The atom for which the prediction of ionic charge has been difficult based on the periodic table is Cobalt.
The ionic charge has been imposed on the atom when there has been a loss or gain of an electron, in order to achieve the stable electronic configuration. The elements on the loss of electron have attained a +ve charge, while the gain of an electron by an atom gives it a -ve charge.
The elements in the periodic table have an oxidation state for the ionic form of the elements. The varying oxidation state element has been able to attain a varying ionic state. Cobalt has been present with varying oxidation states, thus it has been difficult to predict the ionic charge of Cobalt using the periodic table.
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