Download History of the Atomic Theory

History of the Atomic Theory
Chapters 3 and 11
Major Contributors
• Democritus – Greek philosopher who believed
that matter was made up of small indivisible
particles he called atoms
Aristotle – also a Greek philosopher, disagreed with Democritus
and said matter was made up of 4 elements: earth, wind, fire, and
water
(The idea of atomism died for a while. In the
meantime, alchemists worked with matter,
believing they could turn ordinary metals into
gold)
• John Dalton – compiled knowledge about
matter into the first Atomic Theory.
– Dalton’s theory incorporated the works of
other chemists, like Antoine Lavoisier (his
work led to the Law of Conservation of Mass)
and Joseph Proust (his work led to the Law
of Definite Proportions)
Dalton’s Theory
• Each element is made of extremely small,
indivisible particles called atoms. *
• All atoms of a given element are identical to each
other, but different from those of another
element.*
• Atoms are neither created nor destroyed in a
chemical reaction.
• A given compound always has the same ratio of
the atoms that make it up
(* we know today that these two are not true)
• J.J. Thomson – did experiments with the CRT
(which was invented by William Crooks)
– The problem with Dalton’s theory is the idea
that atoms are indivisible spheres
• Based on evidence from his experiments,
Thomson proposed a new model. In this model,
atoms were thought to be a sphere of positive
charge with negative charges randomly
embedded within the sphere. His model is called
the Plum Pudding Model (or Raisin Bun Model)
• Thomson experiments did not provide any
direct evidence of individual negative
particles versus a stream of energy with a
negative charge. R.A. Millikan performed
the Oil Drop experiment and was able to
determine the mass (9.11 x 10-28 grams)
and charge (1.60 x 10-19 Coulombs) of an
individual electron, providing evidence
that electrons are discrete particles.
• Ernest Rutherford decided to test
Thomson’s Plum Pudding Model, by
bombarding gold atoms with alpha
particles which are positively charged (the
Gold Foil experiment). If Thomson’s
model were correct, all of the particles
should have passed straight through.
• What Rutherford found was that 99% of
alpha particles went straight through the
gold foil, but 0.5% deflected, and 0.5%
reflected! The ‘Plum Pudding’ model was
wrong.
• He proposed that All of an atom’s positive
charge is concentrated in a very small core at
the atom’s center, which Rutherford called
the nucleus. The negatively charged
electrons move around the nucleus.
• Henry Moseley – a student of Rutherford’s who
determined the number of positive charges in the
nuclei of most atoms known at the time; said that
all atoms of a given element had the same
number of positive charges (protons); rearranged
Mendeleev’s periodic table
• Niels Bohr, a theoretical physicist,
addressed a fundamental problem with
Rutherford’s model: This model couldn’t
exist because electrons should lose energy
and collapse into the nucleus. He added the
concept of energy in quanta (small packets)
to Rutherford’s Model.
• Main Concepts of Bohr’s Model
1. Electrons assume only certain orbits around
the nucleus. These orbits are stable and called
"stationary" orbits.
2. Each orbit has an energy associated with it.
For example the orbit closest to the nucleus has
an energy E1, the next closest E2 and so on.
3. Light is emitted when an electron jumps from
a higher orbit to a lower orbit and absorbed when
it jumps from a lower to higher orbit. Only
whole number jumps allowed.
• Building on the work Neils Bohr (and other
scientists like Max Planck, Albert Einstein,Louis
deBrogelie, & Werner Heisenberg), Erwin
Schroedinger developed a wave equation to
describe regions outside the nucleus where
electrons are likely to be found. These regions
are called orbitals, and the model developed by
Schroedinger is called the wave mechanical
model.
• James Chadwick was a coworker of
Ernest Rutherford who discovered a
second nuclear particle that was equal in
mass to a proton but had a neutral charge.
He called these particles neutrons.