Dalton atomic theory


After Dalton, in 1807, had put forward the theory that chemical combination consisted of a union between atoms, chemists began their search for the cause and mechanism of the unions. Many ideas were put forward during the following years but, following the discoveries about the structure of the atom, it was realised that the nuclei of atoms were unaffected by chemical combination and that union of atoms must result from interaction between the extra-nuclear electrons. Kossel and Lewis, working independently in 1916, recognised that the atoms of the different noble gases, with the one exception of helium, each had an outer quantum level containing eight electrons they therefore suggested that this arrangement must be connected with stability and inactivity, and that reactions occurred between atoms such that each element attained a noble gas configuration. The rearrangement of electrons into stable octets could occur in two ways (a) by giving or receiving electrons or (b) by sharing electrons.  [c.28]

The atom and bond concepts dominate chemistry. Dalton postulated that atoms retained their identities even when in chemical combinations with other atoms. We know that their properties are sometimes transferable from one molecule to another for example, the incremental increase in the standard enthalpy of formation of a normal hydrocarbon per CHj group is —20.6 1.3 kJmol . We also know that more often there are subtle modifications to the electron density.  [c.316]

However, it was still a long time even after Dalton before anything definite could be known about the internal motions in molecules. The reason was that the microscopic nature of atoms and molecules was a bar to any knowledge of their internal constituents. Furthemiore, nothing at all was known about tlie physical laws that applied at the microscopic level. The first hmts came in the late 19th century, with the classical Maxwell-Lorentz theory of the dynamics of charged particles interacting through the electromagnetic field. The electron was discovered by Thomson, and a little later tlie nuclear structure of the atom by Rutherford. This set the stage in the 20th century for a physical understanding in temis of quantum theory of the constituents of molecules, and the motions of which tliey partake.  [c.53]

In 1804 Dalton formnlated the concept that identified chemical elements with atoms. The notion of atoms, the smallest corpnscles of matter, was not new, of course, and had been around in some form or other since antiquity. Dalton, however, addressed the question to differentiate atoms not only by size (or shape) but also by their weight. To do this, Dalton turned to Proust s law, according to which the relationship of masses according to which two or more elements combine is fixed and not susceptible to continuous variation, and made it the center of his atomic hypothesis. He suggested that chemical combination takes place via discrete units, atom by atom, and that atoms of each element are identical. He also added the concept of multiple proportions that is, when two elements form different compounds the weights in which one element will combine with another are in a simple numerical ratio. Dalton s atomic concept gave the whole body of available chemical information an immediate, easily recognizable meaning. What was also needed, however, was to relate all the atomic weights to a single unit. Dalton chose the atomic weight of hydrogen for this unit. Dalton s atoms also differed fundamentally from Newtonian corpuscles because they were not derived from an attempt to be based on the laws of motion and the attraction of single bodies whose ultimate constituents would be atoms.  [c.29]


See pages that mention the term Dalton atomic theory : [c.65]   
A life of magic chemistry (2001) -- [ c.28 ]