Download Atomic Models Through Time

Atomic Models
Through Time
Democritus
• Theorized that if you were to
cut something in half, then cut
it in half again and again…
• Eventually you would be left
with something too small to be cut.
• Therefore, Democritus gave us “atomos,”
or “uncuttable,” to be the word for the
smallest individual particle of a substance.
John Dalton
• Dalton’s Atomic Theory ~ 1808
– All matter is composed of atoms.
– Atoms of each element are the same size,
mass, etc., but are different from atoms of
other elements.
– Atoms can’t be subdivided (cut) or destroyed.
• Envisioned atoms as “tiny, solid balls”
Cathode Ray Tube
• If you run electricity through a gas filled tube,
– you see streams of particles
• How do we know they are particles?
– A cross left a shadow (something must be
traveling in a straight line), could be light…
– but J.J. Thomson put a magnet next to it and bent
the beam.
• Light is not magnetic!
• Beam had a negative charge
Video
J.J. Thomson
• Thomson’s Atomic Theory ~ 1897
– Discovered negatively charged electrons.
– Envisioned atoms as a “positively charged
sphere with electrons embedded in it.”
• “Muffin” Model
– Positively charged bread
– Negatively charged “blueberries”
floating around (electrons)
Hantaro Nagaoka
• Nagaoka’s Atomic Theory~ 1904
– Proposed a nucleus
– Electrons orbiting a positively
charged sphere in the center.
• “Planetary” Model
– Electrons = planets
– Positive Center Sphere = Sun
Alpha Particle Cannon
• Ernest Rutherford shot fast moving positively
charged alpha particles at thin gold foil…
– If Thomson’s model was
correct, the alpha particles
would pass right through the
foil.
Video
– Instead, some deflected back at large angles!
Alpha Particle Cannon
• Conclusions:
– Thomson’s model was inaccurate
– A new model must be created
1. The atom is mostly empty space
2. There is a dense, positively charged
nucleus in the center
Ernest Rutherford
• Rutherford’s Atomic Theory ~ 1911
– The atom is composed of mostly empty
space.
– Small, dense, positively charged nucleus
containing almost all of the atom’s mass.
• “Peach” Model
– Peach Pit = Nucleus
– Meat = Empty space with electrons
randomly orbiting nucleus.
Refining Atomic Theory
in the Early 1900’s
• Masses of protons are measured.
– Protons are 2000 times more
massive than electrons.
• Charge of protons is determined
to be positive.
– Protons provide equal & opposite
charge to electrons.
Niels Bohr
• Bohr’s Atomic Theory ~ 1913
– Electrons do not “randomly” orbit the
nucleus.
– Electrons move in specific “layers” or
“energy levels” / “shells”.
– Atoms absorb, or give off energy when
electrons move from one “level” to
another.
• Bohr’s Model
– “onion” model
• Electrons move on layers of an onion.
Valence Electrons
• Valence electrons are the electrons located
furthest from the nucleus
– They are a determining factor in chemical
bonding.
– If you visualize the atom like Bohr (an onion), the
valence electrons are the electrons located on the
outer layer.
New Problem
• As scientists find the masses of atoms,
they always have about twice the mass
predicted by the number of protons
– Remember, the mass of electrons is so small
it is insignificant
• So, there must be something else in the
atom…
James Chadwick
• Contribution to Modern Model ~ 1932
• Discovered the neutron
– Same mass as proton
– No electrical charge
• This is why it was not “discovered” earlier
• Existence of neutron explained why
atoms were heavier than the total mass
of the protons and electrons.
Modern Model
• Electron Cloud Model ~
1920’s to present
– Electrons form a
negatively charged cloud
around the nucleus.
– It is impossible to
determine exactly where
an electron is at any given
time. (probability instead)
– Higher probability of
electrons near nucleus.