<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> <Dl> <Dd> 227 89 Ac α (1.38%) → 21.77 y 223 87 Fr β → 22 min 223 88 Ra α → 11.4 d </Dd> </Dl> <Dd> 227 89 Ac α (1.38%) → 21.77 y 223 87 Fr β → 22 min 223 88 Ra α → 11.4 d </Dd>

Where are the most highly reactive elements located on the periodic table