Nickel (Ni), with an atomic number (Z) of 28, is located in Group 10 of the periodic table. Group 10 elements are transition metals and have complex electron configurations.
The electron configuration of nickel is [Ar] 3d^8 4s^2. To determine the number of valence electrons, we look at the outermost energy level, which includes both the 3d and 4s orbitals.
In this case, the 4s orbital is filled with 2 electrons, while the 3d orbital has 8 electrons. The outermost energy level, therefore, contains 10 electrons (2 from 4s and 8 from 3d). Hence, nickel has 10 valence electrons.
Please note that while nickel has 10 valence electrons, it is often considered to have a valency of 2 due to its tendency to lose its 4s^2 electrons to form stable Ni2+ ions.