Download Democritus 440 BCE

Democritus
• Greek philosopher
• Came up with the idea
of an atom
• Named it “atomos ”
meaning a piece of
matter that was
“not able to be divided”
440 BCE
John Dalton
1803
Developed the Atomic theory:
• All substances are made of
atoms. Atoms are small
particles that cannot be
created, divided, or
destroyed.
• Atoms of the same element
are alike, atoms of different
elements are different
• Atoms join together
to make new
substances.
JJ Thomson
• Discovered negatively
charged particles inside
the atom now called
electrons
• Proposed the plumpudding model
1897
Ernest Rutherford
• Tested Thomson’s
theory using Gold-foil
experiment
• Proposed that in the
center of the atom is a
dense, positively
charged part called the
nucleus
1909-1911
Niels Bohr
• Proposed that
electrons move around
the nucleus in certain
paths called energy
levels
• Electrons can jump
from one level to the
next
1913
Schrodinger and Heisenberg
20th century
• Electrons do not travel
in definite paths
• There are regions
where electrons are
likely to be found
called electron clouds