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Introduction to Atomic Theory

The Birth of Chemistry:
Democritus to Dalton – Evolution of the atomic model
Around 440 B.C.E. was the birth of modern Chemistry. Democritus of Thrace put forward the simple,
and at that time, unprovable, notion that all matter can be reduced to a single, solid, spherical unit. This
indivisible piece of matter was named the atom. The idea that the diversity of the substances in the
world was due to combinations of these atoms was a major step forward insofar as we no longer
considered each substance as a single unique element in and of its own right.
This single thought was the foundation of all chemical thought through the start of the Nineteenth
century. In 1803, John Dalton made the first significant expansion on Democritus’ atom with four simple
statements in what has become known as Dalton’s Atomic Theory:
All matter is made of small, indivisible, units of matter known as atoms.
Atoms of any given element are identical
Compounds exist as whole-number ratio combinations of atoms
Chemical reactions occur to create rearrangement of these combinations into new substances
These four postulates gave rise to some of the biggest questions of the next 200+ years.
Why are there differences in properties of an element?
Why do combinations of atoms occur and stay together?
Why do some reactions proceed and others do not?
Why do chemical reactions occur in different manners under different conditions?
Faraday, Hittorf, Crookes, Thompson and Millikan – The most important discovery in
In the early part of the 1800’s the most fundamental precept of an atom, its indivisibility, began to break
Michael Faraday discovered (in 1838) that a strange glowing occurred in tubes filled with low pressure
gases when an electrical current was passed through them. The “rays” emitted were identified by the
fact that they caused gases to glow, in a process known as fluorescence. He noted that the glowing
began slightly in front of what was known as the cathode in his experiment. This became known as
“Faraday Dark Space”. In 1869, Johann Hittorf was able to show that these rays were actually particles
by the shadows he was able to create in his famous Iron Cross experiment.
Investigating Faraday Dark Space later that century (1870’s), William Crookes discovered that the length
of this space could be expanded by creating increasingly lower pressures of gases inside the tube. The
specialized tube that he invented for this experiment is called a “Crookes Tube”. These “cathode rays”
still persisted on a path towards the anode.
Finally, in 1897, J.J. Thompson was able to prove that the path of these particles was altered by the
presence of other electrical potentials. The beam path would deviate towards positively fields and be
repelled by negative ones. Building on the previous discoveries, Thompson identified these particles as
being negatively charged, named them “electrons”, calculated a charge to mass ratio, 1.76 x 1011 C/kg,
and justified their existence in the atom by placing them as studs within a positively charged medium.
His new additions and modifications to Dalton’s theory became known as the Plum Pudding Model.
In 1909, the final physical pieces of evidence were determined. Robert Millikan designed the “Oil Drop
Experiment”. By creating charged droplets of oil within a vacuum chamber he was able to determine the
mass of each droplet and the total amount of electrostatic charge required to completely offset the
gravitational pull. By collecting this data on droplets of varying masses, Millikan determined that the
total amount of charge on each droplet was actually a whole number product of a singular elementary
charge. This charge, 1.606 x 10-19 Coulombs, was the charge of an electron. Coupled with Thompson’s
Charge to Mass ratio, he was able to calculate the mass of an electron as 9.11 x 10 -31 kg.
Nagaoka, Rutherford and Chadwick – Birth of the nuclear atom
When Thompson put forth the Plum Pudding Model in 1903, He actually had three ideas to choose
from. The first was that each electron was paired with a positively charged partner that followed it
everywhere. But with no evidence that a corresponding positive particle was also ejected and the
difficulties in explaining why these neutral pairs would stay together as an atom, this fell by the wayside.
Thompson’s second and third ideas held more merit. The second idea, later known as the Saturnian
Model and endorsed by Japanese physicist, Hantaro Nagaoka, made more sense with electrons orbiting
in rings around a single positively charged center. There was no evidence otherwise to support this so
Thompson took the more conservative stance and simply adjusted the current theory to fit the evidence
he had.
Thompson pursued further experiments to more accurately investigate the atom, but it was not until
1909 when his student, Ernest Rutherford, made the next breakthrough. Working with Hans Geiger and
Ernest Marsden, he developed the Gold Foil Experiment to probe nuclear structure. Positively charged
fragments, called alpha particles, were aimed at a thin sheet of gold foil. According to the Plum Pudding
Model, these particles should bounce off the surface and be scattered backwards. Instead, they found
that most of these alpha particles were able to penetrate through the foil. It took two more years for
Rutherford to finally come to the realization that something along the lines of the Saturnian Model must
be correct. His Nuclear Atom had two parts: a nucleus that contained almost all the mass and positive
charge, and an outer layer of electrons swirling around it. In between the nucleus and electron layer was
a region of empty space that gave the atom its apparent size.
The story of the nuclear atom concludes with James Chadwick and the identification of a third particle.
By the 1920, the idea that all atoms of an element were identical began to break down with the
discovery of “isotopes”. These atoms all had similar properties to the element they represented, but
were found to have different masses. Working with Rutherford and others, Chadwick was able to
identify the neutron – a particle with almost the same mass of a proton that resided in the nucleus and
carrying a zero charge.
Facts, Figures, and Numbers – Getting ready to move on
For about 2500 years, the idea of a single, core-unit of substance has been, and still is, a core question in
our comprehension of matter. It has evolved from a concept to an increasingly complex model
consisting of subatomic particles. Along the way, examples, experiments, and observations have
advanced to broaden the scope of our understanding.
Democritus / Dalton
Thompson / Millikan
Rutherford / Chadwick
Figure 1.1: Progression of Atomic Theory
https://www.sutori.com/item/democritus-atomic-model-442-b-c-democritus-came-up-with-this-model-by-doing ,
https://jjthomsonbyjuntaolin.blogspot.com/p/conclusion.html , http://thehistoryoftheatom.weebly.com/lord-rutherford.html
Mass (kg)
9.10939 x 10-31
1.67262 x 10-27
1.67493 x 10-27
Charge (C)
-1.60218 × 10-19
+1.60218 × 10-19
Mass (amu)
Table 1.1: Summary of values associated with sub-atomic particles.
Appendix 1: Numbers in Science
Demo – Cloud Chamber
Lab – Rutherford’s Gold Foil
Assessment – Online: Atomic Theory Fundamentals (MCQ/FIB/SA)
Assessment – Online: Numbers in Science (Appendix 1)
Relative Charge