Atomic History

What  is  an  Atom?  

•  An  atom  =  smallest  par.cle  of  an  element  that  retains  its  iden.ty  in  a  chemical  reac.on  

Ancient  Greece  

•  The  “stuff”  around  us  is  composed  of  .ny,  indivisible  par.cles;  called  atoms  

•  Four  elements:  Earth,  Air,  Fire,  and  Water  

•  *Not  supported  by  evidence  

John  Dalton  (1803)  

•  All  elements  are  composed  of  atoms,  which  are  indivisible  and  indestruc.ble  (hard  spheres)  

John  Dalton  (1803)  

•  Elements  are  characterized  by  the  mass  of  their  atoms    

– All  atoms  of  the  same  element  are  exactly  alike    (same  mass,  size)    

– Atoms  of  different  elements  are  different    (have  different  masses)    

How  Small  Are  Atoms?  

•  2010  world  popula.on:                      6,900,000,000  

•  Atoms  in  a  penny:  

 29,000,000,000,000,000,000,000  

•  US  Census  Bureau  

J.J.  Thomson  (1897)  

•  Used  cathode  ray  tubes  to  examine  charge  of  atoms                                  

J.J.  Thomson  (1897)  

J.J.  Thomson  (1897)  

J.J.  Thomson  (1897)  

•  Finds  that  atoms  are  not  indivisible  balls,  but  instead  have  a  substructure  

– Atoms  contain  nega.ve  par.cles  called  electrons  

– Total  charge  is  neutral,  atoms  must  have  posi.ve  par.cles  as  well  

J.J.  Thomson  (1897)  

•  “Plum  pudding”  model  of  an  atom:  

Ernest  Rutherford  (1909)  

•  Shot  alpha  par.cles  at  a  thin  foil  of  gold  

•  Expected  results:  

Ernest  Rutherford  (1909)  

Rutherford’s “Gold Foil Experiment”

Observation 1: Most Particles Pass Undeflected

Conclusion: Atom is Mostly Empty Space

Observation 2: Some Particles are Deflected

Conclusion: Atom Must Have a Small Dense Nucleus With a Positive Charge

Why  Was  This  Unexpected?  

•  Rutherford  likened  the  results  to  firing  a  large  ar.llery  shell  at  a  piece  of  paper  and  having  the  shell  come  back  and  hit  you!  

Ernest  Rutherford  (1909)  

•  Most  of  the  atom  is  empty  space    •  Each  atom  contains  a  small,  dense  (contains  

most  of  the  mass),  posi.vely  charged  nucleus    

Ernest  Rutherford  (1909)  

•  The  nucleus  is  surrounded  by  nega.vely  charged  electrons  

•  Model  of  the  atom:  

How  Dense  is  the  Nucleus?  

•  Imagine  the  nucleus  is  the  size  of  the  dot  in  an  exclama.on  point  

•  The  mass  would  be  equal  to  70  automobiles!  

How  Small  is  the  Nucleus?  

•  An  atoms  diameter  is  10,000  .mes  the  diameter  of  the  nucleus  

•  Atom  diameter  =  2  football  fields  

•  Nucleus  diameter  =  nickel  

Niels  Bohr  (1922)  

•  Electrons  orbit  the  nucleus  in  specific  “quan.zed”  energy  levels    

•  Model  of  the  atom:  

Many  researchers  (1950’s)    Wave  Mechanical  /  Electron  Cloud    

•  The  most  probable  place  to  find  an  electron,  most  densely  packed,  is  in  orbitals  of  different  energies  

•  Present  day  model  of  atom:  

Atomic  Timeline  Ancient Greece

1803 1897 1909 1922

Present

1803: Hard Sphere Model

All matter is composed of atoms that are unique for each element

1897: Plum Pudding Model (J.J. Thompson)

Atom has subatomic particles (substructure) with positive and negative charges

1909: Rutherford Model (Ernest Rutherford) “Gold Foil Experiment”

Atom is mostly empty space; contains a small, dense positively charged nucleus

1922: Bohr Model (Niels Bohr)

Electrons are said to orbit the nucleus like planets around the sun

Present: Wave Mechanical/Electron Cloud Model

Electrons are in a cloud around the nucleus; able to move between orbits