Composition of the AtomComposition of the Atom

DemocritusDemocritus’’s Atoms Atom•• Lived around 460 BCELived around 460 BCE•• Can divide something in half andCan divide something in half and

each half has the properties of theeach half has the properties of theoriginaloriginal

•• Keep dividing, eventually reach aKeep dividing, eventually reach apoint where something canpoint where something can’’t bet bedivided further.divided further.

•• He called these pieces thatHe called these pieces thatcouldncouldn’’t be divided t be divided ““atomosatomos””meaning indivisible.meaning indivisible.

•• He believed them to be eternal.He believed them to be eternal.•• Each type of material had its ownEach type of material had its own

unique atoms.unique atoms.

Others thoughtOthers thought4 elements:4 elements:•• EarthEarth•• AirAir•• FireFire•• WaterWater

DaltonDalton’’s Atoms Atom•• Lived in early 1800Lived in early 1800’’ss•• Elements were already knownElements were already known•• Proposed atoms to be Proposed atoms to be solidsolid

spheresspheres..•• Each element composed ofEach element composed of

spheres of the same mass andspheres of the same mass andproperties.properties.

•• Atoms would combine in fixedAtoms would combine in fixedratios to form compounds.ratios to form compounds.

J. J. ThomsonJ. J. Thomson’’s Atoms Atom•• (1897) discovered the first(1897) discovered the first

subatomic particlessubatomic particles•• studied the passage of an electricstudied the passage of an electric

current through a gascurrent through a gas•• The gas gave off rays ofThe gas gave off rays of

negatively charged particlesnegatively charged particles•• Theorized there must be a positiveTheorized there must be a positive

charge to balance the negativecharge to balance the negativechargecharge

•• The negative particle is called theThe negative particle is called theelectronelectron and the positive particle is and the positive particle iscalled the called the protonproton

ThomsonThomson’’s Atomic Models Atomic Model

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•• By 1903, Thomson theorized that the atom wasnBy 1903, Thomson theorized that the atom wasn’’tta solid sphere, but rather a sphere where most ofa solid sphere, but rather a sphere where most ofthe mass and all of the positive charge residedthe mass and all of the positive charge resided(proton). The negative charges (electrons) were(proton). The negative charges (electrons) weredistributed in the sphere.distributed in the sphere.

A.k.a. the A.k.a. the ““plum puddingplum pudding”” model model

Rutherford and ChadwickRutherford and Chadwick’’ss Atom Atom

In 1909 RutherfordIn 1909 Rutherford’’ssatom has a positiveatom has a positivenucleus surroundednucleus surroundedby a cloud of orbitingby a cloud of orbitingelectrons.electrons.

Rutherford suspected that there were other particles in theRutherford suspected that there were other particles in thenucleus of the atom because atomic masses are greater thannucleus of the atom because atomic masses are greater thanthe mass of the protons.the mass of the protons.

In 1932 Chadwick performs experiments andIn 1932 Chadwick performs experiments anddiscovers the discovers the neutronneutron in the nucleus of the in the nucleus of theatom.atom.

BohrBohr’’s Model of the Atoms Model of the AtomIn 1913, In 1913, Niels Niels BohrBohrproposed a model of theproposed a model of theatom that had the electronsatom that had the electronsorbiting the nucleus similarorbiting the nucleus similarto the planets orbiting theto the planets orbiting thesun. The electrons cansun. The electrons canonly go in certain orbits.only go in certain orbits.

Electrons can absorb energy and go to higher orbits or theyElectrons can absorb energy and go to higher orbits or theycan give off energy (radiation) and go to lower orbits.can give off energy (radiation) and go to lower orbits.

This is the model of the atom most people know, but quantumThis is the model of the atom most people know, but quantummechanics has proposed another model of the atom where themechanics has proposed another model of the atom where theproton and neutron are actually composed of smaller particlesproton and neutron are actually composed of smaller particlescalled quarks called quarks ……

Quantum Mechanics Model of the AtomQuantum Mechanics Model of the Atom1924-1926, De1924-1926, De Broglie Broglie, Heisenberg, Schrodinger all, Heisenberg, Schrodinger allcontribute ideas that led to the revision of thecontribute ideas that led to the revision of theelectron orbits of the Bohr model.electron orbits of the Bohr model.

•• DeDe Broglie Broglie - electrons are both particles and- electrons are both particles andwaveswaves

•• HeisenbergHeisenberg - can - can’’t know both the position andt know both the position andvelocity of an electron (by trying to measure thevelocity of an electron (by trying to measure thepath of an electron you alter the path)path of an electron you alter the path)

•• SchrodingerSchrodinger - treats electrons as waves instead - treats electrons as waves insteadof particlesof particles

Quantum ModelQuantum Model