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The History of Atomic Chemistry (The Law of Conservation Energy (This law…
The History of Atomic Chemistry
The Early History of Atom
The ancient Greek philosophers Democritus and Leucippus recorded the concept of the atomos, an indivisible building block of matter, as early as the 5th century BCE but was largely ignored
John Dalton, an English chemist and meteorologist, is credited with the first modern atomic theory based on his experiments with atmospheric gases.
Dalton's Ideas
All atoms of a given element are identical.
The atoms of different elements vary in mass and size.
Atoms are indestructible. Chemical reactions may result in their rearrangement, but not their creation or destruction.
The idea of an indivisible particle was further elaborated upon and explored by a number of scientists and philosophers, including Galileo, Newton, Boyle, Lavoisier, and Dalton.
The History of the Law of Conservation Mass
Antoine Lavoisier described the law of conservation of mass (or the principle of mass/matter conservation) as a fundamental principle of physics in 1789.
The Law of Conservation Energy
This law was amended by Einstein in the law of conservation of mass-energy, which describes the fact that the total mass and energy in a system remain constant.
The law of conservation of mass is useful for a number of calculations and can be used to solve for unknown masses, such the amount of gas consumed or produced during a reaction.
According to the law of conservation of mass, the mass of the products in a chemical reaction must equal the mass of the reactants.
Discovery of Electron
Jordan JJ Thompson
He carried out experiments using cathode rays produced in a discharge tube, and found that the rays were attracted by positively charged metal plates but repelled by negatively charged ones. From this he deduced the rays must be negatively charged.
By measuring the charge on the particles in the rays, he was able to deduce that they were two thousand times lighter than hydrogen, and by changing the metal the cathode was made from he could tell that these particles were present in many types of atoms. He had discovered the electron (though he referred to it as a ‘corpuscle’), and shown that atoms were not indivisible, but had smaller constituent parts.
In 1904, he put forward his model of the atom based on his findings. Dubbed ‘The Plum Pudding Model’ (though not by Thomson himself), it envisaged the atom as a sphere of positive charge, with electrons dotted throughout like plums in a pudding.
Ernest Rutherford(The Discovery of The Nucleus)
Rutherford devised an experiment to probe atomic structure which involved firing positively charged alpha particles at a thin sheet of gold foil. The alpha particles were so small they could pass through the gold foil, and according to Thomson’s model which showed the positive charge diffused over the entire atom, the should do so with little or no deflection.
During the experiment, most of the alpha particles did pass through the foil with little or no deflection. However, a very small number of the particles were deflected from their original paths at very large angles.
The only possible explanation was that the positive charge was not spread throughout the atom, but concentrated in a small, dense centre: the nucleus. Most of the rest of the atom was simply empty space.
Niels Bohr
Bohr’s suggestion of stable energy levels addressed the problem of electrons spiraling into the nucleus to an extent, but not entirely.
Bohr was a Danish physicist who set about trying to solve the problems with Rutherford’s model. He realised that classical physics could not properly explain what was going on at the atomic level; instead, he invoked quantum theory to try and explain the arrangement of electrons.
Electrons Behaviour
Erwin Schrödinger proposed that, rather than the electrons moving in fixed orbits or shells, the electrons behave as waves.Austrian physicist Erwin Schrödinger.
Schrödinger solved a series of mathematical equations to to come up with a model for the distributions of electrons in an atom.
model shows the nucleus surrounding by clouds of electron density. These clouds are clouds of probability; though we don’t know exactly where the electrons are, we know they’re likely to be found in given regions of space.
Schrödinger’s wasn’t quite the last word on the atom. In 1932, the English physicist James Chadwick (a student of Ernest Rutherford) discovered the existence of the neutron, completing our picture of the subatomic particles that make up an atom.
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Dalton’s atomic theory contained the following ideas: