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MiniChem 4: The Atom
Anatomy of the atomic symbol
A mass number
X
atomic symbol
Z atomic number
The atomic symbol of an element is often decorated with the mass number
(A) of one of its isotopic forms (see below) and its atomic number (Z). Z
represents the number of protons (which is also the number of electrons)
and A represents the number of protons + the number of neutrons. The
symbol below represents iron-56 one of the isotopes of iron that contains 30
neutrons. (Iron has 3 other isotopes —iron-54, iron-57 and iron-58— that
have 28, 31, and 32 neutrons, respectively).
56
Fe
26
In periodic tables only the atomic number is reported along with the atomic
mass/weight of the element –note that the atomic mass is expressed in
unified atomic units (see below)– and the representation of the element is
usually as shown below:
26
Fe
55.845
Atomic mass based on 12C
[Ar]3d64s2
Electronic configuration
in the ground state
Isotopes
An atom might have more than one isotope. Hence its atomic mass is the
weighted average of the atomic masses of each isotope:
Marco Ceruso, The C Research Lab, ©2009-2012
Mx =
# isotopes
!
i =1
Wi M iX
and
# isotopes
!
i =1
Wi = 1
The atomic mass of an element is also called atomic weight to recall the
fact that it is a weighted average of the atomic mass of each isotopic form.
Example: Iron (Fe) has 4 naturally occurring isotopes. Their relative abundance
is 5.845% for 54Fe, 91.754% for 56Fe, 2.119% for 57Fe and 0.282% for 58Fe.
The atomic mass of each these isotopes is 53.9396105 u, 55.9349375 u,
56.9353940 u and 57.9332756 u, respectively. Thus the atomic mass of iron
(element) is:
MFe =
0.05845 x 53.9396105
+ 0.91754 x 55.9349375
+ 0.02119 x 56.9353940
+ 0.00282 x 57.9332756
=
55.84515 u
Unified atomic mass units
The atomic mass unit is now the unified atomic mass and its symbol is the
letter ‘u’; 1 u is defined as 1/12th of the mass of an atom of carbon-12 (12C):
1 u
=
1.660 538 782 x 10-27 kg
=
1.660 538 782 x 10-24 g
Before the 1960’s, chemists and physicists had different definitions of the
atomic unit. These differences were due to a different choice of reference,
the ones expressed the atomic masses with respect to that of a single
isotope of oxygen 16O, the others used oxygen as whole (including all of its
isotopes) as a reference. These differences were resolved by choosing carbon12 as a common unified reference.
Marco Ceruso, The C Research Lab, ©2009-2012