Why are the elements 57 through 70, and 89 through 102, found separately at the bottom of the periodic table?

The elements in 57 through 70 are called rare earths. They are also called the lanthanides, because they follow and resemble lanthanum on the periodic table. They differ from each other only in the amount of electrons they have in the fourth level.

The elements in 89 through 102 are called actinides. They are very similar to the lanthanides. They have different numbers of electrons in the fifth level. They are all radioactive and only Ac (actinium), Th, Pa, and U occur in nature.

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