Transcript Development of atomic theory

Development of atomic
theory
Created by :
Angella Santa Monica
VIII-a (03)
• Francis Bacon (1561 - 1626) seems to be
thinking of matter in atomic terms when
he says "Rapid motion of constituent
particles is both a necessary and sufficient
condition for something to be hot."
• Galileo (1564 - 1642) thought the
appearance of new materials in a
chemical change was due to
rearrangement of parts too small to be
seen.
• Robert Boyle ( 1627 - 1691) discarded the
Aristotelian idea of the elements earth, air, fire
and water as constituents of matter. Instead he
used a ‘corpuscular or mechanical hypothesis’,
explaining many physical phenomena in terms
of solid bodies moving, colliding, bouncing, and
having their shapes or sizes changed.
• Isaac Newton (1642 - 1727) in his book Opticks
wrote: "Have not the small Particles of Bodies
certain Powers, Virtues, or Forces, by which they
act at a distance, not only upon the Rays of
Light for reflecting, refracting and inflecting
them, but also upon one another for producing
a great Part of the Phenomena of Nature?"
• Antoine Lavoisier (1743-1794) - Formulated
the Law of Conservation of Matter: "Matter is
neither gained nor lost during a chemical reaction." He
did this by weighing materials before and after
reactions. For example, the weights of the
mercury and oxygen formed by decomposition
of mercuric oxide were compared with the
initial weight of the mercuric oxide.
• Joseph Louis Proust (1754-1826) - Formulated
the Law of Constant Porportions: "In a compound,
the contsitutne elements are always present in a definite
proportion by weight." Like Lavoisier, Proust also
conducted quantitative experiments. He showed
that regardless of how copper carbonate was
prepared in the laboratory, or how it was isolated
from nature, it always contained the same
proportions of copper, oxygen and carbon 5:4:1 parts by weight.
• John Dalton (1766-1844)- Formulated the Law of
Multiple Proportions : "In the formation of two or more
compounds from the same elements, the weights of one element
that combine with a fixed weight of a second element are in a
ratio of small whole numbers (integers) such as 2 to 1, 3 to 1,
3 to 2, or 4 to 3." He had made a quantitative study of
different compounds made from the same elements,
such as carbon monoxide and carbon dioxide. He
found that the weight ratio of carbon to oxygen in
carbon monoxide was 3:4, and the weight ratio of
carbon to oxygen in carbon dioxide was 3:8.
Dalton’s theory
• Here is a summary of Dalton's theory.
• 1. Elements are composed of tiny, separate, indivisible and
indestructible particles. These particles, called atoms, maintain
their identity when the element undergoes physical or chemical
change.
• 2. All atoms of the same element are identical and different from
the atoms of every other element.
• 3. Atoms combine in simple whole number ratios to form
compounds.
• 4. Atoms of the same elements can combine in different ratios to
form more than one compound.
• Berzelius, contributed significantly to the development of
atomic theory. About 1807 he performed a great number of
analyses of chemical compounds, and showed so many examples
of the law of definite proportions that it could no longer be
doubted. He also set about determining atomic weights and his
first table, published in 1828, compared favourably with today's
accepted values.
• Whereas Dalton represented atoms of elements by circles
containing a letter or symbol, Berzelius chose to omit the circle
and just use an initial letter of the Latin name of the element (or
two letters if more than one element began with the same letter).
This led to the system we now use for writing formulae of
elements and compounds and writing chemical equations.
Now, 200 years later, how would you
modify Dalton's theory?
• Think of sub-atomic particles - protons, neutrons,
electrons...
• spontaneous fission of radioactive atoms, nuclear
fission and fusion,
• production of radioactive isotopes in an atomic pile...
So the first part of the theory is no longer accepted.
• Atoms of isotopes of an element are not identical. So
the second part of the theory is no longer accepted.
From Dalton to the Periodic Table
• Modern atomic theory begins with the work of John Dalton, published in
1808. He held that all the atoms of an element are of exactly the same size
and weight (see atomic weight) and are in these two respects unlike the atoms
of any other element. He stated that atoms of the elements unite chemically
in simple numerical ratios to form compounds. The best evidence for his
theory was the experimentally verified law of simple multiple proportions,
which gives a relation between the weights of two elements that combine to
form different compounds.
• Evidence for Dalton's theory also came from Michael Faraday's law of
electrolysis. A major development was the periodic table, devised
simultaneously by Dmitri Mendeleev and J. L. Meyer, which arranged atoms
of different elements in order of increasing atomic weight so that elements
with similar chemical properties fell into groups. By the end of the 19th cent.
it was generally accepted that matter is composed of atoms that combine to
form molecules.
Discovery of the Atom's Structure
•
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In 1911, Ernest Rutherford developed the first coherent explanation of the structure
of an atom. Using alpha particles emitted by radioactive atoms, he showed that the
atom consists of a central, positively charged core, the nucleus, and negatively charged
particles called electrons that orbit the nucleus. There was one serious obstacle to
acceptance of the nuclear atom, however. According to classical theory, as the electrons
orbit about the nucleus, they are continuously being accelerated (see acceleration), and
all accelerated charges radiate electromagnetic energy. Thus, they should lose their
energy and spiral into the nucleus.
This difficulty was solved by Niels Bohr (1913), who applied the quantum theory
developed by Max Planck and Albert Einstein to the problem of atomic structure. Bohr
proposed that electrons could circle a nucleus without radiating energy only in orbits
for which their orbital angular momentum was an integral multiple of Planck's constant
h divided by 2π. The discrete spectral lines (see spectrum) emitted by each element were
produced by electrons dropping from allowed orbits of higher energy to those of lower
energy, the frequency of the photon of light emitted being proportional to the energy
difference between the orbits.
Around the same time, experiments on x-ray spectra (see X ray) by H. G. J. Moseley
showed that each nucleus was characterized by an atomic number, equal to the number
of unit positive charges associated with it. By rearranging the periodic table according
to atomic number rather than atomic weight, a more systematic arrangement was
obtained. The development of quantum mechanics during the 1920s resulted in a
satisfactory explanation for all phenomena related to the role of electrons in atoms and
all aspects of their associated spectra. With the discovery of the neutron in 1932 the
modern picture of the atom was complete.
Kind images of revolution of atom