Final answer:
The first row of transition elements is associated with the filling of the 3d subshell, located in the fourth period and d-block of the periodic table. The filling sequence generally follows the order: 4s, then 3d, up to zinc which completes the subshell with 10 electrons. Transition elements exhibit distinct properties stemming from these d electrons.
Step-by-step explanation:
The first row of transition elements (atomic numbers 21-30) corresponds to the filling of the 3d subshell. These elements are situated in the fourth period of the periodic table, commonly referred to as the first row of the d-block. The electron filling sequence starts with scandium (atomic number 21), where electrons begin to occupy the 3d subshell after the 4s sublevel has been filled (though for potassium and calcium, the 4s sublevel fills before the 3d begins filling). The 3d subshell has a total capacity for 10 electrons, corresponding to the five d orbitals (since for l = 2 [d orbitals], there are 2l + 1 = 5 values of ml).
As we progress through the transition series from scandium (Sc) to zinc (Zn), electrons are added to the 3d subshell. Despite slight deviations due to subshell energy variations, the general pattern of filling is consistent with the order: 4s, then 3d, followed by 4p. This creates a unique set of properties for transition elements, such as varied oxidation states and brightly colored compounds due to d electron transitions.