93 Np

Neptunium (Np)

Element 93 · Actinide · Solid at 25 °C · Period 7 (f-block)

Neptunium was the first transuranium element ever made (1940), and like its planetary namesake it sits just beyond Uranus's element, uranium.

On the periodic table, neptunium sits in the f-block in period 7. It is one of 15 elements classified as an actinide, and at room temperature (25 °C) it is a solid.

The actinides fill the 5f orbitals in the bottom row of the table, and every one of them is radioactive. Only thorium and uranium occur on Earth in significant quantities; the rest are made in reactors or particle accelerators. Their nuclear properties — fission above all — shaped the twentieth century.

Properties of neptunium

Properties of Neptunium
Atomic number (Z)	93
Symbol	Np
Atomic mass	(237) (mass number of the most stable isotope; this element has no standard atomic weight)
Category	Actinide
Group	— (f-block, associated with group 3)
Period	7
Block	f-block
Metal / nonmetal	Metal
State at 25 °C	Solid
Electron configuration	[Rn] 5f4 6d1 7s2
Electronegativity (Pauling)	1.36
Melting point	644 °C (917.15 K / 1,191.2 °F)
Boiling point	3,902 °C (4,175.15 K / 7,055.6 °F)
Density	20.45 g/cm³

Position in the periodic table

Neptunium belongs to the actinides, the detached f-block row shown beneath the main table. These elements all fit between groups 2 and 4 of period 7, and are pulled out into their own row only to keep the table a manageable width.

Within this dataset, it has the #5 highest measured density of the 97 elements with recorded values.

Physical properties

Neptunium melts at 644 °C (917.15 K / 1,191.2 °F) and boils at 3,902 °C (4,175.15 K / 7,055.6 °F), giving it a liquid range of about 3,258 °C at standard pressure.

Its density is 20.45 g/cm³.

Data notes

The atomic mass is shown in parentheses because this element has no standard atomic weight; the number given is the mass number of its most stable known isotope.