The atomic and ionic radii of the transition elements decrease from group 3 to group 6 due to the poor shielding offered by the small number of d-electrons. Those placed between groups 7 and 10 have somewhat similar atomic radii and those placed in groups 11 and 12 have larger radii. This is because the nuclear charge is balanced out by the electron-electron repulsions.
Transition elements or transition metals are elements that have partially filled d orbitals.
"Transition elements as an element having a d subshell that is partially filled with electrons, or an element that has the ability to form stable cations with an incompletely filled d orbital".
In general, any element which corresponds to the d-block of the modern periodic table (groups 3-12) is considered to be a transition element.
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