Neptunium at a glance
- Atomic number
- 93
- Symbol
- Np
- Atomic mass
- 237 u
- Category
- Actinide
- Period
- 7
- Electron configuration
- [Rn] 5f 4 6d 1 7s 2
- State at 25 °C
- Solid
- Melting point
- 643.9 °C
- Boiling point
- 4000 °C
- Density
- 20.45 g/cm³
- Electronegativity
- 1.36
- 1st ionization energy
- 605 kJ/mol
- Oxidation states
- +5
- Discovered
- 1940
Atomic structure
A neutral neptunium atom has 93 protons and 93 electrons, and its most common isotope carries about 144 neutrons. One atom weighs about 3.94 × 10⁻²⁵ kg, so a mole of neptunium (6.022 × 10²³ atoms) weighs 237 g. That nucleus holds 1.55 neutrons for every proton, typical of heavy nuclei, which need extra neutrons to hold together against the protons' repulsion. The electrons fill 7 shells (K 2, L 8, M 18, N 32, O 22, P 9, Q 2), which gives the configuration [Rn] 5f 4 6d 1 7s 2. As a f-block element, both its outer s electrons and its inner f electrons can take part in bonding, which is why it shows more than one oxidation state. Its configuration breaks the simple filling order: one electron moves so that a subshell becomes half-full or full, which is more stable. All of its isotopes are radioactive, so any neptunium on Earth is either freshly made or part of a decay chain.
Physical properties
At room temperature (25 °C) neptunium is a solid. It melts at 643.9 °C and boils at 4000 °C, so it stays liquid across a 3356-degree range. It stays solid in a hot kitchen oven but would melt in a candle flame (about 1000 °C). Its density is 20.45 g/cm³, 20.4 times as dense as water. Its atoms have a radius of about 175 pm (Uranium next door is 175 pm). In the solid, its atoms pack into an orthorhombic lattice.
Chemical behaviour
Its electronegativity is 1.36 on the Pauling scale. Pulling off its outermost electron takes 605 kJ/mol. A bond between neptunium and chlorine would be mostly ionic, with an electronegativity difference of 1.8. Its common oxidation states are +5, so in compounds you will usually meet it as Np⁵⁺. It shares much of its chemistry with the other actinides, especially its neighbours Protactinium, Uranium, Plutonium and Americium. Handle with care: as a pure element it is radioactive.
Where it comes from
It does not occur in nature in useful amounts; it is made in nuclear reactors or particle accelerators. It was first made at Berkeley, California by hitting uranium-238 with neutrons. Its longest-lived known isotope is Np-237, about 2.1 million years. It was first identified in 1940, making it the 92nd element to be discovered (counting the 11 known since antiquity together).