The Periodic Spiral assigns the color green to sodium, aluminum, chlorine, cesium, tantalum, iridium, thallium, and astatine. Only two of these elements have physical associations with this hue. Biochemists use Sodium Green as the NaŅ indicator in fluorescence lifetime microscopy. The name of the element chlorine, a greenish gas, comes from the Greek word khlŰros (green). Thalliumís discoverer, Crookes, named the element for Greek thallos, a young, or green, twig or shoot, after the color of the elementís spectral line and the bright green color it imparts to flames.