Download Dalton`s Atomic Theory

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 (-)1charge, another with (+)2 charge
 Then beam deflected (bent) toward (+) plate
 Calculated
1
2
positive
negative
1
mass
(
of hydrogen atom) so electrons(- charges) are very small
charge 2000
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) particles with (+)
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
3 types of radiation
Alpha particles –(+) charge so go toward (-) charge
Beta particles – (-) charge so go toward (+) charge
Gamma rays – no particle but rays no charge.
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)