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Atomic Theory Timeline
400 B.C. 350 B.C
Democritus Aristotle
1903
1907
1913
Rutherford Millikan Bohr
0
1743-1794
1800
Lavoisier Dalton
1900
Thomson
Democritus
 In 400 B.C. Democritus believed all existence consisted of
two things:
 atoms (coined the term atom from the Greek
word ‘atomos’ meaning indivisible)
 the void (space)
 Democritus’ theory:
 atoms are particles in space (the void)
 atoms always in motion; make life predictable
 matter could not be divided indefinitely; atoms are
indivisible
 all things we see differ because of the shape, size,
and arrangement of atoms
Aristotle
 In 350 B.C. Aristotle and his followers dismissed the idea
of atoms
 Instead modified an earlier theory that matter was made
up of four ‘elements’:
 earth
 fire
 water
 air
 Even though Aristotle was wrong his theory persisted
for 2000 years!
Lavoisier
 During his lifetime, 1743-1794, Antoine Lavoisier established
the Law of Conservation of Matter
 Law= in a chemical reaction, the mass of the reactants is equal to
the mass of the products
 Theory stated that mass is concrete and measurable, rather
than ‘magical’
Dalton
 In 1800 John Dalton was examining gases dissolved in water
and decided that matter was made of individual particles
with spaces between them
 Devised a chemical atomic theory:
 all matter is made of atoms
 atoms of an element are identical
 each element has different atoms
 atoms of different elements combine in constant ratios
to form compounds
 atoms are rearranged in reactions
Thomson
 In 1897, looking at charged particles, J.J Thomson
found particles that had a charge that did not change
regardless of the metal they came from…
 He called these charged particles electrons
(negatively charged particles)
 Later he proposed a model for the composition of the
atom called the ‘raisin bun’ or ‘plum pudding model’
although he was proved to be incorrect
Adding Electrons to the Model
 Materials, when rubbed, can develop a charge difference.
This electricity is called ‘cathode rays’ when passed
through an evacuated tube
 These rays have small mass and are negative
 Thomson noted that these negative subatomic particles
were a fundamental part of all atoms
 1) Dalton’s ‘Billiard Ball’ model (1800-1900):
 atoms are solid and indivisible
2) Thomson’s ‘Plum Pudding’ Model (1900):
 negative electrons in a positive framework
3) The Rutherford Model (around 1910):
 atoms are mostly empty space
 negative electrons orbit a positive nucleus
Rutherford
 Used radiation to unlock clues to the atom; alpha particle
(α) -positive charge and beta particle(β) -negative charge
 Ernest Rutherford short alpha (α) particles at gold foil
 Most particles passed through; therefore, atoms are
mostly empty
 Some positive alpha deflected or bounced back!
 Thus, a ‘nucleus’ is positive and holds most of an atom’s
mass
Millikan
 In 1907 Robert Millikan found that all electrons are
identical
 Millikan was able to find the charge of a single electron
Bohr
 Around 1913 Niels Bohr discovered that electrons orbit
the nucleus in ‘shells’
 Electrons can be bumped up to a higher sell if hit by
an electron or a photon of light
 It is when electrons fall back down energy level(s)
that they release a photon
 There are two types of spectra :
 continuous spectra
 line spectra
 These jumps down from shell to shell account for
the line spectra seen in gas discharge tubes
(through spectroscopes)