(a) The elements are arranged in the Modern Periodic Table according to increasing atomic number. This eliminates the oddity relating to specific elemental pair pairs in Mendeleev\’s periodic table.
(b) Nickel has an atomic number of 28 and cobalt has an atomic number of 27. Therefore, despite having a larger atomic mass than nickel, cobalt will appear before it.
(c) All isotopes of the same elements have different atomic masses but the same atomic number; therefore, they are placed in the same position in the modern periodic table.