How were the patterns in the elements that Mendeleev recognized different from the patterns that John Newlands recognized?
While developing the Periodic Table, there were a few instances where Mendeleev had to place an element with a slightly greater atomic mass before an element with a slightly lower atomic mass. The sequence was inverted so that elements with similar properties could be grouped together. For example, cobalt (atomic mass 58.9) appeared before nickel (atomic mass 58.7).
Further, Mendeleev left some gaps in his Periodic Table. Instead of looking upon these gaps as defects, Mendeleev boldly predicted the existence of some elements that had not been discovered at that time. Mendeleev named them by prefixing, Eka to the name of preceding element in the same group. For instance, scandium, gallium and germanium, discovered later, have properties similar to Eka–boron, Eka–aluminium and Eka–silicon, respectively. The
properties of Eka–Aluminium predicted by Mendeleev and those of the element, Gallium which was discovered later and replaced Eka-Aluminium, are listed as follows:
This provided convincing evidence for both the correctness and
usefulness of Mendeleev's Periodic Table. Further, it was the extraordinary success of Mendeleev's prediction that led chemists not only to accept his Periodic Table but also recognise him, as the originator of the concept on which it is based.
Noble gases were discovered very late because they are very inert and present in extremely low concentrations in our atmosphere. One of the strengths of Mendeléev’s Periodic Table was that, when these gases were discovered, they could be placed in a new group without disturbing the existing order.