History - theory

Original Atomic Model
• What if I take a sheet of paper and
tear it up, and keep tearing it up?
• Will we ever get to a “smallest piece
• Democritus – named the atom, from
“atomos” meaning indivisible
• Matter is made up of indivisible
particles-round spheres
• Dalton - one type of atom for each
Atomic Models through the
Dalton’s Atomic Theory 1808
• All elements are composed of atoms. Atoms
are indivisible and indestructible particles.
• Atoms of the same element are exactly alike.
• Atoms of different elements have different
• Compounds are formed by the joining of
atoms in simple, whole-number ratios.
• In chemical reactions, atoms are rearranged,
but not created or destroyed.
JJ Thomson’s Model
• Discovered electrons (e-)
• Noticed a deflection in a stream of
electrons (Cathode Rays) when a
negative charge was placed near it.
• Atoms have dispersed positive and
negative charges
• No order to the arrangement of the
• “Plum-Pudding” model
Millikan’s Oil Drop Experiment
• The measurement of the electron's charge
1909 and with Thomson's results also a value
for the electron mass was obtained. First
successful scientific attempt to detect and
measure the effect of an individual subatomic
particle. For this and his work on the
photoelectric effect Robert Millikan won the
1923 Nobel Prize in physics.
• Simple
• version
Rutherford’s Model
• Gold Foil Experiment 1911
• Discovered dense positive piece
at the center of the atom (nucleus)
• Shot positive alpha particles at a
sheet of gold foil and noticed “odd”
• Electrons moved around the
nucleus- planetary model
• Atoms are mostly empty space
Gold Foil
• Rutherford Models
Niels Bohr
• Student of Rutherford in 1913
• He wanted to know exactly where
the electrons are, so explains
spectral lines
He proposed that the electrons must
Move like planets around the sun in
circular orbits at different levels
Certain amounts of energy (quanta)
separate one level from another
Bohr’s Model of the Atom
Energy Levels
Bohr Model
•A way of representing electrons in
their “orbits”
•Every atom has its own Bohr model
•Energy Levels: Fill up the inner
levels as you go!
1st level
2nd level
3rd level
4th level
2 e8 e8 e2 e-
Bohr Model
for Lithium
James Chadwick
• He proved the existence of
neutrons.1932 James Chadwick proved
that the atomic nucleus contained a
neutral particle which had been
proposed more than a decade earlier by
Ernest Rutherford.
• Resulting in the solution of the puzzle for
the weight of atoms
The Quantum Mechanical Model
• Energy is quantized. It comes in chunks.
• A quanta is the amount of energy needed to move
from one energy level to another.
• Since the energy of an atom is never “in between”
there must be a quantum leap in energy.
• Schrödinger derived an equation that described the
energy and position of the electrons in an atom.
The Quantum Mechanical Model
• The atom is found inside a
blurry “electron cloud”
• An area where there is a
chance of finding an electron
is called an orbital
• Orbitals do not follow
circular (linear) paths, but
are instead 3-dimensional
• The reason for the “blurry” electron cloud
is that every electron is found within a
region of probability.
• The regions, or orbitals, take a variety of
sizes and shapes.
• These various orbitals stack on top of one
another creating the electron cloud.