Geoscience Reference
In-Depth Information
Table 1.2
The molar energy scale of different physical and chemical processes
Strength
Strength
(kJ mol
−
1
)
T
(K)
1
Bond
(eV/molecule)
6
×
10
6
5.8
×
10
8
7.0
×
10
10
Nuclear
Electronic
6
580
69 800
4.1
393.5
47 400
Kinetic
3
4.2
×
10
−
5
0.040
0.5
1
Equivalent temperature (energy per mol/gas constant).
2
Combustion of one mole (12 grams) of carbon in oxygen.
3
One mole of air (22.4 liters at ambient pressure) at 60 km h
−
1
.
the same in the ocean or in a lake, in granite or limestone, in the Earth's crust or lower man-
tle, on the Moon or on Mars, etc. This probability has not varied measurably over the course
of geological time. The remarkable consistency of radioactive isotopic chronometers of
diverse geologic and planetary objects obtained by using different radioactive isotopes dis-
pels any doubt about the validity of nuclear clocks. The extreme temperature conditions
found in massive stars provide a few exceptions to this rule, but these are of no practical
relevance to our Solar System.
There are a number of decay processes:
1. The
(alpha) process, which is the emission of a helium nucleus (two protons and two
neutrons), is common at high mass. The nucleons (positively charged protons and neu-
tral neutrons) are held together by the short-range attractive strong force. This force is
essentially restricted to adjacent nucleons and therefore varies linearly with their num-
ber. In contrast, the electromagnetic force, which tends to pry the positively charged
protons apart, is weaker but acts over a broader range, so that it involves the entire
nucleus. It varies with the total number of proton pairs and therefore with the squared
number of protons. Overall, for the heavier nuclei, the repulsive force therefore tends
to compensate the attractive force and the mean energy holding the particles together
decreases. A heavy nucleus become knobby and wobbly and some parts tend to detach.
The
α
α
(alpha) process occurs when the sum of the masses of the daughter nuclide and
the
particle is less than the mass of the parent nuclide. It is a consequence of a quantum
property of nuclear particles known as the tunnel effect: although the potential barrier
opposing the ejection of an
α
α
particle is much greater than the energy gain resulting
from separation, there is a non-zero probability that an ejection will occur. For example
147
Sm
143
Nd
→
+ α
.
β
−
(beta minus) process involves the emission of an electron by the parent nucleus.
In a vacuum, a free neutron does not survive more than 15 minutes before it turns into
a proton and an electron. A neutron bound in a nucleus is definitely more stable but,
as indicated above, the lower energy of the proton-neutron interaction with respect to
that of similar nucleons favors a nucleus with an equal number of protons and neutrons.
When
N
2. The
>
Z
, this condition is violated, and excess energy is released by converting a
