|Why is the relative atomic mass of sodium 23amu?|
|Question Date: 2018-08-04|
The relative atomic mass of sodium is not 23
amu (amu = atomic mass unit), it is 23 with no
units (as is suggested by the word "relative").
The relative atomic mass is the ratio
of the average mass of an atom of an element
(given in amu) to 1 amu. For atoms, the
atomic mass one typically uses is essentially the
sum of the masses of the protons and neutrons
which comprise the nucleus of the atom; the
mass of each electron (~1/2000 amu) is negligible
in comparison. Protons and neutrons have
similar masses, approximately 1 amu each.
Most sodium atoms have 11 protons and 12 neutrons,
giving a total of ~23 amu, and dividing by 1 amu
then gives 23 for the relative atomic mass. Note
the use of most in the preceding sentence. The
number of protons determines the chemical identity
of an element, but the number of neutrons can
vary - these are different
isotopes of an element.
The standard atomic weight takes into
account the different
relative abundances and masses of the
isotopes by weighting the mass of isotopes by the
frequency of their occurrence. This is why some
elements have fractional standard weights
(e.g. copper at 63.55).
It's because sodium atoms each have 11 protons,
with a mass of 1, and 12 neutrons, with a mass of
periodic table of elements
Number of Protons/Electrons: 11
Number of Energy Levels: 3
Number of Neutrons: 12
Atomic Mass: 22.98977 amu
Most sodium atoms contain twelve neutrons.
That is the most common isotope of sodium. Because
each neutron weighs one AMU, and because each of
sodium's eleven protons also weighs one AMU,
twelve plus eleven is twenty-three.
It's fairly complicated nuclear physics to
understand why some isotopes are stable and some
aren't, but it is a general trend that the more
protons you have, the more neutrons you need,
and the number of neutrons needed goes up faster
than the number of protons.
Relative atomic mass is calculated by
dividing the atomic mass of an element by what
we call the "unified atomic mass unit".
The unified atomic mass unit is defined as
1/12 of the mass of a carbon atom with 6 protons,
6 neutrons, and 6 electrons. The relative atomic
mass of sodium is calculated by dividing the
atomic mass of sodium by the unified atomic mass
unit, which is 1.66*10-27 kg. The
number of 23 amu means that a normal, or stable,
sodium atom weighs 23 unified atomic mass
units. Since sodium atoms have 11 protons,
this means that a normal sodium atom has 23 - 11 =
12 neutrons. It would also have 11 electrons, but
electrons weigh so little compared to protons and
neutrons that the electron masses can be
approximated to 0. Note that by "stable",
we mean that the sodium atom will not very quickly
change into another element by losing parts of its
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