Geoscience Reference
In-Depth Information
neutron into a proton and an electron. This process calls for a special type of force, the
weak interaction, which is analogous to electromagnetic forces but, as reflected by its
name, much weaker. This interaction is of little relevance to our familiar environment
and is difficult to explain simply. The continuous distribution of electron energy emitted
during this process cast doubt on the quantification of nuclear energy until it was shown
that there was a particle of zero mass, a neutrino, that practically does not interact with
the matter through which it passes, but which can carry considerable energy. Collap
s
-
ing stars lose energy through
ne
utrino emission. For example,
87
Rb
87
Sr
+ β
−
+ ν
→
,
β
−
where
an antineutrino. Beta-minus radioactivity is a common
process when the nucleus has a high neutron/proton ratio. A symmetrical process of
is the electron and
ν
β
+
(beta plus) positron emission (particle with the same mass as an electron but of opposite
charge) takes place when the nucleus has a high proton/neutron ratio (
N
Z
), a situa-
tion rarely found for natural nuclides, which, as we will learn later in
Chapter 12
,
form
in neutron-rich environments.
3. Capture of an electron of the K shell is a less frequent process, and was identified by
von Weizsäcker during his investigation of excess argon-40 in the Earth's atmosphere.
For example,
40
K
<
40
Ar. Electron capture affects the nuclide in much the same
way as positron emission does.
4. Spontaneous fission of some heavy atoms like uranium-238 or plutonium-244 is a rather
rare and very slow process; it forms the basis of the fission-track dating method.
e
−
→
+
A nuclide may be unstable with respect to two decay processes simultaneously and the
probabilities of decomposition by each process are additive. For example, potassium-40
(
40
K) decays dually into
40
Ca by
β
−
emission and into
40
Ar by electron capture; this is
known as a branched mode of radioactive decay.
Exercises
1. Give the first three quantum numbers of an electron in a 3p orbital.
2. Find the electronic formula of the elements K, Hf, and F. Compare the orbital elec-
tronic configuration for K and Rb (s block), Hf and Zr (d block), F and Cl (p block).
For each element, which ionic configuration should be the most stable?
3. Plot the ionic radius and the first ionization energy as a function of the number
Z
of
protons.
4. Which ion of Na
+
,K
+
,Rb
+
,Mg
2
+
,Ca
2
+
has the smallest radius? Why?
5. Why is the ionic radius of Yb (0.99 Å) smaller than that of La (1.16 Å)?
6. Calculate the crystal field stabilization energy (CFSE) in units of
for the common
ions Sc
3
+
,Ti
4
+
,V
5
+
,Cr
3
+
,Mn
2
+
,Fe
2
+
,Co
2
+
,Ni
2
+
,Cu
2
+
, and Zn
2
+
in octahedral
vs. tetrahedral environments in high-spin configuration.
7. The PetDB database (
http://www.petdb.org/
)
collects geochemical data on mid-ocean
ridge basalts (MORB).
Table 1.3
gives the concentrations of the transition elements
of the first row in some glassy samples from the East Pacific Rise (EPR) and also the
