Final answer:
Elements 21 through 30 on the periodic table are known as b. transition metals, which are part of the first transition series in the fourth period and are distinct from rare earth elements, alkaline earth metals, chalcogens, and halides.
Step-by-step explanation:
The elements with atomic numbers 21 through 30 are known as transition metals. These elements are part of the first transition series, which is the first row of the d-block in the fourth period of the periodic table. The rare earth elements consist of a collection of 17 elements, which include the lanthanides plus scandium and yttrium, and are not the same as transition metals. Transition metals are distinguished from alkaline earth metals (group 2 elements), chalcogens (group 16), and halides, which refer to compounds that include halogens.