7 . Four transition series :
a . 3d – transition series . The transition elements with atomic number 21 ( Sc ) to 30 ( Zn ) and having incomplete 3d orbitals is called the first transition series .
b . 4d – transition series . It consists of elements with atomic number 39 ( Y ) to 48 ( Cd ) and having incomplete 4d orbitals . It is called second transition series . c . 5d – transition series . It consists of elements with atomic number 57 ( La ), 72 ( Hf ) to 80 ( Hg ) having incomplete 5d orbitals . It is called third transition series .
d . 6d – transition series . It consists of elements with atomic number 89 ( Ac ), 104 ( Rf ) to 112 ( Uub ) having incomplete 6d orbitals . It is called fourth transition series .
8 . General Characteristics of transition elements :
a . Metallic character : All transition elements are metallic in nature , i . e . they have strong metallic bonds . This is because of presence of unpaired electrons . This gives rise to properties like high density , high enthalpies of atomization , and high melting and boiling points .
b . Atomic radii : The atomic radii decrease from Sc to Cr because the effective nuclear charge increases . The atomic size of Fe , Co , Ni is almost same because the attraction due to increase in nuclear charge is cancelled by the repulsion because of increase in shielding effect . Cu and Zn have bigger size because the shielding effect