Atomic Theory Timeline p.2
Theorist
Theory
Democritus
Around 400BC, Democritus
proposed the idea that all
matter can be cut down until
you find a single indivisible
particle, he called it the
atom
Fire- hot, combustible,
fueled by oxygen, Waterliquid, H2O, Earth- dry, solid,
Air- gas, cold
Structure of matter was
accepted for almost 2000
years.
Matter us made up of atoms
All atoms of an element are
the same
Billiards explain properties
of matter because each
billiard ball in a set has
identical atoms (like pure
gold)
But he could not explain
why some objects attract
each other
Atoms contain negatively
charged electrons
Since atoms are neutral, the
rest of the atom is a
positively charged sphere
Negatively charged atoms
are evenly distributed
throughout the atom
Thomson discovered that
extremely small negatively
charged particles could be
emitted by very hot
materials
Aristotle
John Dalton
The Billiard Ball Model
1807
J.J Thompson
Diagram (if applicable)
Ernest Rutherford
Chadwick
Niels Bohr
-In 1909 Rutherford shot
Alpha rays through a thin
piece of gold foil
-When the experiment was
performed, most of the
particles passed through the
foil unaffected, while a small
number of particles were
deflected at very large
angles
-Rutherford discovered the
proton in 1920
(1932) Through Chadwick’s
experiment that included
projecting alpha particles
towards beryllium, he
discovered that;
-An atom must be an empty
sphere with a tiny dense
central nucleus
-The mass of a neutron is the
same as a proton
-Negatively charged
electrons circle rapidly
through the empty space as
the nucleus
-A neutral atom has the
same # of protons as
electrons
The nucleus has the same
positively charged protons
and neutral particles called
neutrons.
-This model is only useful for
explaining the first 20
elements
-Each electron in an orbit
has a definite amount of
energy
-When an electron returns
to its original orbit, the
energy is given off as light.
Ii is useful for explaining
many chemical and physical
properties
-Positively charged protons
and neutral neutrons are in
the nucleus, and negatively
charged electrons orbit the
nucleus.