TIMELINE
Atomic Theory
By, Varsha, Divya, Avikshit , Aaditya menon , Aditya ram
C. 460 BCE - C. 370 BCE
DEMOCRITUS
Theory :
Proposed the concept of atomism,
suggested that all of matter is
consisted of indivisible &
indestructible particles called
atoms. Democritus also believed
that atoms were uniform & solid.
Also believed atoms moved in
infinite numbers through empty
space until stopped.
Model: Democritus atomic model
envisioned as tiny,solid, & indivisible
particles that combined in various
ways to form different substances
1856 CE - 1940 CE
1766 CE - 1844 CE
SOLID SPHERE - JOHN
DALTON
Discovery :
(1803- 1808) Dalton formulated the modern
atomic theory, which proposed that
elements are composed of atoms and that
chemical reactions involve the
rearrangement of atoms.
Model: Dalton's atomic model described
atoms as indivisible and indestructible
spheres, each with a unique mass and
combining ratio in compounds.Those of given
element are identical. Compounds are
combinations of different types of atoms.
PLUM PUDDING MODEL
JJ THOMPSON
Theory (1897 CE): Thomson discovered
the electron through his cathode ray
experiments and proposed the "plum
pudding" model of the atom.
Model: Thomson's atomic model
depicted atoms as a positively charged
sphere with negatively charged
electrons embedded within, resembling
plums in a pudding.
1885 CE - 1962 CE
PLANETARY MODEL
NIELS BOHR
Theory: Bohr expanded upon Rutherford's
model and proposed the planetary model
of the atom. .
Model: Bohr's atomic model incorporated
the idea of quantized energy levels.
Electrons occupy specific orbits or energy
levels around the nucleus, and they can
jump between these levels by absorbing or
emitting energy. Electron energy was
quantized. Explained emissions spectra of
some elements
1871 CE - 1937 CE
NUCLEAR MODEL - ERNEST
RUTHERFORD
Theory : Rutherford conducted the
famous gold foil experiment and
proposed the nuclear model of the
atom..
Model: Rutherford's atomic model
suggested that atoms have a tiny,
dense, and positively charged
nucleus at the center, with electrons
orbiting around it like planets around
the sun.
1891 CE - 1974 CE
DISCOVERY OF
NEUTRON
JAMES CHADWICK
Theory: Chadwick discovered the neutron,
an uncharged subatomic particle, which
completed the picture of the atomic
nucleus.
Model: Chadwick's discovery led to the
refinement of the atomic model, indicating
the presence of neutral neutrons alongside
protons in the nucleus.
Note: The timeline extends until World War II,
but there were significant advancements in
atomic theory beyond that period as well.
.
COMPARISONS AND
CONTRASTS:
Democritus' model was based on philosophical reasoning, proposing the
existence of indivisible particles. In contrast, the atomic models developed in
the late 19th and early 20th centuries were based on experimental
evidence and scientific principles.
Dalton's model emphasized the idea of atoms as fundamental building blocks
with distinct properties. Thomson's model introduced the concept of
subatomic particles and the electron's presence within the atom.
Rutherford's model further refined the structure of the atom, highlighting
the existence of a small, dense nucleus.
Bohr's model introduced the idea of quantized energy levels and explained
the stability of atoms, which earlier models couldn't account for. It provided
a bridge between classical physics and the emerging field of quantum
mechanics.
Tools and Technologies:
The development of atomic theory required advancements in experimental
techniques and tools, such as the cathode ray tube (used by Thomson), the
gold foil experiment (performed by Rutherford), and spectroscopy (used by
Bohr). These tools enabled scientists to observe and analyze the behavior of
atoms and subatomic particles.
The emergence of more sophisticated instruments, like the electron
microscope and particle accelerators, in later years facilitated further
discoveries and detailed investigations into atomic structure and behavior.
REFRENCES:
OpenStax Chemistry. "2.2 Atomic Theory and Atomic Structure."
OpenStax CNX. Accessed 14 July 2023.
"Democritus." Stanford Encyclopedia of Philosophy. Accessed 14 July
2023.
"John Dalton." Encyclopaedia Britannica. Accessed 14 July 2023.
"J.J. Thomson." NobelPrize.org. Accessed 14 July 2023.
"Ernest Rutherford." NobelPrize.org. Accessed 14 July 2023.
"Niels Bohr." NobelPrize.org. Accessed 14 July 2023.
"James Chadwick." NobelPrize.org. Accessed 14 July 2023.
Heathcote, William. MYP Physics: a Concept Based Approach. Oxford
University Press, 2018. Accessed 14 July 2023