THE HISTORY OF THE ATOMFrom Democritus until now

Made by Jacopo Belviso, Fabrizio Garofalo

and Erica Rio 5A

WHAT IS AN ATOM?

The atom is the smallest particle of anelement; it maintains the chemical properties

DEMOCRITUS (460 A.C – 370 A.C)

Democritus was the firstphilosopher of the history tospeak about atoms. ForDemocrito the atom was thelittlest part of the matter, infactthe same word meansindivisible. This theory was incontrapposition with the one ofAristotle, more famous thanDemocritus, that was verycriticized from the most of theother citizens.

ARISTOTLE (380 B.C. - 320 B.C.)

Aristotle affirms, incontrapposition ofwhat Democritussaid, that theatoms weredivisible endlessly.He was supportedfrom the most ofthe people.

JOHN DALTON (1766 – 1844)

After almost 2000 years, Dalton,recovered the theory of the atom, applying5 different rules: 1. All thematter is composed from very little partindivisible and indisttructible called atoms;

2. All the atoms of the same element areequals and they have the same mass; 3. Atoms of anelement can't become atoms of anotherelement; 4. To form a compound,atoms of an element combined themselveswith atoms of other elements;5. Atoms can't be created or destroyed, butthey can be combined with other elements.

JOHN THOMSON (1856 – 1940)

Thomson created the first model of theatom. He thought that the atom wascomposed from a positive mass in wichwere present the electrones, somecorpuscles of negative charge. Herappresented this model like a sort of plumpudding with the electrons within.

He discovered the charge of the parts of the atomthanks to the cathode ray tube, in wich werescattered the particles in straight line noticing thatthey take different directions when they passedthrough the electric field generated between thetwo plates, charged each other with negative andpositive charge alternately.

ERNEST RUTHERFORD (1871 –1937) Rutherford created a new

model of the atomchanging the previous oneof Thomson. Hedescovered that theelectrones turn around thenucleus, made of a positivemass.

He descovered it using a gold foil, surrounded by a glasscircle, and the chatode ray tube. Scattering the alpha particelstowards the gold foil, he noticed that they bounce back or diverttheir course in the glass circle instead of going straight on likehe thought. So he discovered that the nucleus was formed bythe protons, and the electrones turned around this positivemass following the course of the orbitals.

NIELS BOHR (1885 – 1962)

In fact he noticed that if hewould applied the model ofRutherford to an atomheavier than the hydrogenone, the electrones could beable to overload themselvesfalling into the nucleus.

Bohr, taking ispiration from the model ofRutherford, discovered that the electron turnaround the nucleus in dipendence of thequantic number and just in a specific orbital. He descovered that because the model ofRutherford was wrong in part.

WERNER HEISENBERG (1901 –1976)

He discovered in fact that theelectrons moves theirselveswith a wave motion along theorbitals instead of turningaround them, so we can't saythat the electron is in aparticular place, but we have tosay that there's the probabiltyto find it there.

Heisenberg realized that it's impossible toknow at the same time momentum andvelocity of the atom. This is calleduncertainty principle.

ERWIN SCHRÖDINGER (1887-1961)

Schrodinger showedthat the electrons arewaves and not particels,allowing to the uncertaintyprinciple. Infact he statedthat if you do anexperiment to know theposition of a particel youshould find something, butif not, it means that it is awave that transmit theinformation of theprobability to find theparticel there.

He also said that this waves don't move, butthey are stationary, and the orbitals where theyare can't say us where is the particel but howmuch energy has that wave.

This is called energetic level, and it needs toknow where is the best chance to find theparticel there.

ARNOLD SOMMERFELD (1868-1951)

This theory contrasts theoriginal Bohr model whichsupposed circular orbits.

Arnold Sommerfeld tried to improvethe atomic model of Bohr. In thismodel we supposed that theelectrons were traveling in ellipticalorbits around the nucleus.

PAUL DIRAC (1902-1984)

Paul Dirac made valuablecontributions to thedevelopment of quantummechanics andelectrodynamics. He alsointroduced the Dirac deltafunction, and supposedthe presence of anotherparticle, renamed“magnetic monopole”,equipped with an isolatedmagnetic charge.

WOLFGANG PAULI (1900 – 1958)

Pauli was one of the foundersof quantum mechanics. Heformulated the exclusionprinciple.

According to Pauli, can’t beexist two electrons whichhave the same four quantumnumbers.

JAMES CHADWICK (1891-1974)

James Chadwickwas an Englishphysicist. In 1932 hediscovered theneutron, a neutralparticle of a nucleus.

He bombarding thin beryllium foil with alphaparticles emitted by polonium, he discovered thatfrom the beryllium were emitted secondaryradiation that not resented neither an electric fieldnor a magnetic field.