ARGON is a 5 letter word that starts with A.

1. \Ar"gon\, n. [Gr. ? inactive.] (Chem.)
   A substance regarded as an element, contained in the
   atmosphere and remarkable for its chemical inertness.
   --Rayleigh and Ramsay.
   

2. \Ar"gon\, n. [NL., fr. Gr. ?, neut. of ? inactive; ? priv.
   + ? work.] (Chem.)
   A colorless, odorless gas occurring in the air (of which it
   constitutes 0.93 per cent by volume), in volcanic gases,
   etc.; -- so named on account of its inertness by Rayleigh and
   Ramsay, who prepared and examined it in 1894-95. Symbol, A;
   at. wt., 39.9. Argon is condensible to a colorless liquid
   boiling at -186.1[deg] C. and to a solid melting at
   -189.6[deg] C. It has a characteristic spectrum. No compounds
   of it are known, but there is physical evidence that its
   molecule is monatomic. Weight of one liter at 0[deg] C. and
   760 mm., 1.7828 g.