# Physics:Number density

Short description: Degree of concentration of countable objects

The number density (symbol: n or ρN) is an intensive quantity used to describe the degree of concentration of countable objects (particles, molecules, phonons, cells, galaxies, etc.) in physical space: three-dimensional volumetric number density, two-dimensional areal number density, or one-dimensional linear number density. Population density is an example of areal number density. The term number concentration (symbol: lowercase n, or C, to avoid confusion with amount of substance indicated by uppercase N) is sometimes used in chemistry for the same quantity, particularly when comparing with other concentrations.

## Definition

Volume number density is the number of specified objects per unit volume:

$\displaystyle{ n = \frac{N}{V}, }$

where N is the total number of objects in a volume V.

Here it is assumed that N is large enough that rounding of the count to the nearest integer does not introduce much of an error, however V is chosen to be small enough that the resulting n does not depend much on the size or shape of the volume V because of large-scale features.

Area number density is the number of specified objects per unit area, A:

$\displaystyle{ n' = \frac{N}{A}, }$

Similarly, linear number density is the number of specified objects per unit length, L:

$\displaystyle{ n'' = \frac{N}{L}, }$

## Units

In SI units, number density is measured in m−3, although cm−3 is often used. However, these units are not quite practical when dealing with atoms or molecules of gases, liquids or solids at room temperature and atmospheric pressure, because the resulting numbers are extremely large (on the order of 1020). Using the number density of an ideal gas at 0 °C and 1 atm as a yardstick: n0 = 1 amg = 2.6867774 × 1025 m−3 is often introduced as a unit of number density, for any substances at any conditions (not necessarily limited to an ideal gas at 0 °C and 1 atm).

## Relation to other quantities

### Molar concentration

For any substance, the number density can be expressed in terms of its amount concentration c (in mol/m3) as

$\displaystyle{ n = N_{\rm A}c }$

where NA is the Avogadro constant. This is still true if the spatial dimension unit, metre, in both n and c is consistently replaced by any other spatial dimension unit, e.g. if n is in cm−3 and c is in mol/cm3, or if n is in L−1 and c is in mol/L, etc.

### Mass density

For atoms or molecules of a well-defined molar mass M (in kg/mol), the number density can sometimes be expressed in terms of their mass density ρm (in kg/m3) as

$\displaystyle{ n = \frac{N_{\rm A}}{M} \rho_\mathrm{m}. }$

Note that the ratio M/NA is the mass of a single atom or molecule in kg.

## Examples

The following table lists common examples of number densities at 1 atm and 20 °C, unless otherwise noted.

Molecular number density and related parameters of some materials
Material Number density, n Amount concentration, c Mass density, ρm Molar mass, M
(1027 m−3 = 1021 cm−3) (amg) (103 mol/m3 = mol/L) (103 kg/m3 = g/cm3) (10−3 kg/mol = g/mol)
Ideal gas 0.02504 0.932 0.04158 41.58 × 10−6 M M
Dry air 0.02504 0.932 0.04158 1.2041 × 10−3 28.9644
Water 33.3679 1,241.93 55.4086 0.99820 18.01524
Diamond 176.2 6,556 292.5 3.513 12.01