Final answer:
Transition elements are defined as having D orbitals filled as you move left to right across the periodic table, especially in groups 3-11. These metals are characterized by properties such as high electrical conductivity and the ease of losing valence electrons. The IUPAC, however, specifies transition elements as those with partially filled d orbitals.
Step-by-step explanation:
The elements characterized as having their D orbitals filled with electrons as you move left to right across a period are known as transition elements or transition metals. Transition metals occupy the d block of the periodic table, specifically groups 3-11. These elements have valence electrons that include the ns and (n-1) d electrons. The filling of the D orbitals aligns with the electron configuration where the d sublevels are being filled before the p sublevels of the same principal quantum number.
The transition elements continue to fill their d orbitals up till group 11. However, starting from group 12, elements are considered representative elements because the last electron fills an s orbital. Transition metals are known for their properties such as high thermal and electrical conductivity, and the ability to lose valence electrons, which contribute to their metallic character.
It is essential to note that the IUPAC defines transition elements as those with partially filled d orbitals but often the term transition elements is used more broadly to include elements with filled d orbitals such as Zn, Cd, and Hg.