Question: are all noble gases diatomic?
Answer: The noble gases (helium, neon, argon, krypton, xenon, and radon) are also gases at STP, but they are monatomic. ... At slightly elevated temperatures, the halogens bromine (Br2) and iodine (I2) also form diatomic gases. All halogens have been observed as diatomic molecules, except for astatine, which is uncertain.