*Elements that only have unstable isotopes are listed with the mass number of the isotope that has the longest half-life in parentheses  (to indicate weight).  
Element name Atomic weight
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At STP — standard temperature (75°F) and pressure (14.7 psia)
Periodic Table of the ElementsTap an element, group, or period for its brief description. Tap a state or category to show the elements in that state (at STP — 75°F & 14.7 psia) or category. Tap a temperature to see the change in element states.