According to its periodic table entry, how many electrons does nickel have in its valence level?

10

Explanation:

The atomic number of Nickel is 28: this means that a neutral atom of nickel has 28 electrons.

The electronic configuration of nickel is:

abbreviated, this is written as

where represents the electronic configuration of argon. This means that all the shells until are completed, while the outermost shell:

is the one that gives us the number of valence electrons. Since there are 2 electrons in the 4s orbital and 8 electrons in the 3d orbital, the total number of valence electrons is

2 + 8 = 10