### Avogadro's number

**(N**_{A}, Avogadro constant)
(roughly number of hydrogen atoms in a gram)

**Avogadro's number** (**Avogadro constant**, abbreviated **N**_{A})
is a large number aimed at representing the number of
nucleons (protons and neutrons) in a gram of substance.
No single number can do this with perfect precision
because the mass of atoms and molecules do not
precisely match (the sum of) their **mass numbers**.
*Avogadro's number* is chosen to be a realistic representative,
now having the standard definition of 6.02214076 × 10^{23}.
A **mole (mol)** of a substance is defined as that many molecules.
The number is convenient for chemists, who can relate grams to
elements and molecular formulae, and can be convenient in
calculations for allowing smaller numbers to be used than
particle counts.

Earlier in its history, *Avogadro's number* and
the *mole* were pinned to the standard gram along with some selected
element isotope, but (like the AU) it is now considered more useful
to have a stable, precise number not subject to future change due
to improving measurement.

(*constant,chemistry*)
**Further reading:**

http://en.wikipedia.org/wiki/Avogadro_constant

**Referenced by pages:**

astronomical quantities

Boltzmann constant (k)

ideal gas law

mole (mol)

Index