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Atomic Theory ScientistsThe history of the atom.
HISTORY OF THE ATOM- Democritus
460 BC Democritus develops the idea of atoms
He pounded up materials in his pestle and
mortar until he had reduced them to smaller
and smaller particles which he called
•ATOMOS
•(greek for indivisible)
•This is where we get the word “atoms”
History of the Atom- Aristotle
• Aristotle said there were four elements:
– Earth
– Air
– Water
– Fire
Boyle
• Boyle rejected the idea that there were 4 elements
– Impossible to combine the 4 elements to form a substance
– Impossible to extract these elements from a substance
• Boyle defined an element as any substance that cannot
be decomposed into a simpler substance
HISTORY OF THE ATOM- Dalton
1808
John DaltonDalton's Atomic Theory
1) All matter is made of atoms. Atoms are indivisible
and indestructible.
2) All atoms of a given element are identical in mass
and properties
3) Compounds are formed by a combination of two or
more different kinds of atoms.
4) A chemical reaction is a rearrangement of atoms.
Dalton’s “Billiard Ball Model”:
Atoms are the smallest type of matter and they
are entirely solid like a billiard ball
HISTORY OF THE ATOM- Thomson
Joseph John Thomson
• Found the electron using the cathode ray tube because of
its negative charge
• He found that atoms could sometimes eject a far smaller
negative particle which he called an ELECTRON .
• Also developed the Plum Pudding Model of the atom
Thomson’s Plum Pudding Model
Thompson develops the idea that an atom was made up of
electrons scattered unevenly within an elastic sphere surrounded
by a soup of positive charge to balance the electron's charge
like plums surrounded by pudding.
PLUM PUDDING
MODEL
History of the Atom- Millikan
• Discovered the charge and mass of an electron by his famous oil drop experiment
– The experiment entailed observing tiny charged droplets of oil between two horizontal metal electrodes. This enabled him to measure the charge of an electron
– calculated the mass of an electron, which is 9.1 × 10-
31 kg
Robert Millikan
What’s next?
• Two inferences:
– Something had to account for the mass of the atom since the electron doesn’t contribute to it
– Since the charge on an atom is neutral, there must be a positively charged area to balance out the negative charge of the electron
HISTORY OF THE ATOM- Rutherford
Ernest Rutherford
– He used his Gold Foil experiment to prove Thomson’s plum
pudding model wrong, discovered protons, and developed the
idea of a nucleus
– He passed particles through gold foil, and realized not
everything passed through, so there must be something other
than just a negative charge
– He developed the Nuclear Atom model or the Planetary model
of the atom
– Most of the mass of the atom is in the nucleus and electrons
orbit the nucleus like planets orbit the sun
HISTORY OF THE ATOM
gold foil
helium nuclei
They found that while most of the helium nuclei passed
through the foil, a small number were deflected and, to their
surprise, some helium nuclei bounced straight back.
helium nuclei
History of the Atom- Chadwick
• Chadwick took Rutherford’s research further and found the neutron
• The problem was the atomic number was less than the atomic mass (average mass of the atom). For example, a helium atom has an atomic mass of 4, but an atomic number (or positive charge) of 2.
Since electrons have almost no mass, it seemed that something besides the protons in the nucleus were adding to the mass. One leading explanation was that there were electrons and additional protons in the nucleus as well -- the protons still contributed their mass but their positive charge was canceled out by the negatively charged electrons. idea that there could be a particle with mass but no charge. He called it a neutron, and imagined it as a paired proton and electron.
James Chadwick
Niels Bohr
1913
Niels BohrBohr refined Rutherford's idea by adding
that the electrons were in orbits. Like
planets orbiting the sun. With each orbit
only able to contain a set number of
electrons. This is called Bohr model or
Quantum model of the atom.
Schrodinger
Bohr’s model only fits hydrogen and was proven
incorrect by Schrodinger.
Schrodinger suggested that orbits were not
always circular, but in a pattern. He called them
orbitals.
Schrodinger’s model is called the Quantum
Mechanical Model of the atom
Heisenberg Werner Heisenberg
• The Heisenberg uncertainty principle states that it
is impossible to know simultaneously the exact
position and momentum of a particle.
• That is, the more exactly the position is determined,
the less known the momentum, and vice versa.