[PDF] The atom - Thematic publication - CEA

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Theoretical chemistry

modelling.

Crédit :

D.R. T • The theory that matter is composed of indi- visible elements stretches back to the fifth century BC. • From 1600 to 1800, early philosophizing of extremely small entities (molecules or atoms) to describe matter emerges in treatises by renowned thinkers: Galileo in Il Saggiatore (The

Assayer) or Descartes in Le Monde (The World)

• In 1808, John Dalton borrows the concept of atoms to explain the laws of chemistry. In his atomic theory, Dalton posits that the ultimate particles of a homogeneous body are perfectly alike, but different from body to body. By exten - sion, any and every chemical reaction had to be identifiable as a new arrangement of atoms which cannot, themselves, be changed. • In 1897, Joseph John Thomson shows that cathode rays are composed of massive and negatively-charged particles, i.e. electrons.

He thus theorizes atoms as composed of a

positively-charged matter, as well as being full of electrons.• In 1908, Jean Perrin definitively demonstrates that matter is composed of atoms. •

In 1911, Ernest Rutherford discovers, by shooting particles at a gold foil, that most of the mass of an atom is concentrated in a

tiny-volume nucleus surrounded by electrons, although how they behave remains a mystery. In

1918, the same Rutherford conceptualizes the

idea that each atomic nucleus is composed of protons-particles that are far more massive than electrons, and positively-charged. However, later mass and charge measurements of atomic nuclei demonstrate the existence of neutral protons, which were dubbed neutrons in 1920 and which James Chadwick finally discovered in 1932. • In 1913, Niels Bohr introduced the first model describing electron energy levels. •

In 1964, Murray Gell-Mann and Georg Zweig sketched out an early theory of quarks, which were proven to exist later in 1968.

TIMELINE

© F. Bournaud/CEA-Irfu

                  

The atomic nucleus

protons neutrons electrons electron cloud

Aluminium (Al) atom

Symbolic representation of the components of an atom

Aluminium nucleus

13 electrons

13 protons14 neutrons

REPRESENTATION OF

THE ELECTRON CLOUD OF A LITHIUM ATOM

The lithium atom shown has three protons,

four neutrons and three electrons.

We cannot give the exact position of the three

electrons in the lithium atomWs electron cloud.

In this representation, the electrons are most

likely to be found in the darker areas. This image was produced using mathematical formulae.

Atomic nucleus

Electron cloud

The nucleus has

a diameter that is about 100,000 times smaller than the atom itself practically all an atom's mass is concentrated in the nucleus. in which case it is called an ion. eye3eMn2eimTh tmd aRi2ichR Z which is also the number of electrons, is its atomic number. The number of neutrons is N

The simple calculation NZA

number of nucleons, called the mass number.

These are the numbers that define the

chemical elements. Each element is de- noted by a symbol and its atomic number group 1 1 4 2 5 3 6 72

375914

1116
4813
61015

121718La-Lu

1 34
11

19202122

5723582459256026

612762286329

6430

6531663267336834

6935

703671

3738394089419042914392449345944695479648974998509951100521015310254103

555657-717273747576777879808182838485862

5678910

131415161718

878889-10310410510610710810911011111211311411511611711812

TaH LiBe Na

KCaScTi

LaV CeCr PrMn NdFe PmCo SmNi EuCu GdZn TbGa DyGe HoAs ErSe TmBr YbKr

LuRbSrYZr

AcN b ThMo PaTc URu NpRh PuPd AmAg CmCd BkIn CfSn EsSb FmTe MdI NoXe

LrCsBaHfWReOsIrPtAuHgTiPbBiPoAtRnHe

BCNOFNe

AlSiPSClAr

FrRaRf

DbSgBhHsMtDsRgCn

Uu tFl UupLv

UusUuoMg1,0079

6.94

19.0122

22.989

8

39.098

340.07844.955947.867

138.905

550.9415

140.11

651.9961

140.907

754.9380

144.24

255.845

[145 ]58.9332 150.3

658.6934

151.96

463.546

157.2

565.38

158.925

369.723

162.50

072.64

164.930

374.9216

167.25

978.96

168.934

279.904

173.05

483.798

174.966

8

85.467

887.6288,905891,224

[227 ]92.9064

232.038

hydrogen lithiumberyllium sodium potassiumcalciumscandiumtitanium lanthanumvanadium ceriumchromium praseodymium manganese neodymeiron prométhiumcobalt samariumnickel europiumcopper ga doliniumzinc terbiumgallium dys prosiumgermanium holmiumarsenic er biumselenium thu li umbromine yt terbiumkrypton lutetiumrubidiumstrontiumyttriumzirconium actiniumniobiumthorium molybdenum protactinium technetium uraniumruthenium neptu niumrhodium plutoniumpalladium americiumsilver curiumcadmium ber kéliumindium californiumTin einsteiniumantimony f 26
Fe 55.84
5Iron Sy mbo l (in white and green : no stable isotope)

Atomic m

ass, based on

12CNameAtomic

number alkali metals alkaline earth metals transition metals other metals metalloids other nonmetals halogens noble gases lanthanides actinides

Periodic table of chemical elements

Lanthanides

Actinides

1

H for hydrogen, which has just one pro-

ton, or 26

Fe for iron which has 26). The origi-

nal periodic table devised in 1869 by Dmitri

Mendeleev to classify the atoms according to

their mass and chemical properties has pro- gressively grown into todayWs version.

On Earth, there are 94 chemical elements.

isotopesFor example: ? All the isotopes of hydrogen have just one proton but can have zero, one or two neutrons.

They are hydrogen (the most ubiquitous form),

deuterium and tritium. ? All the isotopes of carbon have 6 protons.

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: 12 C, 13

C and

14 C. 1 electron

Nucleus {1 proton}Hydrogen

1

HDeuterium

2

H or DTritium

3

H or T

1 electron

Nucleus 1 proton

1 neutron 1 electron

Nucleus

1 proton 2 neutrons Symbolic representation of the isotopes of hydrogen          

The chemical properties of an atom depend only

on the number and arrangement of the electrons in its electron cloud; all isotopes of the same element thus share the same chemical proper- ties. However, the slight difference in the mass of their nucleus means that their physical proper- ties are also slightly different.    

A property that describes their

kinetic potential in orbital motion.

An intrinsic property

of the electron, analo - gous to rotation.

The structure of the electron cloud that results

from the distribution of these properties has two consequences. The first is that it dictates how chemical symbols are laid out in MendeleevWs periodic table. The second is that it dictates the type of chemical properties of the different elements.

Certain electron cloud configurations are parti

- cularly stable. Atoms configured like this are not chemically reactive - they are inert.They are the atoms of noble gases, whose symbols are written in the column farthest to the right in MendeleevWs periodic table.

Atoms near the noble gases tend to realign their

electron cloud to make it resemble a noble gas.

They can do this by ionizing, by gaining or lo

- sing one or more electrons, or by establishing a covalent bond with other atoms. They thus share the property of certain electrons.

Where each of the bonded

atoms shares an electron in one of its outer shells to form an electron pair binding the two atoms.Chemical manipulations on a vacuum gas manifold.

© C. Dupont/CEA

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, the atomic force microscope (AFM) that uses the mechanical interaction between the sample and a 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 masses and charges inside the nuclei and mapping these calculations against measurements of certain of their properties. Luminescent effect of silver nanoparticles produced by an electrical current locally injected by the tip of a scanning tunneling microscope.

© CEA

Gravitation

Electromagnetic interaction

Weak interaction

Strong interaction

               

Strong interaction

FERMIONS

2 nd family3 th family1 st family

6 Quarks6 Leptons

Weak interaction

uu d du u

MeV2/3

1/2 up H

1,25 GeV00

Higgs boson

19681964

C

1,27 GeV2/3

1/2 charm

19741970

t

173,2 GeV2/3

1/2 top

19951977

d 4,8 MeV-1/3 1/2 down

19681964

s 104
MeV-1/3 1/2 strange

19681964

b 4,2 GeV-1/3 1/2 bottom 1977-
e

0,511 MeV-1

1/2 electron

18971874

105,7 MeV-1

1/2 muon 1936-

1,777 GeV-1

1/2 tau 1975-
2,2 eV0 1/2 e-neutrino

19561930

0 ,17 MeV0 1/2 -neutrino

19621956

15,5 MeV0 1/2 -neutrino

2000197520121964

e H

15 GeV0

0 Higgs boson

20121964

Electromagnetic interaction

VECTORS

BOSONS

g 0 eV0

1gluons

0 eV0

1photon

w 80,4
GeV1 1 bosons W Z 91,2
GeV0 1 boson Z

19791965

1922 1900

1983196819831968

Mass Gluon

Proton 10

-15 m

Quark 10

-18 m

Electrical

charge

Symbol

Name Date predicted from theoryDate discoveredby experimentSpin

Matter

Molecule 10

-9 m

Atom 10

-10 m

Nucleus 10

-14 m

Neutron

Proton

Electron

Mass

Electric charge

Spin

Strong interaction

FERMIONS

2 nd family3 th family1 st family

6 Quarks6 Leptons

Weak interaction

uu d du u U

2,4 MeV2/3

1/2 up H

1,25 GeV00

Higgs boson

19681964

C

1,27 GeV2/3

1/2 charm

19741970

t

173,2 GeV2/3

1/2 top

19951977

d 4,8 MeV-1/3 1/2 down

19681964

s 104
MeV-1/3 1/2 strange

19681964

b 4,2 GeV-1/3 1/2 bottom 1977-
e

0,511 MeV-1

1/2 electron

18971874

105,7 MeV-1

1/2 muon 1936-

1,777 GeV-1

1/2 tau 1975-
2,2 eV0 1/2 e-neutrino

19561930

0 ,17 MeV0 1/2 -neutrino

19621956

15,5 MeV0 1/2 -neutrino

2000197520121964

e H

125 GeV00

Higgs boson

20121964

Electromagnetic interaction

VECTORS

BOSONS

g 0 eV0

1gluons

0 eV0

1photon

w 80,4
GeV1 1 bosons W Z 91,2
GeV0 1 boson Z

19791965

1922 1900

1983196819831968

Mass Gluon

Proton 10

-15 m

Quark 10

-18 m

Electrical

charge

Symbol

Name Date predicted from theoryDate discoveredby experimentSpin

Matter

Molecule 10

-9 m

Atom 10

-10 m

Nucleus 10

-14 m

Neutron

Proton

Electron

The nucleon assembly can be stable (there are

256 stable nuclei for 80 elements) or, more

often, unstable (approaching 3,000 nuclei).

For each of the unstable nuclei, we define

a radioactive period or half-life T, the time after which half of its radioactive nuclei have decayed. Unstable nuclei want to get back to a stable state, via a decay chain. Cesium (half-life 1.2 s), for instance, becomes stable 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).

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/

la-vallee-de-stabilite/index.php Produced by Frédéric Durillon V Animea 02-2012

RADIOACTIVE DECAY

A specimenWs radioactive activity (expressed in units called becquerels) decreasens over time as its unstable nuclei

progressively decay. For each radioactive isotope, and for each of decay mode it undergoes,n we define a half-life,

or radioactive period, as the time after which half of the radioactive antoms initially present have spontaneously

reacted. For different radioactive nuclides, this half-life period variens wildly over orders of magnitude ranging from

a few milliseconds up to several billion years! Nuclear physics is the study of the atomic nucleus and the interactions involded between its consti - tuents. The charged particle multidetector array Indra, a facility for studying heavy-ion collision.

© P.Stroppa/CEA

Which are nuclei characterized

by their unusual neutron- proton ratios and extremely short-lived lifespans before decaying. The Alice experiment, hosted at CERN, is focused on studying the physics of matter in its extreme states of temperature and density.

© P.Stroppa/CEA

SPIRAL

© P. Stroppa/CEA

Quadrupoles of the Spiral2 linear accelerator.

Large Hadron Collider (LHC) © Commission for Atomic Energy and Alternative Energies, 2017

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