This approach has the advantage of relying on the standard electronic block letters s, f, d and p to distinguish the metals rather than collective names such as active metals, transition metals, and post-transition or poor metals, or numerous alternatives. It avoids uncertainties as to where the transition metals start and end. For the same reason, it does not necessarily fix which d-block elements are noble metals. Individual authors can replace the block labels with their preferred class names. While bringing the s-and f-block metals under one umbrella may seem questionable the similarities between the two sets are well documented in the literature. Thus, the lanthanides are described as behaving largely as if they were trivalent versions of the +2 cations Ca 2+ , Mg 2+ of group 2 . More generally, associates the chemistry of the lanthanides in the +3 state with the chemistry of the alkali and alkaline earths. The actinides are similarly reactive noting the early members show a wider range of oxidation states. Further: "Many significant trends are apparent in the structures of the halides and their physical and chemical properties…The majority of pre-transition metals (Groups 1, 2) together with Group 3, the lanthanides and the actinides in the +2 and +3 oxidation states form halides that are predominately ionic in character, whereas the non-metals and metals in high oxidation states (≥ +3) tend to form covalent molecular halides." The nonmetals are parsed into three well-known sets: halogen, metalloid, and noble gas. The remaining "left-over" nonmetals are referred to as "unclassified" nonmetals.