Availability of electron orbitals in metals and metalloids

Outer Orbitals empty orbitals which make little or no contribution to bonding, being too high in energy 

Valence Orbitals, which may or may not be fully occupied and which are important in bonding 

Inner (Core) Orbitals, which are filled and are too low in energy to contribute significantly to the bonding 

Thus although all elements possess /-orbitals, only in the case of rf-block transition elements do they have both suitable energy for bond formation and contain electrons. 

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