$Nd ^{2+}=$ ___________
The question asks about the electronic configuration of the neodymium ion with a 2+ charge, represented as \(Nd^{2+}\). Neodymium is a lanthanide element with the atomic number 60. In its neutral state, the electronic configuration of neodymium is:
\([Xe] 4f^4 6s^2\)
When neodymium forms a 2+ ion, it loses two electrons. Typically, electrons are removed first from the outermost orbitals, which are the highest energy level orbitals. For neodymium, the 6s electrons are the highest in energy and thus removed first. Therefore, the loss of two electrons from Nd results in the following electron configuration:
Hence, the configuration of \(Nd^{2+}\) is:
\([Xe] 4f^4\)
Thus, the correct option is: $4 f ^4$.
To conclude, the electron configuration confirms that the neodymium ion with a 2+ charge has the electronic configuration \(4f^4\).