The formula of salt is XCl₂, in which X has 26 electrons in this salt, thus, the X is metal - Ni as it gives two electrons to chloride.
In this salt, the number of electrons in X has 26. It also has 2 positive charges on it as in this salt. Which meant X must have 28 electrons and, therefore, 28 protons.
Protons are represents the atomic number of atoms, 28 is the atomic number of Nickle (Ni). The X is Nickle in this salt.
Learn more about the Oxidation state:
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