4.1 Studying Atoms

Democritus(Greek philosopher 500 B.C.)– 1st to mention the word “atom” (means “cannot be divided”)

  • All matters are made of smallest particles called “atoms” (atomos: Greek word for indivisible)
  • Different matters have different atoms

Dalton’s Atomic Theory(England 1803 – 1st scientist proved the existence of atom through experiments)

Methods:

  • Studied behaviors of gases
  • Concluded that a gas is made of particles.
  • Measured the masses of elements that combine to make a compound

Theory:

  • All elements are made of atoms()
  • Atoms are indivisible. (×)
  • All atoms of the same element have the same atoms (same mass) ; atoms of different elements have different atoms (different masses) (×)
  • Compounds have a fixed ratio of composition of elements. (Proust’s Law of Definite Composition)compounds are made of more than one kind of atom; atoms in a compound always combine in same way. ()
  • During chemical reactions atoms are neither destroyed nor created but simply rearranged. (Lavoisier’s Law of Conservation of mass)()

Thomson’s Model of the Atom (England 1897 – discovered electrons)

Experiments:

  • used electric current = flow of charged (+/-) particles
  • Used cathode ray tube – sealed glass tube – most air removed – metal disks at each side– passed electric current in the tube  straight beam
  • Put metal plates on each side – one with (-)[1]charge, another with (+)[2] charge
  • Then beam deflected (bent) toward (+) plate
  • Calculated ( of hydrogen atom) so electrons(- charges) are very small
    Thomson’s Conclusion:
  • Atom is made of subatomic particles (smaller particles in an atom)  particles in the beam
  • Subatomic particles have (+) and (-) charges

-Particles in the beam must be (-) because the beam was deflected toward the (+) plate.

-But atom is neutral. So, there must be (+) particles.

Thomson’s Model:

  • “plum pudding”(or “cupcake”) model – (+) and (-) charged particles are evenly scattered in the atom.

Rutherford’s atomic theory(1911 – discovered nucleus)

Experiment

  • Hypothesized (based on Thomson’s “plum pudding” model) (alpha) particleswith (+) charge from U (uranium) would go straight through gold.
  • Gold foil experiment:


Result

  • Most particles went straight.
  • 1 out of 20,000 particles bounced back (deflected by more than 90°) (+) charge is not evenly scattered in the atom


Conclusion

  • Something positive and dense is at the center.--> a small positive thing at the center of an atom.-“nucleus”(where particles bounced) - most of atom is empty space (where particles went through)
  • Most atom is not positive as Thomson believed. Only a small part is positive.

Rutherford’s model

Nucleus’ volume ×1012= atom’s volume

(marble: Yankee stadium=nucleus: atom)

[1]positive

[2]negative