Complex sentence: Although scandium and yttrium are always the first two elements in group 3 the identity of the next two elements is not settled; these are either lanthanum and actinium; or lutetium and lawrencium.
Split into simpler sentences: Although scandium and yttrium are always the first two elements in group 3 the identity of the next two elements is not settled. They are either lanthanum and actinium; or lutetium and lawrencium.