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Neptunium is a chemical element with the symbol Np and atomic number 93. A silvery radioactive metallic element, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a component in neutron detection equipment. Neptunium is also found in trace amounts in uranium ores.


Name, Symbol, Number neptunium, Np, 93
Chemical series actinides
Group, Period, Block n/a, 7, f
Appearance silvery metallic
Standard atomic weight (237) g·mol-1
Electrons per shell 2, 8, 18, 32, 22, 9, 2
Physical properties
Phase solid
Density (near r.t.) 20.45 g·cm−3
Melting point 910K
(637°C, 1179°F)
Boiling point 4273K
(4000°C, 7232°F)
Heat of fusion 3.20 kJ·mol-1
Heat of vaporization 336 kJ·mol-1
Heat capacity (25°C) 29.46 J·mol-1·K-1
Vapor pressure
P(Pa) 1 10 100 1 k 10 k 100 k
at T(K) 2194 2437        
Atomic properties
Crystal structure 3 forms: orthorhombic,
tetragonal and cubic
Oxidation states 6, 5, 4, 3
(amphoteric oxide)
Electronegativity 1.36 (Pauling scale)
Ionization energies 1st: 604.5 kJ/mol
Atomic radius 175 pm
Magnetic ordering  ?
Electrical resistivity (22 °C) 1.220 µΩ·m
Thermal conductivity (300K) 6.3 W·m-1·K-1
CAS registry number 7439-99-8


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