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bohr model Philogiston Theory

Phlogiston Theory

According to the phlogiston theory, propounded in the 17th century, every combustible substance consisted of a hypothetical principle of fire known as phlogiston,
which was liberated through burning, and a residue. The word phlogiston was first used early in the 18th century by the German chemist Georg Ernst Stahl. Stahl
declared that the rusting of iron was also a form of burning in which phlogiston was freed and the metal reduced to an ash or calx. The theory was superseded
between 1770 and 1790 when the French chemist Antoine Lavoisier showed that burning and rusting both involved oxygen and concluded that both ash and rust
were compounds of oxygen. Lavoisier's oxidization theory has been accepted by scientists from about 1800 to the present day.

The theory of phlogiston was predominantly German in origin, with much early work done in Mainz, though it was widely believed through much of the eighteenth
century -- two of the most prominent followers of the theory, Johann Joachim Becher and Georg Ernst Stahl (who first used the name phlogiston in 1700), were
Swedish. Phlogiston was not only widespread but deep-seated, and gave way to the atomic theory only slowly.

Phlogiston theorists identified three essences which comprise all matter: sulfur or terra pinguis, the essence of inflammability; mercury or terra mercurialis, the
essence of fluidity; and salt or terra lapida, the essence of fixity and inertness. In this respect phlogiston theory is similar to the ancient alchemical notions of earth,
air, fire, and water. The terra pinguis was renamed phlogiston. In this view, metals were made of a "calx" (or residue) combined with phlogiston, the fiery principle,
which was liberated during combustion, leaving only the calx. Air, according to the theory, was merely the receptacle for phlogiston; all combustible or calcinable
substances, in fact, were not elements but compounds containing phlogiston. Rusting iron, for instance, was believed to be losing its phlogiston and thereby returning
to its elemental state.

Phlogiston theory was widely supported throughout the eighteenth century, although it came under increasing attack as empirical research pointed up its difficulties.
When it was determined that some metals actually gained mass when burnt, partisans explained it by giving phlogiston a negative mass. Even Priestley believed in the
theory until his death, convinced that his discovery of oxygen was "dephlogisticated air." It was up to Lavoisier to realize the significance of his discovery.

Lavoisier made a symbolic break with phlogiston theory by burning all textbooks that supported the theory, just as Paracelsus had destroyed his copies of the works
of the medieval medical authorities. His theory of oxidation soon replaced phlogiston theory, and remains a part of modern chemistry.

Although he exaggerated its importance, Lavoisier was the first to understand the significance of
Priestley's work on oxygen, and is considered by some to have discovered the element. He
disproved phlogiston theory by demonstrating that oxygen is required for combustion, rusting, and
respiration. He combined his chemical abilities with an interest in zoology to produce pioneering
work on anatomy and physiology.

phlogiston theory , hypothesis regarding combustion. The theory,
advanced by J. J. Becher late in the 17th cent. and extended and
popularized by G. E. Stahl, postulates that in all flammable materials there is
present phlogiston, a substance without color, odor, taste, or weight that is
given off in burning. APhlogisticated@ substances are those that contain
phlogiston and, on being burned, are Adephlogisticated.@ The ash of the

burned material is held to be the true material. The theory received strong
and wide support throughout a large part of the 18th cent. until it was
refuted by the work of A. L. Lavoisier, who revealed the true nature of
combustion. Joseph Priestley, however, defended the theory throughout his
lifetime. Henry Cavendish remained doubtful, but most other chemists of the
period, including C. L. Berthollet, rejected it.


Phlogiston Theory



The failure to understand combustion was an insurmountable obstacle to real progress in
chemistry. Any theory of chemical change must be able to explain combustion and phlogiston
was the first real attempt to do so.

The fact that wood turns to ashes and metals become soft powders when heated and can be
changed back into metals in the presence of charcoal is hard to reconcile without imagining the
addition or subtraction of some substance.

Phlogiston was that substance.

The theory was simple, and although having serious contradictions, was better than no theory at
all. Besides, despite the quantitative work of Galileo and Newton, the ... more

bohr model

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A brief history of atomic theory

In the beginning of the 1800s John Dalton, an English scientist did work some
          work on gases, which lead him to the creation of a complex system of
          symbols for all known elements at the time. He took all the information he had
          collected, along with the Laws of Conservation of Mass, Definite Composition
          and Multiple Proportions and updated Aristotles theory of matter with the
          Atomic Theory of Matter, which stated: - All matter is composed of tiny,
          indivisible particles called atoms. - Atoms of an element have identical
          properties. - Atoms of different elements have different properties. - Atoms of
          two or more elements can combine in constant ratios to form new
          substances. In the late 1800s a man named J. J. Thomson did some
          experiments, whos results did not agree with Daltons Atomic Theory.
          Thomson passed electricity though gases, my his experiments, he theorized
          the existence negatively charged subatomic particles he called electrons.
          From this theory Thomson created a model of a atom which had the electrons
          placed evenly inside the atoms. In the early 1900s a Japanese scientist
          named H. Nagaoka designed an atom model as a large sphere surrounded
          by a ring of negatively charged electrons. Also, during the early 1900s
          (1898-1907) a physicist named Ernest Rutherford worked on experiments to
          test current atom models. His experiments involved shooting rays of alpha
          particles (small positively charged particles) though very thin pieces of gold
          foil. Based on Thomsons model, Rutherford hypothesized that the alpha
          particles would travel through the gold foil mostly unaffected by the gold. He
          was right. Most of the particles did pass through, but a small amount of
          particles were deflected. From this Rutherford hypothesized that the atoms
          must have a small positively charged core, the nucleus, which is surrounded
          by mainly empty space, which contains the electrons. In 1914 Rutherford
          made up the word proton, which were subatomic particles that had a
          positive charge. A student of Rutherfords, a man named H. G. J. Moseley
          was the one who gathered the empirical support for Rutherfords work. In his
          experiments he used X-rays to show that the positive charge in the nucleus
          grows by one, from each element to the other. From this Moseley devised the
          concept of Atomic Number. In 1932, James Chadwick established that the
          nucleus must contain heavy neutral particles as well as positive ones, this was
          to explain the entire mass of the atom. He called the neutral subatomic
          particles neutrons. I Danish scientist named Niels Bohr created a theory
          explaining the periodic law. Bohr took the Quantum Theory of Energy,
          proposed by Max Planck (in 1900), and the relationship between the sudden
          end of the periodic table. Using this, periodic law, and some experimental
          evidence, Bohr hypothesized the following: - Each electron has a fixed
          quantity of energy related to the circular orbit in which the election is found. -
          Electrons cannot exist between orbits, but they can move to unfilled orbits if a
          quantum of energy is absorbed or released. - The higher the energy level of
          an electron, the further it is from the nucleus. - The maximum number of
          electrons in the first three energy levels is 2, 8, and 8. - An atom with a
          maximum number of electrons in its outermost level is stable, that is, it is
          unreactive. Bohrs theory was developed mathematically, so as to explain the
          visible spectrum of hydrogen gas, as well as to predict other lines of ultraviolet
          and infrared light. One of the great things about Bohrs theory is that it
          explains periodic law. The theory states that properties of elements can be
          explained by the way that their electrons are arranged. Due to the fact that
          orbits can only contain certain amounts of electons. Quantum mechanics is a
          highly mathematical theory developed in the 1920s that describes the
          positioning of electrons as patterns of probability, instead of distinct orbits. It
          explains the positioning of protons, and atomic numbers. It describes the
          energy levels of electrons outside the nucleus, stability of atoms, and the
          amounts of electrons that can exist at certain levels. It is a very complex
          theory which explains much about atomic ... more

bohr model

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