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In 1871, Mendeleev published an updated form of periodic table (shown at left), as well as giving detailed predictions for the elements he had earlier noted were missing, but should exist.[69] These gaps were subsequently filled as chemists discovered additional naturally occurring elements.[70] It is often stated that the last naturally occurring element to be discovered was francium (referred to by Mendeleev as eka-caesium) in 1939.[71] However, plutonium, produced synthetically in 1940, was identified in trace quantities as a naturally occurring primordial element in 1971,[72] and by 2011 it was known that all the elements up to californium can occur naturally as trace amounts in uranium ores by neutron capture and beta decay.[6]