Density is defined as the mass per unit volume. It is an intensive property, which is mathematically defined as mass divided by volume:
ρ = m/V
In other words, the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance. The standard SI unit is kilograms per cubic meter (kg/m3). The Standard English unit is pounds mass per cubic foot (lbm/ft3).
The density (ρ) of a substance is the reciprocal of its specific volume (ν).
ρ = m/V = 1/ρ
Specific volume is an intensive variable, whereas volume is an extensive variable.
The SI system’s standard unit for specific volumes is cubic meters per kilogram (m3/kg). The standard unit in the English system is cubic feet per pound-mass (ft3/lbm).
Density in the periodic table
Density – Atomic Mass and Atomic Number Density
Since the density (ρ) of a substance is the total mass (m) of that substance divided by the total volume (V) occupied by that substance, it is obvious, the density of a substance strongly depends on its atomic mass and also on the atomic number density (N; atoms/cm3),
- Atomic Weight. The atomic mass is carried by the atomic nucleus, which occupies only about 10-12 of the total volume of the atom or less, but it contains all the positive charge and at least 99.95% of the total mass of the atom. Therefore it is determined by the mass number (number of protons and neutrons).
- Atomic Number Density. The atomic number density (N; atoms/cm3), which is associated with atomic radii, is the number of atoms of a given type per unit volume (V; cm3) of the material. The atomic number density (N; atoms/cm3) of a pure material having an atomic or molecular weight (M; grams/mol) and the material density (⍴; gram/cm3) is easily computed from the following equation using Avogadro’s number (NA = 6.022×1023 atoms or molecules per mole):
Densest Materials on the Earth
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 103 kg/m3
- Iridium – 22.4 x 103 kg/m3
- Platinum – 21.5 x 103 kg/m3
- Rhenium – 21.0 x 103 kg/m3
- Plutonium – 19.8 x 103 kg/m3
- Gold – 19.3 x 103 kg/m3
- Tungsten – 19.3 x 103 kg/m3
- Uranium – 18.8 x 103 kg/m3
- Tantalum – 16.6 x 103 kg/m3
- Mercury – 13.6 x 103 kg/m3
- Rhodium – 12.4 x 103 kg/m3
- Thorium – 11.7 x 103 kg/m3
- Lead – 11.3 x 103 kg/m3
- Silver – 10.5 x 103 kg/m3
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, 269Hs, has a half-life of approximately 9.7 seconds. It has an estimated density of 40.7 x 103 kg/m3. The density of Hassium results from its 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 103 kg/m3.
The density of various Materials – Examples
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 = r0 . A1/3
where r0 = 1.2 x 10-15 m = 1.2 fm
For example, natural uranium consists primarily of isotope 238U (99.28%). Therefore the atomic mass of the uranium element is close to the atomic mass of the 238U isotope (238.03u). The radius of this nucleus will be:
r = r0 . A1/3 = 7.44 fm.
Assuming it is spherical, its volume will be:
V = 4πr3/3 = 1.73 x 10-42 m3.
The usual definition of nuclear density gives for its density:
ρnucleus = m / V = 238 x 1.66 x 10-27 / (1.73 x 10-42) = 2.3 x 1017 kg/m3.
Thus, the density of nuclear material is more than 2.1014 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.