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John dalton
From 1814 to 1819, Irish chemist William Higgins claimed that Dalton had plagiarised his ideas, but Higgins' theory did not address relative atomic mass.However, recent evidence suggests that Dalton's development of thought may have been influenced by the ideas of another Irish chemist Bryan Higgins, who was William's uncle. Bryan believed that an atom was a heavy central particle surrounded by an atmosphere of caloric, the supposed substance of heat at the time. -
groups of three elements
A German scientist called Johann dobereiner put forward his law of triads each of the triads were a group of three elements the appearance and reactions of the elements in a triad were similar to each other -
Period: to
groups of three elements
at this time scientist had begun to find out the relative atomic masses of the elements dobereiner discovered that the relative atomic mass of the middle of the element in each triad were close to the average of the relative atomic masses of the other two element this gave scientists the clue that relative atomic masses were important when arranging the elements -
John newlands
An English scientist called John Newlands put forward his law of octaves in 1864. He arranged all the elements known at the time into a table in order of relative atomic mass. When he did this, he found that each element was similar to the element eight places further on. For example, starting at Li, Be is the second element, B is the third and Na is the eighth element. -
Dimitri mendeleve
In 1869, just five years after John Newlands put forward his law of octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. he did some other things that made his table much more successful.He realised that the physical and chemical properties of elements were related to their atomic mass in a 'periodic' way, and arranged them so that groups of elements with similar properties fell into vertical columns in his table -
Period: to
Dimitri medeleve cont.
He was also able to work out the atomic mass of the missing elements, and so predict their properties. And when they were discovered, Mendeleev turned out to be right.For example, he predicted the properties of an undiscovered element that should fit below aluminium in his table. When this element, called gallium, was discovered in 1875, its properties were found to be close to Mendeleev's predictions.Two other predicted elements were later discovered, lending further credit to Mendeleev's table -
henry moseley
Using atomic number instead of atomic mass as the organising principle was first proposed by the British chemist Henry Moseley in 1913, and it solved anomalies like this one. Iodine has a higher atomic number than tellurium. So, even though he didn't know why, Mendeleev was right to place it after tellurium after all!
You need to be able to locate the positions of the:
• alkali metals (group 1)
• halogens (group 7)
• noble gases (group 0)
• transition metals
