At 20 °C and approximately 1 atm pressure, how many of the familiar elements are gases?

Explanation

At 20 °C and approximately 1 atm, the usual gaseous elements are H₂, N₂, O₂, F₂ and Cl₂, together with helium, neon, argon, krypton, xenon and radon. That is five diatomic elements and six monatomic noble gases, giving eleven. A temperature must be specified when counting gaseous elements.