Chemistry Reference
In-Depth Information
oxides, so that oxygen was more convenient and the
amu
was defined as the 16
th
part of the atomic mass of oxygen. Refined methods were used for exact measure-
ments of atomic weights, correcting some erroneous data that had been confusing
in Mendeleev's time. T. W. Richards was awarded the 1914 Chemistry Nobel Prize
(distributed in 1915) for his work on atomic mass determinations. However, after
1912 when the newly invented mass spectrometer revealed the existence of
17
O and
18
O stable isotopes along with the predominating
16
O (99.66 %), physicists based
their determinations on the atomic weight of
16
O, whereas chemists continued to
use the old definition of
amu
, involving the weighted average of all three stable
oxygen isotopes; thus chemists had a unit which was more massive by 1.00028 than
the physicists' unit. It was only in 1961 that IUPAC, IUPAP, and IUB agreed on
redefining the “unified atomic mass unit” as the 12
th
part of the mass of a
12
C atom.
After the SI unit “kg” was redefined in 2012, the present-day definition of atomic
weights remains based on carbon atoms and Avogadro's number (
N
A
) as 1 dalton
(Da)
0.001/
N
A
(in kg) (Holden
2004
). However, there is an ongoing discussion
on the compatibility between the three related SI units (mole, kilogram, and dalton)
after fixing the Planck constant and Avogadro's constant (Leonard
2012
).
After discussing the role of carbon in the thermonuclear cosmic formation of
helium from hydrogen, it is not surprising that carbon is the
4th most abundant
element in the universe by mass
, after
1
H,
2
He, and
8
O. One should not forget that
2 and 8 are magic numbers of nucleons (protons and neutrons), so that
2
He,
1
8
=
O are
double-magic nuclei.
Carbon is the
15th most abundant element in the Earth's crust
, and it is the
2nd
most abundant element by mass in the human body
(18.5 %) after oxygen (65 %); it
is followed by hydrogen (10 %) and nitrogen (3 %).
The most important chemical reaction occurring naturally on this planet is the
photo-assimilation of carbon dioxide. In this reaction (catalyzed by chlorophyll in
plants and algae, or by phycocyanin in cyanobacteria) the energy of sun's photons is
driving the conversion of carbon dioxide and water vapor into glucose and oxygen.
6CO
2
+
6H
2
O
+
energy
−→
C
6
H
12
O
6
+
O
2
All the oxygen in our atmosphere originates in this reaction, and the energy re-
leased in the reverse reaction allows animals feeding on plants to proceed with their
metabolism. Glucose and other simple raw materials such as phosphate, ammonia,
and mineral salts are processed by living cells into all constituents of living cells:
other carbohydrates, lipids, proteins, and polynucleotides.
Carbon dioxide from the atmosphere is replenished by the well-known
carbon
cycle
with the participation of carbon sources from the geosphere, hydrosphere,
biosphere, and pedosphere. The amount of carbon (in Tt, teratonnes, 10
15
kg) in
the major reservoirs is 0.7 for the atmosphere, 40 for the hydrosphere, and 75,000
for the geosphere (60,000 for the sedimentary carbonates and 15,000 for kerogens).
Even carbon from the calcium/magnesium carbonate rocks is recycled via volcanic
emissions triggered by plate tectonics. Throughout most of our planet's history,
volcanoes provided the highest turnout of CO
2
, but after the industrial revolution,
