The History of Atomic Structure

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Transcript The History of Atomic Structure

The History of Atomic Structure
Aliese, Aoife, Lauren
In short...
Democritus + Greek philosophers
John Dalton
Dmitri Mendeleev
J.J. Thomson
Ernest Rutherford
Max Planck
Niels Bohr
Edwin Schrodinger
James Chadwick
Democritus and ancient Greek
philosophy, circa 400-500 BCE
• Greek philosophy was about 150 years old, having emerged early in the
sixth century BC, centred in the city of Miletus on the Ionian coast in
Asia Minor (now Turkey). The earliest known Greek philosopher was
Thales of Miletus.
• Democritus was one of the two philosophers of atomic theory. He
elaborated a system originated by his teacher Leucippus into a materialist
account of the natural world.
• The idea of the atom was strongly opposed by Aristotle and others.
Because of this, the atom receded into the background. Although there
is a fairly continuous pattern of atomistic thought through the ages, only a
relative few scholars gave it much thought.
• Due to complex circumstances beyond the scope of this lesson, the
Catholic Church accepted Aristotle's position and came to equate
atomistic ideas with Godlessness. For example, "Democritus of Abdera
said that there is no end to the universe, since it was not created by any
outside power."
Points that ancient Greek philosophers put forth that were to be the structure of the
future atomic theory:
Point #1 - All matter is composed of atoms, which are bits of matter too small to be
seen. These atoms CANNOT be further split into smaller portions.
Point #2 - There is a void, which is empty space between atoms.
Point #3 - Atoms are completely solid.
Point #4 - Atoms are homogeneous, with no internal structure.
Point #5 - Atoms are different in:
1) ...their sizes. See the Democritus quote just below.
2) ...their shapes. According to Aristotle: "Democritus and Leucippus say that
there are indivisible bodies, infinite both in number and in the varieties of
their shapes...." Democritus says of atoms: "They have all sorts of shapes and
appearances and different sizes.... Some are rough, some hook-shaped, some
concave, some convex and some have other innumerable variations."
3) ...their weight. Again from Aristotle: "Democritus recognized only two basic
properties of the atom: size and shape. But Epicurus added weight as a third.
For, according to him, the bodies move by necessity through the force of
The atomists held that there are two fundamentally different kinds of realities
composing the natural world, atoms and void. Atoms, from the Greek adjective
atomos or atomon, ‘indivisible,’ are infinite in number and various in size and
shape, and perfectly solid.
They move about in an infinite void, repelling one another when they collide or
combining into clusters by means of tiny hooks and barbs on their surfaces,
which become entangled.
Clusters of atoms moving in the infinite void come to form kosmoi or worlds as
a result of a circular motion that gathers atoms up into a whirl, creating clusters
within it; these kosmoi are impermanent.
Our world and the species within it have arisen from the collision of atoms
moving about in such a whirl, and will likewise disintegrate in time.
John Dalton 1803
Dalton's theory was based on the premise that the atoms of different elements could
be distinguished by differences in their weights. He stated his theory in a lecture to
the Royal Institution in 1803. The theory proposed a number of basic ideas:
All matter is composed of atoms
Atoms cannot be made or destroyed
All atoms of the same element are identical
Different elements have different types of atoms
Chemical reactions occur when atoms are rearranged
Compounds are formed from atoms of the constituent elements.
Using his theory, Dalton rationalised the various laws of chemical combination which
were in existence at that time. However, he made a mistake in assuming that the
simplest compound of two elements must be binary, formed from atoms of each
element in a 1:1 ratio, and his system of atomic weights was not very accurate - he
gave oxygen an atomic weight of seven instead of eight.
Despite these errors, Dalton's theory provided a logical explanation of concepts, and
led the way into new fields of experimentation.
• He proceeded to calculate atomic weights from percentage compositions of
compounds, using an arbitrary system to determine the likely atomic
structure of each compound. If there are two elements that can combine,
their combinations will occur in a set sequence. The first compound will
have one atom of A and one of B; the next, one atom of A and two atoms of
B; the next, two atoms of A and one of B; and so on.
Dmitri Mendeleev 1869
Dmitiri Mendeleev, who was born in Tobolsk, Siberia in 1834, contributed greatly to
atomistic theory through his discoveries concerning elements.
Mendeleev was the first to organize elements into a Periodic Table, the elemental
organizational system which is still used today. Although Mendeleev’s table consisted
of only 63 elements (his table excluded all of the Noble Gases, which had not yet
been discovered), his Periodic Table demonstrated that alternate organizational
systems could be applied to atoms and elements, rather than the conventionally
accepted atomic weight system which had been originally proposed by the ancient
He also developed a series of laws, known as the Laws of Period, which determined
that the properties of elements were actually functions of their atomic weights.
Ultimately, Mendeleev’s advances concerning elements and periodic organization
opened up new possibilities for atomistic theory and shed new light on many aspects
of chemical elements.
Ultimately, Mendeleev’s advances concerning elements and periodic organization
opened up new possibilities for atomistic theory and shed new light on many aspects
of chemical elements.
• Mendeleev took his concept even further by stating that it was possible to
predict the properties of undiscovered elements. He then proceeded to
make predictions for three new elements (eka-aluminum, eka-boron and
eka-silicon) and suggested several properties of each, including density, radii,
and combining ratios with oxygen, among others.
• The science world was perplexed, and many scoffed at Mendeleev's
predictions. It was not until November, 1875, when the Frenchman Lecoq
de Boisbaudran discovered one of the predicted elements (eka-aluminum)
(which he named Gallium) that Dmitri's ideas were taken seriously. The
other two elements were discovered later and their properties were found to
be remarkably similar to those predicted by Mendeleev. These discoveries,
verifying his predictions and substantiating his law, took him to the top of
the science world. He was 35 years old when the initial paper was presented.
Joseph John Thomson 1897
• In 1897, a British physicist named Joseph John Thompson discovered the
particle we now know as the electron.
• He discovered this through conducting a series of experiments designed to
study the nature of electric discharge in a high-vacuum cathode-ray tube, an
area being investigated by numerous scientists at the time.
• Thomson interpreted the deflection of the rays by electrically charged plates
and magnets as evidence of ‘bodies much smaller than atoms’ that he
calculated as having a very large value for the charge-to-mass ratio.
Ernest Rutherford 1898
• In 1898, Ernest Rutherford published his atomic
theory describing the atom as having a central positive
nucleus surrounded by negative, orbiting electrons.
• This model suggested that most of the mass of the
atom was contained in the small nucleus, and that the
rest of the atom was mostly empty space.
• Rutherford discovered this through conducting his
famous gold foil experiment. This experiment involved
firing radioactive particles through thin metal foils
(mostly gold) and detecting them using screens coated
with zinc sulphide.
• Rutherford found that although the vast majority of
particles passed straight through the foil approximately
1 in 8000 were deflected leading him to his theory that
most of the atom was made up of 'empty space'.
Max Planck 1900
• In 1900 Max Planck made a profound
discovery in modern physics / Quantum
Theory. He showed (from purely formal /
mathematical foundations) that light must be
emitted and absorbed in discrete amounts if
it was to correctly describe observed
phenomena (i.e. Blackbody radiation).
• Prior to then light had been considered as a
continuous electromagnetic wave, thus the
discrete nature of light was completely
Niels Bohr 1922
• In 1913 he completed his theory of atomic structure.
• Rutherford’s atomic theory described an atomic model with all the
mass concentrated in a nucleus with electrons circling the nucleus in a
fixed orbit.
• This theory was shown incorrect by using Maxwell’s equations, which
states since the electrons are moving in a circular motion, they are
accelerating. Accelerating electrons means they are emitting radiation
and therefore losing energy and would eventually spiral in motion
toward the nucleus and collapse.
• Bohr’s insight was that he declared an electron could orbit the
nucleus but only in discrete orbits which didn’t emit radiation. An
electron moves to a higher orbit, with a larger radius, by absorbing
radiation (a photon) and in contrast will emit a photon of energy
when the electron moves to a lower orbit with a smaller radius.
• Bohr proposed that the outer
orbits could accommodate
more electrons than the inner
orbits. In total, the atomic
structure theory that Bohr
proposed included an atom
which was 1/10,000 the size of
the atoms proposed by other
scientists. In 1922,
• Niels Bohr received the Nobel
Prize in Physics for his research
in the atomic structure. This
atomic theory was a
combination of Rutherford’s
work and ideas of the atom,
with Planck’s Quantum Theory.
Edwin Schrodinger 1930
• In 1930, Edwin Schrodinger published an atomic theory that stated that rather than
electrons being distributed within an electron configuration of shells and energy levels,
they were arranged in orbitals which were systematically distributed within Electron
• He defined an orbital as: “The region of space that surrounds a nucleus in which two
electrons may randomly move.”
• This was known as the Quantum Model of Electrons, and described more of the
chemical phenomena than the simple Particle or Corpuscular Model.
• Edwin Schrodinger realised that this combined with the fact that electrons are
constantly moving, meant that electrons could not be given a definite position within
the atom.
James Chadwick 1932
• James Chadwick proved the
existence of neutrons in 1932.
Chadwick's research greatly
increased understanding of the
structure of the atomic nucleus.
Before his discovery, scientists
knew of the existence of protons
in the nucleus, but not neutrons.
Chadwick won the 1935 Nobel
Prize in Physics. His work led to
the fission of uranium 235, and
the subsequent development of
the atomic bomb.