Atomic mass unit
The
atomic mass unit (
amu),
unified atomic mass unit (
u), or
Dalton named after the
chemist John Dalton, is a small
unit of
mass used to express
atomic masses and molecular masses. It is defined to be 1/12 of the mass of one
atom of
Carbon-12. Accordingly,
1 u = 1/N
A gram = 1/(1000 N
A) kg
where N
A is
Avogadro's number.
The symbol "amu" can sometimes be found, particularly in older works. Atomic masses are often written without any unit and then the atomic mass unit is implied.
In
biochemistry and
molecular biology literature (particularly in reference to
proteins), the term Dalton is used, with the symbol "Da". Because proteins are large molecules, they are typically referred to in kilodaltons, or "kDa".
It is not an
SI unit of mass, although it is accepted for use with SI. See SI website link below.
- 1 u ≈ 1.6605402 x 10-27 kg
See
1 E-27 kg for a list of objects which have a mass of about 1 u.
The unit is convenient because one
hydrogen atom has a mass of approximately 1 u, and more generally an
atom or
molecule that contains
n protons and
neutrons will have a mass approximately equal to
n u. This is only a rough approximation however, since it doesn't account for the mass contained in the binding energy of the
nucleus (in fact, does not account for variations in that mass relative to the total mass, compared with this ratio for C-12).
Another reason the unit is used is that it is much easier to compare masses of atoms and molecules (determine
relative masses) than to measure their
absolute masses, because masses in kilograms are inconveniently small numbers.
Avogadro's number (N
A) and the
mole are defined so that one mole of a substance with atomic or molecular mass 1 u will have a mass of precisely 1
gram. As an equation:\n:1 u = 1 gram/mole\nor equivalently\n:1 gram = N
A u
For example, the molecular mass of
water is 18.01508 u, and this means that one mole of water has a mass of 18.01508 grams, or conversely that 1 gram of water contains N
A/18.01508 ≈ 3.3428 × 10
22 molecules.
Measuring Relative Atomic Masses
The relative atomic mass is measured with a mass spectrometer. After placing a sample of the element to be measured in the mass spectrometer it is bombarded with electrons which turns the atoms into positive ions. An electric field is then used to accelerate these positive ions, afterwhich the ions are deflected using a magnetic field. As a result the various isotopes are separated out due to the ions of lighter isotopes being deflected more than those heavier. This produces a mass spectrum.
This spectrum provides two things:
- Relative isotopic masses in the sample\n# Abundances of the isotopes
Using Mess Spectrum Data to Calculate Relative Atomic Mass
A simple calculation may be used to calculate the relative atomic mass of the sample. This is demonstrated in the following example.
{| border="1" cellpadding="2"\n|Ion\n|Relative Mass\n|Percentage Abundance\n|-\n|11C+\n|11\n|70%\n|-\n|13C+\n|13\n|30%\n|-\n|}
Therefore, the relative atomic mass of the Carbon sample is:
(70/100 x 11) + (30/100 x 13)
7.7 + 3.9
= 11.6 [this is not the true atomic mass of carbon, it is merely illustrative]
External link
\n*SI website
\n \n \n
Category:Nuclear chemistry\nCategory:Units of mass
\n\n\n\n\n\n
