Are Molecules Fundamental?

• chemists of 1800's found that atoms in all compounds come in fixed ratios: H₂O, CO₂
  
  
  
• spectral line fingerprints from base "elements"
• • emitted if hot, in a spark, or in a gaseous discharge
           experiment on H, He lines + charts
           Applications: diagnostics, forensics, medicine
  • absorption lines if a cool cloud between
          (e.g. sun's, earth's atmosphere)

All complex molecules made up of atoms of basic "elements"

• held together by (residual) electric force
  
  
• atom smallest piece of anything with same characteristics