Early History of
450 BC – Empedocles
"There are 4 elements;
Earth, Air, Fire and Water"
400 BC – Democritus
matter could be cut into smaller and smaller
pieces until it reached an atom
“atomos” meaning indivisible
No evidence – based on thought
350 BC – Aristotle
The Greek philosopher
opposed Democritus' idea of atoms and dismissed
it as worthless and promoted the 4 element
Aristotle had great influence so the 4 element
model lasted for more than 2000 years in the
Believed that elements were all basically the
same, but with different levels of purity.
Gold was the purest element, and alchemists tried
to change different metals into gold.
Considered to be serious science up until the
They used equipment to do experiments
Found uses of elements, not explaining their
Contributions to the
Modern Atomic Theory
Robert Boyle (1660)
"There are not just 4 elements!“
Controlled experiments, gas law
Joseph Priestly (1773)
Discovered oxygen, also discovered pop!
Contributions to the
Modern Atomic Theory (cont’d)
Antoine Lavoisier (1778)
made first balance, leads to;
Law of Conservation of Mass
Joseph Proust (1790)
Law of Definite Proportions
Beginning of Scientific Method , start
to explain behaviour
John Dalton (1805)
Combined several ideas and experiments into one
Used to predict the outcome of other
His work supported:
The Law Conservation of Mass
The Law of Definite Composition
The Law of Multiple Proportions
Model: Indivisible billiard ball
J.J. Thomson (1897)
Used the work of other scientists that
showed that atoms contain charges, and his
own work with cathode ray tubes.
Atoms are positive spheres, with negative
particles (electrons) embedded in
Model: Raisin Bun
Ernest Rutherford (1911)
Student of Thomson – disagreed.
Gold Foil Experiment:
Alpha radiation (positive) were shot at a piece
of gold foil.
Most of the alpha particles passed through
the foil, suggesting that most of matter is
Some particles were deflected back at angles;
they had come in contact with something very
Gold Foil Experiment:
97% was made up of empty space
3% showed a very DENSE positive space
Detection was possible due to radioactivity 12
Leading to…..Nuclear Model:
proposed the name "proton" for the positively
charged particles and the opposite charged
particles were named “electrons”
Chadwick worked with Rtuherford and discovered
the "neutron“ using a mass spectrometer. This
Model: Positive nucleus with electron cloud
Problem with Rutherford
If electrons are orbiting
around a nucleus, they are
19th century physics - if
Result: would eventually
collapse into the nucleus and
cause a black whole effect
Matter is fairly stable,
Rutherford’s model is not.
Max Planck (1900)
Matter, at the atomic level, can absorb or emit only
discrete quantities of energy; the energy of an atom is
quantized, not continuous!
A "quantity" or "package" of energy is now called a
Analogy: Money Analogy. All money can be expressed in terms
of the smallest “quantum”, the penny. You do continuously
lose money, you lose is packages i.e. $5 here, $3.25 there
that are all multiples of the 1 cent quanta.
This was a radical hypothesis! It made Planck
Here comes Einstein
Albert Einstein concluded light
consists of photons which are
essentially a quantum of energy.
Different wavelengths (λ)
represent different energies
Explained the “Photoelectric
Effect” (metal gives off
electrons when bombarded with
heat) won noble prize
Niels Bohr (Model for H Atom)
The energy of the electron is directly proportional to its
distance from the nucleus.
When light is absorbed, the electron jumps from a lower
energy level to a higher energy level. (exicted state)
When light is emitted, the electron falls from a higher
energy level to a lower energy level. (ground state)
Saw EMR spectrum (different colours = different
wavelengths = different energies, thus electrons are in
different energy levels).
Electrons must absorb or emit a specific quantum
of energy (like stairs).
Electrons do not continuously give off energy as
they orbit. They exist in constant energy states.
Since elements have a different number of
electrons, each element will have a different line
spectra (like a fingerprint)
Model: Protons and neutrons in nucleus with
Present Day Model
Bohr model only supported by
hydrogen (single electron system)
Louis de Broglie (1923)
Made the hypothesis that a particle can act like
a wave and particle and that each has a specific
quantum of energy.
If it can be a wave it can be described
Proved this mathematically, proven
experimentally later by G.P. Thomson
Used de Broglie’s idea to describe electrons as
Developed wave equations to describe the
energy and behaviour of electrons.
These equations give electrons a specific number
(like a postal code). Each number is the general
orbital (n) where each electron is found, the
orbital shape (l), orientation in space (ml) and
Schrodinger’s Cat Analogy
Had a box sealed with a
Now placed a vial of
poison gas, which would
instantly kill the cat.
The vial is hooked up to an apparatus which is
wired into a Geiger counter, a device used to
detect radiation. The radioactive atom is placed
near the Geiger counter and left there for
exactly one hour.
Schrodinger’s Cat Analogy (cont’d)
If the atom decays, then the Geiger counter will
detect the radiation, break the vial, and kill the
cat. If the atom does not decay, then the vial
will be intact and the cat will be alive.
After the one-hour period, the atom is in a state
where it is both decayed and not-decayed.
However, given how we've constructed the
situation, this means that the vial is both broken
and not-broken and, ultimately, according to the
Copenhagen interpretation of quantum physics
the cat is both dead and alive.
(Heisenberg Uncertainty Principle)
There is no experimental evidence that an
electron has a definite path, or orbit, around the
nucleus as previously thought (Bohr).
Can’t know exact momentum and position of the
electron at the same time
Now we use the term orbital (NOT orbit) to
describe the region of space where electrons may
Schrodinger's wave equations can be manipulated
to produce a 3D probability distribution of the
electron in an orbital specified by quantum
numbers. (electron probability density) 23