| 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. |
Classical
