Chapter 7 The Structure of Atoms and Periodic Trends - 7-6 Periodic Trends and Chemical Properties - Applying Chemical Principles - Questions - Page 280: 3

a) Image attached b) The most common oxidation state is +3. The Electron Configuration of Gd$^{3+}$ is [Xe]$4f^{7}$

a) Image attached b) The most common oxidation state is +3 because it corresponds to the loss of the two $6s$ and one $5d$ electrons. If we use noble gas notation we get the Electron Configuration of Gd$^{3+}$ to be [Xe]$4f^{7}$