Dalton's Atomic Theory
Dalton's fascination with gases gradually led him to formally assert that every form of matter (whether solid, liquid or gas) was also made up of small individual particles. He referred to the Greek philosopher Democritus of Abdera's more abstract theory of matter, which had centuries ago fallen out of fashion, and borrowed the term "atomos" or "atoms" to classify the particles. Dalton created the first chart of atomic weights, in an article he wrote for the Manchester Literary and Philosophical Society in 1803. Seeking to expand on his theory, he readdressed the subject of atomic weight in his book A New System of Chemical Philosophy, published 1808. In A New System of Chemical Philosophy, Dalton introduced his belief that atoms of different elements could be universally distinguished based on their varying atomic weights. In so doing, he became the first scientist to explain the behavior of atoms in terms of the measurement of weight. He also discovered the fact that atoms couldn't be created or destroyed. Dalton's theory additionally examined the compositions of compounds, explaining that the tiny particles (atoms) in a compound were compound atoms. In A New System of Chemical Philosophy, Dalton also wrote about his experiments proving that atoms consistently combine in simple ratios. Which means that the molecules of an element are always made up of the same proportions, with the exception of water molecules. In 1810 Dalton published an appendix to A New System of Chemical Philosophy. In it he elaborated on some of the practical details of his theory: that the atoms within a given element are all exactly the same size and weight,while the atoms of different elements look and are different from one other. Dalton eventually composed a table listing the atomic weights of all known elements.His atomic theories were quickly adopted by the scientific community at large with few objections. 1) All matter is made of atoms. Atoms are indivisible and indestructible. 2) All atoms of a given element are identical in mass and properties 3) Compounds are formed by a combination of two or more different kinds of atoms. 4) A chemical reaction is a rearrangement of atoms. |
J J Thomson's Atomic Theory
In 1897 the British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electron in 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. Later he estimated the value of the charge itself. In 1904 Thomson suggested a model of the atom as a sphere of positive matter in which electrons are positioned by electrostatic forces. His efforts to estimate the number of electrons in an atom from measurements of the scattering of light, X, beta, and gamma rays initiated the research trajectory along which his student Ernest Rutherford moved. Thomson's last important experimental program focused on determining the nature of positively charged particles. Here his techniques led to the development of the mass spectrograph. His assistant, Francis Aston, developed Thomson's instrument further and with the improved version was able to discover isotopes—atoms of the same element with different atomic weights—in a large number of nonradioactive elements. J. J. Thomson considered that the structure of an atom is something like a raisin bread, so that his atomic model is sometimes called the raisin bread model. He assumed that the basic body of an atom is a spherical object containing N electrons confined in homogeneous jellylike but relatively massive positive charge distribution whose total charge cancels that of the N electrons. The schematic drawing of this model is shown in the following figure. Thomson's model is sometimes dubbed a plum pudding model. |
Ernest Rutherford's Atomic Theory
Ernest Rutherford publishes 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 came to this conclusion following the results of his famous gold foil experiment. This experiment involved the firing of radioactive particles through minutely thin metal foils (notably gold) and detecting them using screens coated with zinc sulfide 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'. |
Niels Bohr's Atomic Theory
Bohr concentrated the majority of his effort on researching the structure of the atom, and in 1913 he completed his theory of atomic structure. This atomic theory was a combination of Rutherford’s work and ideas of the atom, with Planck’s Quantum Theory. 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. Each orbit corresponds to an angular momentum value relating to Planck’s constant (h) divided by 2pi. Insights regarding radiation and atoms were taken from Planck’s Quantum Theory. Bohr proposed that the outer orbits could accommodate more electrons than the inner orbits. 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. |
Modern Atomic Theory
Modern Atomic Theory Bohr's atomic model is important because it introduces the idea of the quantum in explaining atomic particles.However, Bohr's model needed to be changed because it didn't explain atoms morecomplicated than hydrogen. It took almost another decade to develop the more complete atomic theory. The modern atomic theory is demonstrated by the cloud model. De Broglie suggested that particles could sometimes behave as waves and waves could sometimes bahave as particles - the wave/particle duality of nature.Erwin Schrodinger developed the equation which is used today to understand atoms and molecules - the Schrodinger Equation (1926)
Essential conclusion from the Schrodinger Equation Energies are quantized: Atoms and molecules cannot have any energy but only certain energies. This means that energies are "quantized". The orbitals, associated with each energy, determine where the electrons are located: Each orbital is determined by a quantum number call the angular momentum quantum number "l". This quantum number can take on the values l=0 (s-orbital), l=1 (p-orbital), l=2 (d-orbital), l=3 (f-orbital) etc. In the modern atomic model, electrons are found in orbitals within each energy level. |
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