1. Symbol As A highly poisonous metallic element having several allotropic forms of which the brittle, crystalline gray metallic form is the most common. The less stable yellow allotrope has a molecular structure, As4. Arsenic and its compounds are used in insecticides, weed killers, semiconductor dopants, and various alloys. Atomic number 33; atomic weight 74.9216; valence 3, 5. Gray arsenic melts at 817°C (at 28 atm pressure), sublimes at 616°C, and has a specific gravity of 5.75. See Periodic Table.
2. Arsenic trioxide.
adj. ar·sen·ic (är-sĕnĭk)
Of or containing arsenic, especially with valence 5.
[Middle English arsenik, from Old French, from Latin arsenicum, from Greek arsenikon, yellow orpiment, alteration of Syriac zarnīkā, from Middle Persian *zarnīk, from Old Iranian *zarna-, golden; see ghel-2 in the Appendix of Indo-European roots.]
The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2018 by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
The American Heritage Dictionary Blog
Check out our blog, updated regularly, for new words and revised definitions, interesting images from the 5th edition, discussions of usage, and more.