The History of the Atom

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The History of the Atom
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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

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Why?

• The eminent philosophers of the time, Aristotle
  and Plato, had a more respected, (and ultimately
  wrong) theory.

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.
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Dalton 1803

• Father of the atomic theory
• Resurrected Democritus's idea of atoms
• He thought the atom looked like a billard ball!

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How do the ideas compare?
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The big question.

• If it wasnt Daltons idea to begin with. Why is
  he called the father of the atomic theory?

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Daltons 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.

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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!

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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.
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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).

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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.

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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

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How did he make his discovery?

• His experiment is now famous!

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• http//chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHE
  RFD.html

• http//chemmovies.unl.edu/ChemAnime/RUTHERFD/RUTHE
  RFD.html

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What else did Rutherford discover?

• Rutherford proved that the majority of the atom
  is empty space!

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Bohr-1913

• Neils Bohr discovered that the electrons move in
  energy levels

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The Bohr Model

• The Bohr Model of the atom is still used today in
  science classes!

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Heisenburg- 1927

• Thomson was correct- the electrons move in energy
  levels but they move so quickly that you cant be
  certain where they are
• Heisenburg discovered that electrons move so
  quickly they form an electron cloud!

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What does his model look like?
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Chadwick-1932

• Even though both protons and electrons had been
  discovered, scientists still couldnt determine
  why the atom had such a large mass.
• Chadwick discovered the answer when he discovered
  the neutron

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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)

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It didnt 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
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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

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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.

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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.

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There is still more to be discovered.
What will the model we know change with the new
discoveries?