Britannica Nobel Prize: Neptunium: Overview and basic data.
ChemGlobe: Neptunium: Electronic, thermal, and steric data along with an isotope table.
ChemicalElements.com: Neptunium: Basic information, atomic structure, and isotopes table.
EnvironmentalChemistry.com: Neptunium: Atomic structure, chemical and physical properties, and table of nuclides.
It's Elemental: Neptunium: Basic physical and historical information.
LANL: Neptunium: Basic information and history.
Lenntech: Neptunium: Physical data, chemical properties, health and environmental effects.
Neptunium: Data tables and historic information.
Visual Elements: Neptunium: General and physical information, source, and key isotopes.
WebElements: Neptunium: Extensive information on history, uses, compounds, and properties.
Wikipedia: Neptunium: Notable characteristics, history, occurrence, isotopes, and weapons applications.