Lanthanoids are otherwise known as rare earth elements, and are found in the f-block of the periodic table. They are comprised of the 15 elements from Lanthanum (atomic number 57) to Lutetium (atomic number 71). Commonly referred to as “inner transition metals,” lanthanoids are characterized by their shiny surfaces and low densities.
Actinoids are also found in the f-block of the periodic table, and are comprised of the elements from Actinium (atomic number 89) to Lawrencium (atomic number 103). They are sometimes referred to as “inner transition metals” as well, but are heavier and much more radioactive than lanthanoids.