The element with the symbol “Os” has atomic number 76. That element is osmium, a rare, hard, brittle, bluish-white transition metal in the platinum group.
An atomic number identifies the number of protons in an atom’s nucleus. Every osmium atom therefore contains 76 protons; changing that number would create a different element. The symbol Os comes from the Greek word “osme,” meaning smell, referring to the sharp odor of some osmium compounds.
Osmium is famous for having the highest density among stable elements under commonly cited conditions. It is found in tiny quantities alongside other platinum-group metals and is used in specialized alloys, electrical contacts, and extremely durable instrument tips.
The nearby choices can cause confusion because several are also transition metals. Platinum has atomic number 78, hafnium has 72, and ytterbium has 70. Atomic number 76 belongs specifically to osmium, not to any of those neighboring elements.