrino
Weak interaction w
bosons W
Z
boson Z
1/2 1/2 1/2 1 1

Illustration: Fabrice Mathé

1930 1956 1956 1962 1975 2000 1968 1983 1968 1983

THE STANDARD MODEL Mass 125 GeV 0 Electrical

OF PARTICLE PHYSICS charge

This standard model is the theory describing the elemen-

Symbol
H
Higgs
tary particles of matter and the particles that mediate the Name boson Spin
0
fundamental interaction forces between them all at scales
Date 1964 2012 Date discovered
down below 10-15 m. predicted by experiment
Some of these particles have been observed and studied for from theory
decades. Others, like the Higgs boson predicted in 1964
but not discovered until 2012 at the CERN (see page 23),
are only beginning to be studied.

From infinitely large to infinitely small 1 > The atom

20 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS

THE VALLEY OF STABILITY

Nuclides are classified as a map that charts
a valley of stability where the stable nuclides
lie along the valley floor. The plot of unstable
nuclides from the sides of the valley down
to the floor depicts the different types of
radioactivity.

See the animated version at http://irfu.cea.fr/

STABILITY OF ATOMIC NUCLEI la-vallee-de-stabilite/index.php
Atomic nuclei composed of Z protons and N Produced by Frédéric Durillon – Animea 02-2012
neutrons are only bound together by strong
interaction, which manifests as the exchange
The nucleon assembly can be stable (there are
of π mesons between nucleons, just as Hideki
256 stable nuclei for 80 elements) or, more
Yukawa had predicted back in 1935 (a predic-
often, unstable (approaching 3,000 nuclei).
tion that made him the first Japanese physicist
For each of the unstable nuclei, we define
to receive the Nobel Prize in Physics in 1949).
a radioactive period or half-life T, the time
We would later learn that the π mesons are
after which half of its radioactive nuclei have
composed of one quark and one antiquark of
decayed. Unstable nuclei want to get back
the same particle family. Neutrons and protons
to a stable state, via a decay chain. Cesium
share out the energy of the nucleus and thus get
(half-life 1.2 s), for instance, becomes stable
propelled with extremely rapid motion.
neodymium by changing into barium (half-life
14.5 s), lanthanum (half-life 14.2 min), cerium
(half-life 33 h) and praseodymium (half-life
13.5 d).

RADIOACTIVE DECAY
A specimen’s radioactive activity (expressed in units called becquerels) decreases over time as its unstable nuclei
progressively decay. For each radioactive isotope, and for each of decay mode it undergoes, we define a half-life,
or radioactive period, as the time after which half of the radioactive atoms initially present have spontaneously
reacted. For different radioactive nuclides, this half-life period varies wildly over orders of magnitude ranging from
a few milliseconds up to several billion years!

From infinitely large to infinitely small 1 > The atom

 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS 21

NEW NUCLEI TO SYNTHESIZE

AND STUDY
Frédéric Joliot and Irène Joliot-Curie made the
discovery of artificial discovery back in 1933,
and a host of atomic nuclei have been synthe-
sized since. While nuclear physics centers at
Dubna in Russia, Darmstadt in Germany and
Berkeley in the USA focus on synthesizing
high-atomic-number nuclei, here in France
NUCLEAR PHYSICS the CEA/CNRS center of Ganil, National Large
Heavy Ion Accelerator (Grand accélérateur
Nuclear physics is the study of the atomic nucleus
national d’ions lourds) in Caen is a facility that
and the interactions involded between its consti-
investigates the stability of the nuclei produced
tuents.
in an effort to better understand how strong
interaction holds nucleons glued together.
It studies the nucleus as a collection of
The Ganil explores two lines of inquiry:
nucleons that move with attraction, the mecha-
studying stable nuclei in various states of
nisms underpinning their attraction forces and
excitation, and producing and studying exotic
the influence of quarks on their properties and
nuclei. The Ganil Which are nuclei characterized
behaviors. Its method of inquiry is to probe
went live in 1983 by their unusual neutron-
nuclei with an atom-scale-adapted ‘micro- proton ratios and extremely
and its extension, short-lived lifespans before
scalpel’ a beam of accelerated particles that decaying.
Spiral2, started up
is used to see what proportions of the particles
in February 2012, will soon be operational.
are deflected or absorbed. It also lets us see
how the nuclei react by ejecting nucleons,
producing other particles, and so on.

Over the past few decades, as technology has

moved forward, bringing increasingly sharp and
wide-ranging observations, so has the nuclear
modeling developed to explain them, bringing
increasingly complex explanations.
The models themselves have also evolved,
spurred by the power of computer simulation.
They have moved on to complex structures
where the nucleons form stable aggregates
© P.Stroppa/CEA

inside the nucleus or, in other cases, make

up a diffuse halo surrounding a denser core.
A revolution opening up a whole new world of
nuclear physics. The charged particle multidetector array Indra, a facility
for studying heavy-ion collision.

From infinitely large to infinitely small 1 > The atom

22 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS

© P.Stroppa/CEA
The Alice experiment, hosted at CERN, is focused on studying
the physics of matter in its extreme states of temperature and
density.

NUCLEAR MATTER
Strong interaction can form atomic nuclei up to
© P. Stroppa/CEA

a mass number (number of nucleons) of up to

300. However, it is possible to force the nucleons
temporarily into higher-number assemblies by
Quadrupoles of the Spiral2 linear accelerator. accelerating heavy ions (like lead ions) with
colossal energy densities (several TeV).
SPIRAL (in-line-beam radioactive ion production
system) is a Ganil facility built in 2001 and used to When two heavy nuclei collide, their nucleons
produce and accelerate exotic nuclei. Exotic nuclei meld during the split second of the impact, and
are characterized by their very unusual neutron–
proton ratios and extremely short-lived lifespans the conditions prevailing inside quark-gluon
before decaying. Exotic nuclei are a vital focus of plasma formed resemble the temperature and
study in many subfields of nuclear physics, but pressure conditions marking the state of the
also astrophysics, chiefly to understand how atomic
nuclei are formed in stars and supernovas. Although universe milliseconds after the Big Bang (the
physicists already know how to synthesize exotic Big Bang nucleosynthesis scenario). The plasma
nuclei in the laboratory, the Spiral facility means
they can now produce exotic nuclei in large quanti-
that forms is unstable, and the vast majority of
ties, accelerate them, observe their collisions with the energy injected to form it transforms into
other nuclei and thereby understand their structure. a huge array of all kinds of particles that are
To explore the boundaries even further, the second- instantaneously detected. Proton–lead colli-
generation Spiral2 facility will be able to produce
exotic nuclei a thousand times more powerfully sions help distinguish what happens in a cold
than previously possible. plasma compared to a hot plasma with lead-
The objective is to continue producing these synthe- lead collisions. What makes these collisions so
tic nuclei in order to discover the nature of these interesting for research is that they test out the
particles and understand the laws governing how
they behave. But this time, in contrast to the first- mechanisms of primordial nucleosynthesis by
generation accelerator, it will also allow to produce comparing today’s measurements against the
and study heavier exotic nuclei, and maybe, for the
first time ever, even “super-heavy” nuclei.
result of what happened 13.7 billion years ago.
This is what is done at the LHC since 2012 with
heavy ion collisions.

From infinitely large to infinitely small 1 > The atom

 > THE ATOMIC NUCLEUS, A WHOLE NEW WORLD, A WHOLE NEW PHYSICS 23

THE HIGGS BOSON

© Cern

Event display of a proton proton collision in a transverse view of the Atlas detector.

At the Large Hadron Collider (LHC), it is sometimes protons, hydrogen nuclei, that circulate in densely-packed
bunches, with 100 billion protons in each bunch! The collider is a 27 km circumference circular tunnel
embedded 100 m underground beneath the France-Switzerland border. These proton bunches travel at 11,000
revolutions per second. Two beams travel in opposite directions and collide every 25 ns at four locations around
the accelerator ring, corresponding to the positions of four particle detectors. The beam energy deliverable, at
7 to 8 TeV, recreates temperature and pressure conditions similar to those prevailing in the universe
shortly after the Big Bang.
Of the 6 million billion LHC proton–proton collisions produced from 2010 to 2012, the Atlas and CMS
experiments each recorded around 10 billion interesting collisions. Thanks to this colossal accumulation
of data, isolated events stack up and emerge the signal from the background noise. In July 2012, around
400 collisions signatured events signaling a particle consistent with the Higgs boson. The Higgs boson
was predicted back in 1964 by theoretical physicists François Englert, Robert Brout and Peter Higgs, and
its discovery earned them the 2013 Nobel Prize in Physics.

From infinitely large to infinitely small 1 > The atom

THE COLLECTION

1 > The atom

2 > Radioactivity
3 > Man and Radiation
4 > Energy
5 > DNA
6 > Nuclear reactors
7 > The nuclear fuel cycle
8 > Microelectronics
9 > The Lasers: a concentrate of light
10 > Medical Imagery
11 > Nuclear Astrophysics
12 > Hydrogen
13 > The Sun
14 > Radioactive waste
15 > The Climate
16 > Numerical Simulation
17 > Earthquakes
18 > The Nanoworld
19 > Energy for the twenty-first century
20 > Chemistry for energy

© Commission for Atomic Energy and Alternative Energies, 2017

Communication Direction
Headquarters Building
91191 Gif sur Yvette cedex - www.cea.fr
ISSN 1637-5408.