Final answer:
Selenium has more in common with sulfur because they are in the same group in the periodic table and have similar chemical properties, unlike bromine which is in a different group with different reactivity.
Step-by-step explanation:
Selenium has more in common with sulfur than bromine, as it is in the same family below sulfur in the periodic table. Both selenium and sulfur are nonmetals in group 16, which means they have similar chemical properties due to the same number of electrons in their outer shell and a comparable readiness to bond. The selenol group in selenium is more fully ionized at physiological pH compared to the thiol group in cysteine, indicating that substitution of selenium for sulfur, as occurs in certain enzymes, leads to drastic reductions in catalytic activity. This showcases the importance of selenium's unique chemistry over sulfur, despite the general similarity in reactivity attributable to their positioning in the periodic table.
The atomic number of selenium is 34, which places it in period 4 and group 16, signifying that it is a nonmetal, although some of its properties may be similar to a semimetal due to its position close to the metal-nonmetal dividing line. This contrasts with bromine, which is a halogen in group 17 known for its reactivity as an oxidizing agent, although it's less reactive than chlorine.
Historically, similarities between elements like sulfur and selenium have been exploited in medical science to create drugs by mimicking chemical structures, as demonstrated by Gertrude Elion's work using sulfur-based compounds to interrupt cell replication, which led to treatments for leukemia and earned her a Nobel Prize in 1988.