Background:
In Mendeleev's Periodic Table, the rare earth elements were initially placed separately at the bottom of the table due to their similarities in chemical properties. These elements were later rearranged and integrated into the main body of the table.

Cerium:
Cerium was one of the elements that was initially placed in the rare earths group in Mendeleev's Periodic Table. It is a lanthanide metal with the atomic number 58. Cerium exhibits similar chemical properties to other rare earth elements, which led to its classification in the rare earths group.

Reasons for Placement:
1. Similar Properties: Cerium shares many chemical properties with other rare earth elements, such as lanthanum, praseodymium, and neodymium. These similarities made it suitable for placement in the rare earths group.

2. Position in Periodic Table: Cerium's position in the periodic table, along with its atomic number and electron configuration, also contributed to its initial placement in the rare earths group.

Reclassification:
Over time, as more information about the rare earth elements became available, their placement in the periodic table was refined. The rare earth elements were eventually integrated into the main body of the table based on their atomic numbers and properties.

Conclusion:
Cerium was initially placed in the rare earths group in Mendeleev's Periodic Table due to its similarities with other rare earth elements. However, as the understanding of these elements improved, they were reclassified and integrated into the main body of the periodic table based on their atomic numbers and properties.