Answer:
Atomic orbital energy ordering.
Explanation:
The transition metals constitute the d-block. The answer to your question has to do with the energy ordering of atomic orbitals. Specifically, the 3s orbitals are lower in energy than 3p, which are lower in energy than 4s, which are lower in energy than 3d (remember the principal quantum number for d orbitals is one minus the principle quantum number of the shell so n = 4 level's d orbitals are the 3d orbitals). According to the Aufbau principle, atomic orbitals are filled with electrons from the lowest energy up. So the orbitals would have to be filled in the order of 3s, 3p, 4s, and then 3d. Magnesium has its last valence electron residing in the 3s orbital and Aluminum has its last valence electron residing in the first 3p orbital (specifically the 3px orbital, which is aligned horizontally in the 3d plane. The three p orbitals for all principle quantum levels are px, py, pz, with the x, y, z describing the orientation in the 3d plane). The 3d has not yet been reached in terms of energy ordering. This is why there are no transition elements between magnesium and aluminum, in terms of electronic structure.
