Download 3. all atoms of a given element are identical in mass and other

History of the Atom
Objective: know the historical development of the atom by reading and
presenting facts.
In groups of 4 or 5, you will read a portion of a test book or
article about a scientist who made a major contribution to
the history of the atom.
We will do the first one together as a class.
Who: Democritus
When: 2500 years ago or 488 B.C.E
What was their contribution: Defined the word "atomos" or tiny
indivisible, solid objects that make up all matter in the universe. Our
current word of atoms comes from his definition
If possible, how did they determine their contribution: he used logic and
never performed an experiment, as a result the theory was rejected
until 2000 years later
Additional Information deemed important: believed the atomos were of
different varieties of different shapes, sizes and masses (elements)
Who: John Dalton
When: Late 1700s to early 1800s (1803)
What was their contribution: Developed the modern atomic theory and the
concepts of atomic weight
If possible, how did they determine their contribution: Experimentation and
proposing the law of multiple proportions (when two elements react to form more
than one substance the ratio of the masses is in small whole numbers, 1:1, 2:1)
Additional Information deemed important: Dalton's Theory
1. matter is made of tiny particles - atoms
2. atoms are indivisible; during reactions atoms are rearranged, they do
not break apart and are not created nor destroyed
3. all atoms of a given element are identical in mass and other
properties (no longer true)
4. atoms of different elements differ in mass and other properties
5. atoms of one element can combine with atoms of another element to
form compounds
Who: J.J Thomson
When: 1897
What was their contribution: discovered the proton and electron
created the plum-pudding model of the atom
If possible, how did they determine their contribution: The use of a
cathode ray; the negative particles were attracted to the positive
portion
Additional Information deemed important: His plum-pudding model was
disproved by Rutherford years later
Who: Ernest Rutherford
When: 1912
What was their contribution: used gold-foil experiment to come up with
the positively charged nucleus and disproved the plum-pudding model
If possible, how did they determine their contribution: he used the goldfoil experiment - negative particles fired at gold-foil, thought they would
pass through but several particles bounced back (indicating a positive
center to an atom)
Additional Information deemed important: He was able to detect
positive and negative portions of an atom, but was unable to find the
neutrons (that came later)
Who: Henry Moseley
When: 1912
What was their contribution: theorized that the elements on the periodic
table should be arranged by atomic number (protons) instead of atomic
mass
If possible, how did they determine their contribution: used x-ray
spectra (wavelengths to determine the atomic number
Additional Information deemed important: he was able to adjust the
previous periodic table
Who: Niels Bohr
When: 1913
What was their contribution: modern atomic model (Bohrs Model)
If possible, how did they determine their contribution: through quantum
theory of atoms or there are energy level in atoms and atoms are in a
stationary or stable state; to change states, energy must be added or
released
Additional Information deemed important:became a nuclear physicists,
helped develop the idea of fission and the atomic bomb; became an
advocate for the peaceful use of nuclear energy
Who: James Chadwick
When: 1932
What was their contribution: discovery of the neutron
If possible, how did they determine their contribution: when working
with beryllium and using the conservation of energy and momentum he
was able to detect the neutron
Additional Information deemed important: was a major contributor in
the assembly of the atomic bomb
History of the Atom Timeline