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