Nota Terkini

Friday, May 22, 2009

[Chemistry Form 4] Elements

Initially, Antoine Lavoisier classified "elements" into four groups based on their chemical properties. He included oxygen, nitrogen, hydrogen, heat and light under gases. His classification was unsuccessful because his table consisted of non-elements and compounds.

Then, J.W. Dobereiner introduced the law of triads. He divided the elements into groups of three with similar chemical properties. For example, one group comprises lithium, sodium and potassium, and the other chlorine, bromine and iodine. His attempt was unsuccessful because the classification was limited to a few elements only.

Further progress was made when John Newlands arranged the elements according to increasing relative atomic volume. He proposed the law of octaves, which stated that elements with similar properties recurred after every eight elements. However, his pattern only worked for the first few elements.

Lothar Meyer plotted the graph of atomic volume against the atomic mass of each element and found that elements that have similar chemical properties occupied equivalent position in the graph. For example Li, Na, K, Rb (alkali metals) occupied the tips of the curve plotted.

Dmitri Mendeleev arranged the elements according to increasing relative atomic mass in horizontal groups called periods. Elements with similar chemical properties were arranged in vertical columns called groups.

When the pattern began to go wrong, he would leave a gas in his table. He claimed that the gaps were for elements that had not yet been discovered. He even changed the order round when similar elements didn't line up.

In the early stages, other chemists doubted his ideas but when the element germanium, which was discovered in 1886, matched his prediction, Mendeleev's periodic table was accepted.

Years later, Henry Moseley contribute further to the development of the modern periodic table. He arranged the elements according to increasing proton number and had elements with the same number of valence electrons listed under the same group.

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