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The History of the Atom
Democritus 400 BC
• Greek philosopher
• Believed matter could be
cut and cut until you get
the smallest possible
piece
• The smallest piece would
be called atomis
• The eminent
philosophers
of the time,
Aristotle and
Plato, had a
more
respected,
(and
ultimately
wrong) theory.
Why?
Aristotle and Plato favored the earth, fire, air
and water approach to the nature of matter.
Their ideas held sway because of their
eminence as philosophers. The atomos idea
was buried for approximately 2000 years.
Dalton 1803
• Father of the atomic
theory
• Resurrected
Democritus's idea of
atoms
• He thought the atom
looked like a billard ball!
How do the ideas compare?
The big question….
• If it wasn’t Dalton’s idea to
begin with…. Why is he
called the father of the
atomic theory?
Dalton’s Atomic Theory
• 1) All elements are composed of atoms, which are
indivisible and indestructible particles.
• 2) All atoms of the same element are exactly alike; in
particular, they all have the same mass.
• 3) All atoms of different elements are different; in
particular, they have different masses.
• 4) Compounds are formed by the joining of atoms of two
or more elements. In any compound , the atoms of the
different elements in the compound are joined in a
definite whole-number ratio, such as 1 to 1, 2 to 1, 3 to 2,
etc.
What does it mean?
• Each element is made of a
different kind of atom
• Atoms combine to form
compounds.
• When they form
compounds they combine in
specific ratios- such as
water is always 2 hydrogens
and 1 oxygen!
Thomson- 1897-1898
Thomson agreed with
Dalton that everything
was made of atoms
He discovered the
electron
He proved that the
atom was not
indestructible and was
not indivisible- instead
it could be broken down
into smaller parts.
It was a great idea but…..
• Thomson faced two major problems:
• 1. How could he account for the mass of the
atom when the electron was only about
1/1000 the mass of the hydrogen atom (the
more modern figure is 1/1836)
• 2. How could he create a neutral atom when
the only particle he knew about was
negatively charged (the electron).
The plum pudding model
• He predicted that something
else would be discovered to
account for the mass of the
atom
• He predicted that the atom
would have a positive part.
Rutherford-1908-1911
• Ernst Rutherford proved that
the atom did have a positive
part (Thomson's idea)
• He discovered the positive
nucleus of the atom
• He discovered the proton
How did he make his discovery?
• His
experiment is
now famous!
•
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What else did Rutherford discover?
• Rutherford proved that the majority
of the atom is empty space!
Bohr-1913
• Neils Bohr discovered
that the electrons move
in energy levels
The Bohr Model
• The Bohr Model of the atom is still used today
in science classes!
Heisenburg- 1927
• Thomson was correct- the
electrons move in energy
levels but they move so
quickly that you can’t be
certain where they are…
• Heisenburg discovered
that electrons move so
quickly they form an
electron cloud!
What does his model look like?
Chadwick-1932
Even though both protons and
electrons had been discovered,
scientists still couldn’t
determine why the atom had
such a large mass.
Chadwick discovered the answer
when he discovered the
neutron
Scientists through time discovered the
atom has three main subatomic particles
• The proton is positive and has an atomic
mass of 1 amu (and is in the nucleus)
• The electron is negative and has
relatively no mass at all (and orbits the
nucleus)
• The neutron is neutral and has a mass of
1 amu (and is in the nucleus)
It didn’t end there…
• Kendall- Friedman and Taylor
• 1968-1969
• Discovered that protons and neutrons
were made of smaller particles
• The particles are called quarks
The proton
The neutron
What do we now know?
Atoms are the building block of all matter
• An Atom is the smallest part of an
element which can take part in a
chemical reaction.
• The atom consists of three fundamental
particles- the proton, the neutron and
the electron
The main subatomic particles
• The Proton has mass of approximately 1
atomic mass unit and a positive charge,
• The Neutron has a mass of approximately
1 atomic mass unit and no charge
• The Electron has a mass 1/1840 of the
proton and a negative charge.
Atoms make up elements
• Each element is made up of one kind of
atom only. Just over one hundred
different kinds of atoms are known.
• Ninety of these elements are naturally
occurring (i.e. they are found on the
earth), and the remainder have been
made artificially in nuclear reactors.
There is still more to be discovered….
What will the model we know
change with the new
discoveries?