Döbereiner's triads Groups of three elements

1. Döbereiner's triads Groups of three elements A German scientist called Johann Döbereiner put forward his law of triads in 1817. Each of Döbereiner's triads was a group of three elements. The appearance and reactions of the elements in a triad were similar to each other. Alkali formers Salt formers Atomic masses At this time, scientists had begun to find out the relative atomic masses of the elements. Döbereiner discovered that the relative atomic mass of the middle element in each triad was close to the average of the relative atomic masses of the other two elements. This gave other scientists a clue that relative atomic masses were important when arranging the elements.

2. Newlands' octaves A table in order of relative atomic mass John Newlands (1837 - 1898) 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. Part of Newlands' table Regular repeats Newlands' table showed a repeating or periodic pattern of properties, but it had problems. For example, he put iron in the same group as oxygen and sulphur, which are two non-metals. As a result, his table was not accepted by other scientists.

3. Mendeleev's periodic table Another table in order of relative atomic mass Dmitri Mendeleev (1834 - 1907) In 1869, just five years after John Newlands put forward his law of octaves, a Russian chemist called Dmitri Mendeleev published a periodic table. Mendeleev also arranged the elements known at the time in order of relative atomic mass, but 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. Part of Mendeleev's periodic table Gaps and predictions Sometimes this method of arranging elements meant there were gaps in his horizontal rows or 'periods'. But instead of seeing this as a problem, Mendeleev thought it simply meant that the elements which belonged in the gaps had not yet been discovered. 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.

4. The Modern Periodic Table • Mendeleev's table needed one important modification before it became the modern periodic table – the use of atomic number to order the elements. • All atoms of the same element contain the same number of particles called protons, and this is called the element's atomic number. • Mendeleev put the elements in order of their relative atomic mass, and this gave him some problems. For example, iodine has a lower relative atomic mass than tellurium, so it should come before tellurium in Mendeleev's table. In order to get iodine in the same group as other elements with similar properties such as fluorine, chlorine and bromine, he had to put it after tellurium, so breaking his own rules. • Henry Moseley (1887 - 1915) • 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 • The modern periodic table