Here's why:

* Nickel's electron configuration: Nickel has an atomic number of 28, meaning it has 28 electrons. Its electron configuration is [Ar] 3d⁸ 4s².

* Losing electrons to form an ion: To achieve a stable configuration, nickel loses its two 4s electrons, becoming Ni²⁺.

* Full outer shell: This leaves nickel with the electron configuration [Ar] 3d⁸, which is similar to the stable configuration of Argon. The 3d shell can hold up to 10 electrons, and now nickel has a full outer shell.

Important Note: This applies to the *ionic* form of nickel. In its *neutral* state, nickel's outer shell is not full, as it has two electrons in the 4s shell.