How many valence electrons does nickel have?

Hint: The valence electrons are those electrons that are present in the outermost shell of an atom. The nickel has an atomic number as ${\text{28}}$. One can find out the outermost shell electrons according to the electronic configuration of the nickel-based on the number of electrons present in it.

Complete step by step answer:
1) First of all, we will learn about the concept of valence electrons. The valence electrons are the electrons that take part in the formation of a covalent bond with other atoms. The valence electrons are the electrons present in the outermost shell of an atom.
2) Now let us see how we can find out the valence electrons present in the nickel atom. The nickel is a transition element that is present in the period ${\text{4}}$ of the periodic period. To find out the electrons present in the outermost shell of the nickel atom we first need to find out the electronic configuration of the nickel atom. The nickel atom has an atomic number as ${\text{28}}$ which means there are ${\text{28}}$ electrons present in it.
3) The electronic configuration of the nickel atom is as below,
$Ni = 1{s^2},2{s^2},2{p^6},3{s^2},3{p^6},3{d^8},4{s^2}$
4) Now the electronic configuration of the nickel shows that the outermost orbital which is ${\text{4s}}$ has two electrons which are involved in the bonding when nickel forms covalent bonds with other elements.
Therefore, we can say that nickel has two valence electrons.

Note:
The valence electron is the basis for an element's bonding behavior. The elements which have the same valence electrons show similar chemical properties and similar structural properties such as bonding.