A History of the AtomA History of the Atom
A long time to study a little thingA long time to study a little thing
IntroductionIntroduction
The PhilosophersThe Philosophers
Ancient GreeksAncient GreeksFormed ideas of nature based on their Formed ideas of nature based on their
experiencesexperiencesDid no experiments, just thoughtDid no experiments, just thought
Everything made of :Everything made of :
Earth Air
FireWater
Aristotle Aristotle (384–322 B.C.) (384–322 B.C.)
DemocritusDemocritus(460 – 370 B.C.)(460 – 370 B.C.)
Greek philosopher (not scientist) Idea of the atom (smallest particle)Named the atomDifferent kinds of atoms have different sizes Different kinds of atoms have different sizes
and shapes.and shapes.Apparent changes in matter result from Apparent changes in matter result from
changes in the groupings of atoms and not from changes in the groupings of atoms and not from changes in the atoms themselves.changes in the atoms themselves.
Democritus and AristotleDemocritus and Aristotle
The AlchemistsThe Alchemists
Rearrange the basic elementsRearrange the basic elementsWanted to turn lead in goldWanted to turn lead in gold
ElementsElements
We now know that there are 92 basic We now know that there are 92 basic elementselements
The type of element is determined by the The type of element is determined by the number of protons in the atomnumber of protons in the atom
Modern elemental theoryModern elemental theory
John DaltonJohn Dalton(1766-1844)(1766-1844)
First true atomic theoryAll matter is composed of atoms.All atoms of a given element are
identical and different from those of any other element.
Atoms combine in simple whole number ratios to form compounds.
In a chemical reaction, atoms are separated, combined, or rearranged.
Dalton’s Particle TheoryDalton’s Particle Theory
By the late 1800s…By the late 1800s…Cathode ray tube (aka Crook’s tube) inventedCathode ray tube (aka Crook’s tube) invented
JJ ThomsonJJ Thomson(1897)(1897)
Cathode rays made of Cathode rays made of particles smaller than particles smaller than atomsatoms
First subatomic particlesFirst subatomic particlesDiscovered electronPlum pudding model of
the atom
J. J. ThomsonJ. J. Thomson
Plum pudding modelPlum pudding modelAtom is a sphereAtom is a spherePositive charge is evenly distrubutedPositive charge is evenly distrubutedNegatively charged electrons spread Negatively charged electrons spread
randomly through the sphererandomly through the sphere
Questions raised by ThomsonQuestions raised by Thomson
If electrons are particles smaller than If electrons are particles smaller than atoms, are there other particles?atoms, are there other particles?
What makes up most of the mass of the What makes up most of the mass of the atom?atom?
If electrons are negatively charged, why If electrons are negatively charged, why are atoms neutral?are atoms neutral?
The proton/electron atomThe proton/electron atom
Ernest RutherfordErnest Rutherford(1911)(1911)
Aimed a beam of “alpha particles” at Aimed a beam of “alpha particles” at a thin gold foil a thin gold foil
The beam made a bright spot on a The beam made a bright spot on a fluorescent screenfluorescent screen
Lead block
Uranium
Gold Foil
Florescent Screen
What Rutherford expectedWhat Rutherford expected
What Rutherford expectedWhat Rutherford expected
What Rutherford sawWhat Rutherford saw
What Rutherford sawWhat Rutherford saw
+
Rutherford concludedRutherford concluded
There had to be a heavy central core to the atom: the nucleus
Nucleus contains the protons
Rutherford and the Structure of Rutherford and the Structure of AtomsAtoms
James ChadwickJames Chadwick(1932)(1932)
Experimenting with radiation Experimenting with radiation sourcessources
Discovered a new particle with no charge : the neutron
Niels Bohr Niels Bohr (1913)(1913)
Student of Rutherford and ThomsonStudent of Rutherford and ThomsonElectrons travel in orbitals around the Electrons travel in orbitals around the
nucleusnucleusElectron can move from one orbital to Electron can move from one orbital to
another with input or release of energyanother with input or release of energyMore on this later…More on this later…
Bohr’s atomic modelBohr’s atomic model
Structure of the atomStructure of the atom
99.97% of 99.97% of mass in mass in nucleusnucleus
Most of Most of the volume the volume is empty is empty spacespace
Electrons Electrons in cloudin cloud
How big is an atom?How big is an atom?Simulate the size of a hydrogen atom:Simulate the size of a hydrogen atom:
Nucleus : place a baseball on the 50 yard line Nucleus : place a baseball on the 50 yard line of Reliant Stadiumof Reliant Stadium
Electron : put a grain of sand on the back row Electron : put a grain of sand on the back row of the highest sectionof the highest section
Size of AtomsSize of Atoms
Atomic NumberAtomic Number
Atomic MassAtomic Mass
IsotopesIsotopes
Atomic Number, Mass, & Atomic Number, Mass, & IsotopesIsotopes