John Dalton was the originator of atomic theory, which theory provided scientists with new ways of seeing the physical world. Atomic weights and fixed ratios of atoms inside compounds provided researchers with the knowledge to explore chemical compositions of matter.
Dalton’s new information on atomic weights allowed a Russian chemist, Dmitry Mendeleyev, to create the periodic table of elements. This table provided the means for discovering new elements, and Dalton also proposed symbols for the elements. The symbols would represent the atom combination in compounds, but these were later changed to the abbreviations known today. Dalton’s theory had been accepted at the time, but it opened new questions about atom interactions. The structure and interaction of atoms refined atomic theory.
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Until Dalton, the line between physics and chemistry was almost unnoticeable, but his atomic theory distinguished and divided the two disciplines. In physics and chemistry, the atom is a cornerstone providing an understanding of matter and force, especially in quantitative analysis. Quantitative analysis refers to the measurement of “how much” of a certain substance is within a sample. Dalton’s theory was the beginning of nuclear energy and fusion research still used today. The theory also provides for the study of the quantitative relationships between substances in chemical reactions, or stoichiometry. It provides for structural theory and arrangements of atoms for influencing properties. Dalton’s models eventually turned into the understanding of 3-dimensional qualities of molecules and the affect of structure on properties. Dalton’s theory did have some inaccuracies, but it did provide the foundation for future generations of scientists. Atomic theory is the idea that matter is made up of little units called atoms. In 1897, the British scientist J.J. Thomson discovered that atoms are in fact made up of smaller particles. Today atomic theory refers to matter being made up of units that are indivisible only some of the time. Dalton concluded that evaporated water exists in air as an independent gas. He wondered how water and air could occupy the same space at the same time, when obviously solid bodies can’t. If the water and air were composed of discrete particles, Dalton reasoned, evaporation might be viewed as a mixing of water particles with air particles. He performed a series of experiments on mixtures of gases to determine what effect properties of the individual gases had on the properties of the mixture as a whole. While trying to explain the results of those experiments, Dalton developed the hypothesis that the sizes of the particles making up different gases must be different. John Dalton published the first table on relative atomic weights that included six elements viz. hydrogen, oxygen, carbon, nitrogen, phosphorous and sulfur. He stated the atomic weight of hydrogen to be 1. It was not confirmed as to how he derived the atomic weights. However, his laboratory notebook indicated that he found out the relative weights from the studies of the compounds – water, carbon dioxide and ammonia. In his notebook, the elements were represented by symbols. He was the first person to propose the idea of identifying an element by its symbol. Later on, elements are represented by their abbreviations, for example, ‘H’ is the abbreviation of the element hydrogen. While the concept of atoms was thus being made indispensable, the ancient belief that they were probably structure less and certainly indestructible came under devastating attack. J.J. Thomson’s discovery of the electron in 1897 soon led to the realization that the mass of an atom largely resides in a positively charged part, electrically neutralized by a cloud of much lighter electrons. A few years later Ernest Rutherford and Frederick Soddy showed how the emission of alpha and beta particles from radioactive elements causes them to be transformed into elements of different chemical properties. By 1913, with Rutherford as the leading figure, the foundations of the modern theory of atomic structure were laid. It was determined that a small, massive nucleus carries all the positive charge whose magnitude, expressed as a multiple of the fundamental charge of the proton, is the atomic number. An equal number of electrons carrying a negative charge numerically equal to that of the proton form a cloud whose diameter is several thousand times that of the nucleus around which they swarm. The atomic number determines the chemical properties of the atom, and in alpha decay a helium nucleus, whose atomic number is 2, is emitted from the radioactive nucleus, leaving one whose atomic number is reduced by 2. In beta decay the nucleus in effect gains one positive charge by emitting a negative electron and thus has its atomic number increased by unity. Rutherford’s gold foil experiment, performed in conjunction with Geiger and Marsden, provided evidence for the nucleus due to the scattering of alpha particles. The protons. Further work by Chadwick revealed the existence of neutrons within the nucleus of the atom. The atomic number describes the number of protons in the nucleus. For a neutral atom this is also the number of electrons outside the nucleus. Subtracting the atomic number from the atomic mass number gives the number of neutrons in the nucleus. Isotopes are atoms of the same element they have the same number of protons, or the same atomic number which have a different number of neutrons in the nucleus. Isotopes of an element have similar chemical properties. Radioactive isotopes are called radioisotopes. Most of the elements in the periodic table have several isotopes, found in varying proportions for any given element. The average atomic mass of an element takes into account the relative proportions of its isotopes found in nature. A nuclear binding force holds the nucleus of the atom together. The nuclear mass defect, a slightly lower mass of the nucleus compared to the sum of the masses of its constituent matter, is due to the nuclear binding energy holding the nucleus together. The mass defect can be used to calculate the nuclear binding energy, with E = mc2. The average binding energy per nucleon is a measure of nuclear stability. The higher the average binding energy, the more stable the nucleus. The basic difference between Rutherford’s Atomic Theory and the Plum Pudding Model has to do with the fact that the Plum Pudding Model theorized that an atom was made up electrons “the plums” surrounded by a positively charged mass “the pudding”. Rutherford later proved that this wasn’t the case and theorized that atoms were comprised of a very small nucleus surrounded by electrons. The basic tenets of that statement are still held to be true in the modern era. The chief difference is the fact that Niels Bohr, when theorizing the design of the atom, included a thesis for the forces that help to hold the atomic structure together. The Bohr model is still widely accepted by contemporary scientists. The Bohr model of the atom was established in 1913 as the leading theory of atomic structure. Because of this, it can be truthfully said that Rutherford’s Atomic Theory was an extremely short-lived phenomenon. Despite its brief time frame, however, Rutherford’s theory was extremely important because Niels Bohr wouldn’t have been able to develop his own model without the established background that Rutherford’s Atomic Theory laid out for him.
www.angelfire.com/sc2/atomtheory/
www.iun.edu/~cpanhd/C101webnotes/…/dalton.html
www.angelfire.com/sc2/atomtheory/
www.chemprofessor.com/atomic.htm
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