7 Mendeleev’s Periodic Table Produced a table based on atomic weights but arranged 'periodically' with elements with similar properties under each other.
8 Mendeleev’s Periodic Table Gaps were left for elements that were unknown at that time and their properties predicted (the elements were gallium, scandium and germanium).The order of elements was re-arranged if their properties dictated it, eg, tellerium is heavier than iodine but comes before it in the Periodic Table.
10 Modern Periodic TableOrganized according to mass number rather than atomic massesThis solved the “mass discrepancy” issue with certain elements where different isotopic abundances caused masses to be out of orderE.g. Ar and KCo and NiTe and I
16 Alkali Metals Li, Na, K, Rb, Cs, Fr These atoms each have one electron in their valence energy levelThey lose one electron to form a +1 ionMost active metals on the periodic tableSo reactive that they are never found as free elements
17 Periodic Table Some Metals Alkaline Earth Metals
18 Alkaline Earth Metals Be, Mg, Ca, Sr, Ba, Ra The atoms each have two electrons in their valence energy levelThey lose two electrons to form +2 ion