Sodium metal on dissolution in liquid ammonia gives a deep blue solution due to the formation of :

The ground state energy of hydrogen atom is – 13.6 eV. The energy of second excited state of He⁺ ion in eV is :

The 71^st electron of an element X with an atomic number of 71 enters into the orbital :

The difference in the number of unpaired electrons of a metal ion in its high spin and low-spin octahedral complexes is two. The metal ion is :