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Democritus
• Ancient Greek philosopher and
mathematician
• Born 460 BC
• Died 370 BC
• Considered the father of atomic theory
for his idea that the world was made up
of small, indivisible parts called “atomos”.
• “Atomos” is the Greek word for
“uncuttable.” This is where we get the
current word “atom.”
• His ideas were not popular, as most
thinkers of the time held that the world
was made of four elements (earth, water,
air, fire) and that combinations of these
elements made up everything.
• It was only much later in science that his
ideas were confirmed.
Democritus’ Atom: a featureless sphere
with properties determined by size.
John Dalton
• British chemist, physicist, and
meteorologist
• Born 1766 AD
• Died 1844 AD
• Formulated the first modern version of
atomic theory:
1. All matter is made of indivisible
units called atoms.
2. All atoms of the same element are
identical.
3. Compounds (molecules) are formed
when two or more atoms bond
together.
4. Chemical reactions are the
rearrangement of atomic bonds.
• Color-blindness was often called
“Daltonism” due to his own color
blindness and the research he did on it.
Dalton’s Atom: like Democritus he
envisioned atoms as small, featureless
spheres.
J. J. Thomson
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British physicist
Born 1856 AD
Died 1940 AD
Credited with the discovery of the
electron, invalidating Dalton’s first atomic
law.
• Using cathode ray tubes, he was able to
determine that when a strong electric
current was run through it, the metals
would give off a small, highly charged
particle.
• He is also known for his discovery of
isotopes, invalidating Dalton’s second
atomic law.
Thomson’s Atom: also known as the
“plum pudding” model, Thomson
believed that a mass of positive charge
was embedded with small, negative
electrons.
Ernest Rutherford
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New Zealand physicist
Born 1871 AD
Died 1937 AD
Conducted a key experiment to test the
structure of the atom by firing alpha
particles at a gold foil.
1. If Thomson’s model was correct,
then the alpha particles should pass
straight through.
2. Instead, Rutherford had some alpha
particles bounce right back.
3. This led him to hypothesize that
there was a small positive nucleus.
Rutherford’s atom: a small, positive
nucleus with electrons orbiting around
it like planets around a sun.
Niels Bohr
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Danish physicist
Born 1885 AD
Died 1962 AD
One of the fathers of modern quantum
mechanics
• Considered both a scientist and
philosopher
• Was trying to solve two problems:
1. Why don’t negative electrons get
pulled into a positive nucleus?
2. Why does metal emit the same
wavelengths of light every time it is
heated?
• In order to solve these, he proposed his
new version of the atom.
Bohr’s Atom: electrons don’t exist in any
orbit, but in specific orbitals around the
atom. They cannot exist between
these.
Werner Heisenberg
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German physicist
Born 1901 AD
Died 1976 AD
One of the fathers of modern quantum
mechanics
• Commonly known for his “uncertainty
principle.”
• “Heisenberg’s Uncertainty Principle”
states that it is impossible to measure the
location and velocity of a particle at the
same time.
• Because of this uncertainty, electrons
were better viewed as “clouds of charge”
or “probability” rather than solid balls.
Heisenberg’s atom: electrons still exist
in set levels like Bohr, but rather than
having a specific location they are
spread out.