The electronic configuration of Gadolinium (Atomic number 64) is

The electronic configuration of La (Z = 57) is Xe 5d1 6s2. Therefore, further addition of electrons occurs in the lower energy 4f-orbital till it is exactly half-filled at Eu (Z = 63). Thus, the electronic configuration of Eu is Xe 4f7 6s2. Therefore, addition of next electron does not occur in the more stable exactly half-filled 4f7shell but occurs in the little higher energy 5d-orbital. Thus, the electronic configuration of Gd (Z = 64) is Xe 4f7 5d1 6s2.Note…In a f-block element the number of (n-1)d electrons are always 0 or 1 ;