The principal and most common oxidation state of all lanthanoids is +3. Although some elements in the series show +2 and +4 oxidation states in solution or in solid compounds (often linked to the extra stability of empty, half-filled, or filled $f$-subshells, i.e., $f^0, f^7, f^{14}$), the +3 state is universally predominant. Ions in other states often act as strong oxidising or reducing agents to revert to the stable +3 state .