Why are there no transition elements in periods 1 and 2?

Transition elements are defined as d-block elements that have incomplete d subshells in their neutral or ionized states. In periods 1 and 2, the d subshell is not populated; period 1 contains only the 1s orbital, while period 2 fills the 2s and 2p orbitals but does not include d orbitals. Transition metals appear starting from period 3, where the 3d subshell begins to fill. Thus, the absence of transition elements in the first two periods is due to the lack of available d orbitals.