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The f-block consists of the two series, lanthanoids (the fourteen elements following lanthanum) and actinoids (the fourteen elements following actinium). . Because actinium closely resembles the actinoids, it is usually included in any discussion of the actinoids for which the general symbol An is often used. The chemistry of actinoids is complex. The complication arises partly owing to the occurrence of a wide range of oxidation states in these elements and partly because their radioactivity creates special problems in their study.