\(\mu \,\, = \,\,\sqrt {2(2\,\, + \,\,2)} \,\, = \,\,2.84\,\,BM\)
$(a)$ $\left( NH _{4}\right)_{2}\left[ Ce \left( NO _{3}\right)_{6}\right]$
$(b)$ $Gd \left( NO _{3}\right)_{3}$ અને
$(c)$ $Eu \left( NO _{3}\right)_{3}$