Number density explained

Number density should not be confused with Natural density.

The number density (symbol: n or ρ_N) 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:$$n\; =\; \backslash frac,$$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:$$n\text{'}\; =\; \backslash frac,$$Similarly, linear number density is the number of specified objects per unit length, L:$$n$$ = \frac,

Column number density is a kind of areal density, the number or count of a substance per unit area, obtained integrating volumetric number density along a vertical path:$$n\text{'}\_c\; =\; \backslash int\; n\; \backslash ,\; \backslash mathrms.$$It's related to column mass density, with the volumetric number density replaced by the volume mass density.

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 and as a yardstick: is often introduced as a unit of number density, for any substances at any conditions (not necessarily limited to an ideal gas at and).

Usage

Using the number density as a function of spatial coordinates, the total number of objects N in the entire volume V can be calculated as$$N\; =\; \backslash iiint\_V\; n(x,\backslash ,y,\backslash ,z)\backslash ,\backslash mathrmV,$$where dV = dx dy dz is a volume element. If each object possesses the same mass m₀, the total mass m of all the objects in the volume V can be expressed as$$m\; =\; \backslash iiint\_V\; m\_0\; n(x,\backslash ,y,\backslash ,z)\backslash ,\backslash mathrmV.$$

Similar expressions are valid for electric charge or any other extensive quantity associated with countable objects. For example, replacing m with q (total charge) and m₀ with q₀ (charge of each object) in the above equation will lead to a correct expression for charge.

The number density of solute molecules in a solvent is sometimes called concentration, although usually concentration is expressed as a number of moles per unit volume (and thus called molar concentration).

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$$n\; =\; N\_c$$where 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 ρ_m (in kg/m³) as$$n\; =\; \backslash frac\; \backslash rho\_\backslash mathrm.$$Note that the ratio M/N_A is the mass of a single atom or molecule in kg.

Examples

The following table lists common examples of number densities at and, unless otherwise noted.

Molecular^[1] number density and related parameters of some materials

Material	Number density, n		Amount concentration, c	Mass density, ρm	Molar mass, M
	(10^27 m^−3 = 10^21 cm^−3)	(amg)	(10^3 mol/m^3 = mol/L)	(10^3 kg/m^3 = g/cm^3)	(10^−3 kg/mol = g/mol)
	0.02504	0.932	0.04158	41.58 × 10^−6 M	M
	0.02504	0.932	0.04158	1.2041 × 10^−3	28.9644
	33.3679	1,241.93	55.4086	0.99820	18.01524
	176.2	6,556	292.5	3.513	12.01

See also

• Columnar number density

Notes and References

1. For elemental substances, atomic densities/concentrations are used