ChemGlobe: Neptunium
- Electronic, thermal, and steric data along with an isotope table.
- http://partisans.spurious.biz/~pkremer/projects/chemglobe/ptoe/_/93.html

LANL: Neptunium
- Basic information and history.
- http://periodic.lanl.gov/elements/93.html

Wikipedia: Neptunium
- Notable characteristics, history, occurrence, isotopes, and weapons applications.
- http://en.wikipedia.org/wiki/Neptunium

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

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

Visual Elements: Neptunium
- General and physical information, source, and key isotopes.
- http://www.chemsoc.org/viselements/pages/neptunium.html

It's Elemental: Neptunium
- Basic physical and historical information.
- http://education.jlab.org/itselemental/ele093.html

ChemicalElements.com: Neptunium
- Basic information, atomic structure, and isotopes table.
- http://www.chemicalelements.com/elements/np.html

Britannica Nobel Prize: Neptunium
- Overview and basic data.
- http://www.britannica.com/nobel/micro/420_27.html