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Development of the periodic table.

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Development of the periodic table The periodic table developed gradually by contributions from various scientists. This eventually led to the modern periodic table. 1. Dobereiner began with the development by grouping elements based on their similarities and classified them using their relative atomic mass. Dobereiner also identified a 'triad' in the list of elements. Calcium, strontium and barium had similar chemical properties. He noticed that the atomic weight of strontium fell midway between the weights of calcium and barium. (Ca 40.1 + Ba 137.5) � 0.5 = 88.7 Sr = 87.6 2. In order of increasing atomic weights, Newlands pointed out that every eighth element had similar properties, e.g. Hydrogen and Lithium (see fig.1). He called this the 'Law of Octaves'. 3. Mendeleev placed elements according to their increase in atomic mass. He left gaps for undiscovered elements. Mendeleev predicted the properties of the unknown elements based on their 'periodic properties', (see fig. 2). Two of these elements he named 'eka- aluminium' and 'eka- silicium'. 4. Reyleigh and Ramsey reported the discovery of noble gases. ...read more.


The properties of gallium make it hard to classify it as either a metal or a non metal as it has characteristics of both. Like aluminum, it dissolves in both acids and alkalis, evolving hydrogen. This is because both elements are in the same group; therefore their reactivity with other molecules is similar. Aluminum and gallium with Hydrogen ions: 2Al (s) + 6H+ (aq) 2Al �+ (aq) + 3H2 (g) 2Ga (s) + 6H+ (aq) 2Ga �+ (aq) + 3H2 (g) Aluminum and gallium with hydroxide ions: 2Al (s) + 2OH- (aq) + 6H2O (l) 2 [Al(OH)4] - (aq) + 3H2 (g) 2Ga (s) + 2OH- (aq) + 6H2O (l) 2 [Ga(OH)4]- (aq) + 3H2 (g) (Article 1) Very few metals behave in this way which also shows gallium is unusual. This supports the idea of aluminum and gallium being very similar in properties, clearly proving Mendeleev's predictions to be correct. Techniques to increase knowledge of chemical elements There are two main in ways in which chemists have increased their knowledge on chemical elements. ...read more.


Through this technique, chemists found that Nuclei repel because they have a positive charge and that chemical reactions involve loss or gain of electrons. Scientists can strip off the electron shells from even very heavy atoms. 'This allows them to study quantum electrodynamics the most precise theory in physics to unprecedented levels of accuracy' (GSI in profile). The elements that are unstable and have a very short half-life and have decayed long ago are only known to us through UNILAC accelerator. Fig 6, Particle beam target Discovering to synthesizing elements The discovery of elements began before the time of Newland's table. Mendeleev created a table with spaces for new elements to be discovered. The remaining elements were found in the last 140 years, including the noble gases. Techniques such as atomic spectroscopy supported the discovery and production of new elements. Through time, the development of understanding of the arrangement of electrons in atoms has helped to explain the patterns of behaviour and properties of elements. Scientists began to synthesize elements, (turn an element into another). New ways of producing elements were discovered such as the UNILAC accelerator. "... the exception proves the rule." (article 1) Sources Websites: www.hull.ac.uk/hull/chem-web/lectures/06217b/atomic-spec.html www.cerncourier.com/triari/article/38/9/9/1 http://fccjmail.fccj.org/~ethall/period/period.htm Books: Chemical Ideas (Salters) Farshea Saail 0095 1 ...read more.

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