Download History of the Atom

From Democritus to Chadwick
Ancient History
Democritus
 An Ancient Greek Philosopher
 Claimed that all things were made up of tiny particles
called atoms
Aristotle
 Claimed that all matter is made up of 4 elements




Earth
Water
Fire /Heat
Air
John Dalton: Billiard Ball Model
 Claimed that all matter was made up of tiny
particles that could not be broken down into
anything smaller -- called atoms
 Model: Billiard Ball
 Hard, indestructible ball
Dalton’s 5 Postulates (Ideas)
 All matter is made up of small particles called
atoms
 Atoms cannot be created, destroyed or divided
into smaller particles
 All atoms of the same element are identical in
mass and size
 All atoms of different elements differ from each
other in mass and size
 Compounds are created when atoms of different
elements link together in different proportions
J.J. Thomson: Plum Pudding Model
(Raisin Bun Model)
 JJ discovered that matter had separate negative
particles (electrons)
 Claimed that an atom could be broken down
into positive and negative parts.
 The tiny electrons (negatively charged
particles) were embedded within a “cloud” of
protons (positively charged particles)
 Model: Raisin Bun
 Raisins= electrons
 Bun = protons
 Dominant model for many years.
Ernest Rutherford
 A Scientist working for JJ Thomson
 Famous for “Gold Foil Experiment”
 Fired alpha particles (positively charged helium
nuclei) at a thin piece of gold foil
 Expected that the heavy, positively charged
particles would pass straight through the atoms of
gold in the foil (based on Raisin bun model).
 NOT WHAT HE FOUND…WATCH…
Rutherford’s Results
Alpha particles
 Went straight through
 Went through but on angles,
 Even bounced back (straight/on angles)
Conclusions
 The positive charge and mass of an atom must
be concentrated in a very small area at the
centre of an atom -- called the “nucleus”
 Electrons were a problem: Rutherford left them
in clouds around the nucleus which can’t work
as they would fall into the nucleus.
Rutherford Model
Electron
Clouds
Nucleus
Niels Bohr
 A Scientist who used the solar system to model
the atom!
 Claimed that the electrons must orbit the
nucleus in fixed energy levels -- these levels are
called shells
 The nucleus contains protons
Chadwick
 A Scientist working for Rutherford
 Discovered that the nucleus contains protons and ALSO
NEUTRONS
 Neutrons are neutral (do not carry a charge)
 These are what we call “Bohr-Rutherford diagrams”
Bohr-Rutherford Diagrams
Atom Parts
 http://www.youtube.com/watch?v=O5iaw5WNuB0
Protons
 Positively charged particles
 Located in the nucleus (centre) of an atom
 Using the Periodic Table: the Atomic
Number (Z) tells how many protons exist
in an atom
 Symbol: p+
Electrons
 Negatively charged particles
 Smallest Sub-atomic Particle
 Located outside of the nucleus (centre)
 Orbit around the nucleus in rings (shells)
 Using the Periodic Table: # of Electrons are equal
to # of Protons in atoms (not ions)
 For atoms the # of electrons is equal to the Atomic
Number (Z)
 Symbol: e-
Neutrons
 Have a neutral charge or “no charge”
 Located in the nucleus (centre) of an atom
 Atomic Mass = protons and neutrons together
 Using the Periodic Table: neutrons are equal to
Atomic Mass (A) - Atomic Number (Z)
 Symbol: no
Atomic Info
Atom quick representation
A
Z
X
Example: sodium is element #11
and has a mass of 23.
It can be represented as
23
11
Na
It has 11 protons, 11 electrons and 12
neutrons! (23-11 =12)
Class/Home work
 Pg 233 #3, 4, 5 AND pg 240 #1, 3, 6