Since **nucleons** (**protons** and **neutrons**) make up most of the mass of ordinary atoms, the density of normal matter tends to be limited by how closely we can pack these nucleons and depends on the internal atomic structure. The **densest material** found on earth is the **metal osmium. Still, its** density pales by comparison to the densities of exotic astronomical objects such as white** dwarf stars** and **neutron stars**.

**List of densest materials:**

- Osmium – 22.6 x 10
^{3}kg/m^{3} - Iridium – 22.4 x 10
^{3}kg/m^{3} - Platinum – 21.5 x 10
^{3}kg/m^{3} - Rhenium – 21.0 x 10
^{3}kg/m^{3} - Plutonium – 19.8 x 10
^{3}kg/m^{3} - Gold – 19.3 x 10
^{3}kg/m^{3} - Tungsten – 19.3 x 10
^{3}kg/m^{3} - Uranium – 18.8 x 10
^{3}kg/m^{3} - Tantalum – 16.6 x 10
^{3}kg/m^{3} - Mercury – 13.6 x 10
^{3}kg/m^{3} - Rhodium – 12.4 x 10
^{3}kg/m^{3} - Thorium – 11.7 x 10
^{3}kg/m^{3} - Lead – 11.3 x 10
^{3}kg/m^{3} - Silver – 10.5 x 10
^{3}kg/m^{3}

It must be noted, plutonium is a manufactured isotope and is created from uranium in nuclear reactors. But scientists have found trace amounts of naturally-occurring plutonium.

If we include manufactured elements, the densest so far is** Hassium**. **Hassium** is a chemical element with the symbol **Hs** and atomic number 108. It is a synthetic element (first synthesized at Hasse in Germany) and radioactive. The most stable known isotope, ** ^{269}Hs**, has a half-life of approximately 9.7 seconds. It has an estimated density of

**40.7 x 10**. The density of Hassium results from its

^{3}kg/m^{3}**high atomic weight**and the significant decrease in

**ionic radii**of the elements in the lanthanide series, known as

**lanthanide and actinide contraction**.

The density of Hassium is followed by **Meitnerium** (element 109, named after the physicist Lise Meitner), which has an estimated density of** 37.4 x 10 ^{3} kg/m^{3}**.

## The density of Nuclear Matter

**Nuclear density** is the density of the nucleus of an atom. It is the ratio of mass per unit volume inside the nucleus. Since the atomic nucleus carries most of the atom’s mass and the atomic nucleus is very small compared to the entire atom, the nuclear density is very high.

The nuclear density for a typical nucleus can be approximately calculated from the size of the nucleus and its mass. **Typical nuclear radii** are of the order **10**^{−14}** m**. Nuclear radii can be calculated according to the following formula assuming spherical shape:

r = r_{0} . A^{1/3}

where r_{0} = 1.2 x 10^{-15 }m = 1.2 fm

For example, **natural uranium** consists primarily of isotope ^{238}U (99.28%). Therefore the atomic mass of the uranium element is close to the atomic mass of the ^{238}U isotope (238.03u). The radius of this nucleus will be:

r = r_{0} . A^{1/3} = 7.44 fm.

Assuming it is spherical, its volume will be:

V = 4πr^{3}/3 = 1.73 x 10^{-42} m^{3}.

The usual definition of nuclear density gives for its density:

ρ_{nucleus} = m / V = 238 x 1.66 x 10^{-27} / (1.73 x 10^{-42}) = **2.3 x 10 ^{17} kg/m^{3}**.

Thus, the density of nuclear material is more than 2.10^{14} times greater than that of water. It is an immense density. The descriptive term *nuclear density* is also applied to situations where similarly high densities occur, such as within neutron stars. Such immense densities are also found in neutron stars.