ChemGlobe: Arsenic
   Electronic, thermal, and steric data along with an isotope table.
   http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/33.html

ChemicalElements.com: Arsenic
   Basic information, atomic structure, and isotopes.
   http://www.chemicalelements.com/elements/as.html

EnvironmentalChemistry.com: Arsenic
   Atomic structure, chemical and physical properties, and table of nuclides.
   http://environmentalchemistry.com/yogi/periodic/As.html

It's Elemental: Arsenic
   Basic physical and historical information.
   http://education.jlab.org/itselemental/ele033.html

LANL: Arsenic
   History, properties, and uses.
   http://periodic.lanl.gov/elements/33.html

Lenntech: Arsenic
   Physical data, chemical properties, health and environmental effects.
   http://www.lenntech.com/Periodic-chart-elements/As-en.htm

Periodic Table of the Elements: Arsenic
   Thermal properties, ionisation energies, and reduction potentials.
   http://www.ktf-split.hr/periodni/en/as.html

Radiochemistry of Arsenic
   Full text of the monograph by J. G. Cuninghame (Atomic Energy Research Establishment, Harwell, Didcot, Berkshire, England). [PDF]
   http://lib-www.lanl.gov/cgi-bin/getfile?rc000052.pdf

USGS - Minerals Information: Arsenic
   Overview and a number of publications in PDF format.
   http://minerals.usgs.gov/minerals/pubs/commodity/arsenic/

Visual Elements: Arsenic
   Source, biological role, physical information, and key isotopes.
   http://www.chemsoc.org/viselements/pages/arsenic.html

Wikipedia: Arsenic
   Details the properties of the element, including its history, applications, and characteristics.
   http://en.wikipedia.org/wiki/Arsenic