624-574
BC
Thales
of
States
that water is the simplest
substance on Earth
500-428
BC
Anaxgoras
and Empedocles
Changes in
matter are due to changes in
indivisible particles
484-428
BC
Empedocles
Separated
indivisible matter into four
elements; earth, water, fire and air.
460-370
BC
Democritus
Matter is
made of small indivisible particles
which have different form, position and arrangement.
Democritus named these particles atoms.
384-322
BC
Aristotle
Gathered
all theories up to date and was
responsible for formalizing the knowledge into one idea.
Aristotle was leery that Democritus’s atoms
didn’t account for the great variance of matter.
1214-1294
Rodger
Bacon
Taught
that in order to understand the
natural universe there must first be observation, allowing for evidence
to come
from the natural world.
1704
Isaac
Newton
The
universe is a mechanical universe with
small solid masses in constant motion
1803
John
Dalton
Developed
an atomic theory stating that
spherical atoms had measurable properties of mass
1832
Michael
Faraday
Split
molecules with electricity by means of
electrolysis, developed laws of electrolysis
1859
J.
Plucker
Built
the first cathode ray tube, used for gas discharge
1873
James
Clerk Maxwell
Magnetic
and electric fields filled “empty”
space in atoms
1879
Sir
William Crookes
Studied
the properties of cathode rays and
found that they exhibit negative charge and mass.
1886
E.
Goldstein
1895
Wilhelm
Roentgen
Used
cathode ray tubes to observe that nearby
chemicals glowed and the penetrating rays coming from the cathode ray
tube were
not affected by magnetic fields. He
named these rays, X-Rays.
1896
Henri
Becquerel
Discovered
that some chemicals spontaneously
decompose and give off penetrating rays while working with X-rays and
photography paper.
1897
J.J.
Thomson
Discovers
the electron and used cathode ray
tubes to determine the charge to mass ratio of an electron to be 1.759
x 108
Coulombs/gram.
J.J.
Thomson
Found
canal rays were associated with a
proton, H+.
1898
Marie
Skodowska Curie
Named the
spontaneous decay process of
uranium and thorium to be radioactivity.
1899
Ernest
Rutherford
Discovers
alpha and beta rays emitting
from radium.
1900
Soddy
Discovered
that radioactive elements have
isotopes, and half lives. Made
preliminary calculations concerning the energy released during decay.
Max
Planck
Used
quanta, discrete units of energy
to explain glowing hot matter.
Pierre
Curie
While
working on radioactive
substances, Curie discovers gamma rays.
1903
Nagaoka
Saturnian
model of the atom where there is
flat rings of electrons revolving around a positively charged particle.
1904
Abegg
Inert
gases have stable electron
configurations.
Ernest
Rutherford
Discovers
alpha rays are heavily positively
charged particles.
1905
Albert
Einstein
Publishes
theory on special relativity and
states that matter can be converted into energy.
1906
Hans
Geiger
Used
an electrical device to click
when hit with an alpha particle.
1909
R.A.
Millikan
Performed
an oil drop experiment to determine
the charge and the mass of an electron to be 1.602 x 10-19 C
and
9.11 x 10-28 g respectively.
1911
Ernest
Rutherford
Performed
the gold foil experiment to
determine that the nucleus is a small, dense, and positively charged
part of
the atom, based on the assumption that electrons are on the outside of
the
nucleus. Develops the plum pudding model of the atom.
1913
Neils
Bohr
Publishes
paper regarding combining nuclear
and quantum theories.
1914
H.G.J.
Moseley
Determined
the charge of the nuclei of most
atoms using X-ray tubes. Stated that the
atomic number is equal to the number of protons in the nucleus.
1919
Aston
Used mass
spectra to prove the existence of
isotopes.
Ernest
Rutherford
First
artificially induced nuclear
reaction. Used nitrogen and alpha
particles to obtain an oxygen isotope and protons.
1922
Neils
Bohr
Created a
model of successive orbital shells
based o an equation of fixed radii and quantum numbers.
1923
Louis Victor de
Broglie
Electrons
are similar to both particles and
waves.
1925
Werner
Heisenberg, Max Born and Erwin Schodinger
Develop
quantum mechanics
1927
Werner
Heisenberg
Used the
frequencies based on spectra lines
to describe atoms. The Principle of
Indeterminacy states that you can not know the both the position and
the
velocity of a particular particle at any one moment in time.
1930
Erwin
Schrodinger
Saw
electrons as a continuous cloud and
developed wave mechanics to be a mathematical model for the atom.
Paul
Dirac
Suggested
the existence of
anti-particles. Found the first
anti-electron, positron.
1932
James
Chadwick
Discovered
neutral atomic particle with
similar mass to a proton, the neutron by using alpha particles.