The ordering of elements in the periodic table is not solely determined by their average atomic masses. Instead, it is based on a combination of factors, including the element's atomic number (the number of protons in its nucleus) and its electronic configuration.
The periodic table is organized in a way that groups elements with similar chemical properties together in the same column (group) and arranges them in order of increasing atomic number from left to right across each row (period). The atomic number serves as the primary factor for determining the position of an element in the periodic table.
In the case of nickel (Ni) and cobalt (Co), nickel has an atomic number of 28, while cobalt has an atomic number of 27. Since atomic number takes precedence in determining the element's position, nickel is placed after cobalt in the periodic table.
While the atomic mass of an element can provide information about the average mass of its isotopes, it does not dictate the order of elements in the periodic table. The periodic table is primarily structured based on atomic number and the resulting electronic configurations, which play a crucial role in determining an element's chemical behavior and properties.