<P> Rubidium and caesium were the first elements to be discovered using the spectroscope, invented in 1859 by Robert Bunsen and Gustav Kirchhoff . The next year, they discovered caesium in the mineral water from Bad Dürkheim, Germany . Their discovery of rubidium came the following year in Heidelberg, Germany, finding it in the mineral lepidolite . The names of rubidium and caesium come from the most prominent lines in their emission spectra: a bright red line for rubidium (from the Latin word rubidus, meaning dark red or bright red), and a sky - blue line for caesium (derived from the Latin word caesius, meaning sky - blue). </P> <P> Around 1865 John Newlands produced a series of papers where he listed the elements in order of increasing atomic weight and similar physical and chemical properties that recurred at intervals of eight; he likened such periodicity to the octaves of music, where notes an octave apart have similar musical functions . His version put all the alkali metals then known (lithium to caesium), as well as copper, silver, and thallium (which show the + 1 oxidation state characteristic of the alkali metals), together into a group . His table placed hydrogen with the halogens . </P> <P> After 1869, Dmitri Mendeleev proposed his periodic table placing lithium at the top of a group with sodium, potassium, rubidium, caesium, and thallium . Two years later, Mendeleev revised his table, placing hydrogen in group 1 above lithium, and also moving thallium to the boron group . In this 1871 version, copper, silver, and gold were placed twice, once as part of group IB, and once as part of a "group VIII" encompassing today's groups 8 to 11 . After the introduction of the 18 - column table, the group IB elements were moved to their current position in the d - block, while alkali metals were left in group IA . Later the group's name was changed to group 1 in 1988 . The trivial name "alkali metals" comes from the fact that the hydroxides of the group 1 elements are all strong alkalis when dissolved in water . </P> <P> There were at least four erroneous and incomplete discoveries before Marguerite Perey of the Curie Institute in Paris, France discovered francium in 1939 by purifying a sample of actinium - 227, which had been reported to have a decay energy of 220 keV . However, Perey noticed decay particles with an energy level below 80 keV . Perey thought this decay activity might have been caused by a previously unidentified decay product, one that was separated during purification, but emerged again out of the pure actinium - 227 . Various tests eliminated the possibility of the unknown element being thorium, radium, lead, bismuth, or thallium . The new product exhibited chemical properties of an alkali metal (such as coprecipitating with caesium salts), which led Perey to believe that it was element 87, caused by the alpha decay of actinium - 227 . Perey then attempted to determine the proportion of beta decay to alpha decay in actinium - 227 . Her first test put the alpha branching at 0.6%, a figure that she later revised to 1% . </P>

The most reactive metals on the periodic table are located