Selenium (Se) is a member of Group 16 (the chalcogens) in the periodic table, alongside oxygen (O), sulfur (S), tellurium (Te), and polonium (Po). The element that is most chemically similar to selenium is sulfur. Both selenium and sulfur share similar chemical properties due to their similar electron configurations and the fact that they are in the same group.
Tellurium is also chemically similar to selenium, as it is directly below selenium in the periodic table, making it another close analog. However, in terms of chemical properties and behavior, sulfur is often considered the most similar.