The electronic configurations of Eu(Atomic No 63),
Gd(Atomic No 64) and Tb (Atomic No. 65) are
(1) [Xe]``4f^76s^2``, [Xe]``4f^8 6s^2`` and [Xe]``4f^85d^16s^2``
(2) [Xe]``4f^75d^16s^2``, [Xe]``4f^7 5d^1 6s^2`` and [Xe]``4f^96s^2``
(3) [Xe]``4f^65d^16s^2``, [Xe]``4f^75d^16s^2`` and [Xe]``4f^85d^16s^2``
(4) [Xe]``4f^76s^2``, [Xe]``4f^75d^16s^2 ``and [Xe]``4f^96s^2``