(a) In the Modern Periodic table elements are arranged in the increasing order of their atomic number. This removes the anomaly regarding certain pairs of elements in Mendeleev's periodic table.
(b) Atomic number of cobalt is 27 and nickel is 28. Hence cobalt will come before nickel even though its atomic mass is greater.
(c) All isotopes of the same elements have different atomic masses, but same atomic number; therefore they are placed in the same position in the modern periodic table.