Why are electrons restricted to specific orbitals?

The answer lies in the concept of quantum mechanics. Every single electron of an atom has its own unique set of quantum numbers. It is stated in the Pauli Exclusion principle that no two electrons would have the same quantum numbers. That is why each electron occupies a specific orbital. In addition to the Pauli Exclusion Principle, the other laws and principles that help set the quantum numbers are the Aufbau Principle and the Hund's rule.

All the elements are arranged by melting points on the periodic table. in row they go from metals to gasses. in the row from sodium to argon, sodium is more reactive than the one on its right, magnesium.