2
1 Nuclear Chemistry
1.3 Nuclear Structure
In order to completely establish the constitution of an atomic nucleus, it is necessary to know the nature of the constituents, forces binding them together, and laws
which govern their behavior. The fundamental nuclear constituents are protons and
neutrons, and laws governing their interactions are those of quantum mechanics.
However, the precise nature of the nuclear forces is not yet fully understood. Much
of the known information are embodied in several nuclear models; each of these has
advantages, but none of them are able to explain all of the available experimental
data of a nucleus.
1.4 Shell Model
Among the various nuclear models, the shell model is more popular because it can
explain most of the nuclear behavior of the atom and has helped to synthesize new
isotopes, which were unknown to exist. The basis of this model follows almost similar
concepts as the arrangement of electrons in an atom. Therefore, it may be easier to
explain the shell model by comparing it with an atomic electron structure. Hence, it
would be perhaps better to briefly revive the basic knowledge of an atomic structure.
An atom consists of electrons and it revolves around the nucleus with different orbitals
depending upon the number of electrons present in the atom. Protons and neutrons
are present in the nucleus. The various orbitals designated for electrons are s which
can accommodate a maximum of 2 electrons, p which can house a maximum of
6 electrons, d which can accommodate a maximum of 10 electrons, and f orbital
accommodating 14 electrons as its maximum capacity. Atoms with total number of
electrons, i.e., 2 (Helium), 10 (Neon), 18 (Argon), 38 (Krypton), 54 (Xenon), and 86
(Radon) are the most inert and stable elements, and other elements containing less
or more number of electrons are usually less stable. These numbers are also referred
to as a magic number.
Like the electron arrangement in an atom, the shell model also assumes the revolution of proton or neutron in some specific orbital such that not more than 2 nucleons
(i.e., either proton or neutron) would be present in one orbital. As nucleons are added
to the nucleus, they first occupy the lowest energy as allowable by the Pauli Exclusion Principle. This means that each nucleon possesses a unique quantum number
which portrays its motion. When the orbital of the shell is completely filled, the
nucleus attains high stability. It is assumed that like electrons, nucleons possess spin
(
1
2
) and they possess a quantum number like l and m. It is also assumed that within
the nucleus, separate orbitals exist for protons and neutrons.
Thus, the first two protons will be filled in the zero level (i.e., 0, 0, 0, +
1
2
and
−
1
2
= 2 protons). Likewise the next six protons will be filled in level one. In this
fashion, protons can be filled in all other possible levels. Considering the arrangement
of six shells, the number nucleon distributed would follow as shown here:
1 Nuclear Chemistry
1.3 Nuclear Structure
In order to completely establish the constitution of an atomic nucleus, it is necessary to know the nature of the constituents, forces binding them together, and laws
which govern their behavior. The fundamental nuclear constituents are protons and
neutrons, and laws governing their interactions are those of quantum mechanics.
However, the precise nature of the nuclear forces is not yet fully understood. Much
of the known information are embodied in several nuclear models; each of these has
advantages, but none of them are able to explain all of the available experimental
data of a nucleus.
1.4 Shell Model
Among the various nuclear models, the shell model is more popular because it can
explain most of the nuclear behavior of the atom and has helped to synthesize new
isotopes, which were unknown to exist. The basis of this model follows almost similar
concepts as the arrangement of electrons in an atom. Therefore, it may be easier to
explain the shell model by comparing it with an atomic electron structure. Hence, it
would be perhaps better to briefly revive the basic knowledge of an atomic structure.
An atom consists of electrons and it revolves around the nucleus with different orbitals
depending upon the number of electrons present in the atom. Protons and neutrons
are present in the nucleus. The various orbitals designated for electrons are s which
can accommodate a maximum of 2 electrons, p which can house a maximum of
6 electrons, d which can accommodate a maximum of 10 electrons, and f orbital
accommodating 14 electrons as its maximum capacity. Atoms with total number of
electrons, i.e., 2 (Helium), 10 (Neon), 18 (Argon), 38 (Krypton), 54 (Xenon), and 86
(Radon) are the most inert and stable elements, and other elements containing less
or more number of electrons are usually less stable. These numbers are also referred
to as a magic number.
Like the electron arrangement in an atom, the shell model also assumes the revolution of proton or neutron in some specific orbital such that not more than 2 nucleons
(i.e., either proton or neutron) would be present in one orbital. As nucleons are added
to the nucleus, they first occupy the lowest energy as allowable by the Pauli Exclusion Principle. This means that each nucleon possesses a unique quantum number
which portrays its motion. When the orbital of the shell is completely filled, the
nucleus attains high stability. It is assumed that like electrons, nucleons possess spin
(
1
2
) and they possess a quantum number like l and m. It is also assumed that within
the nucleus, separate orbitals exist for protons and neutrons.
Thus, the first two protons will be filled in the zero level (i.e., 0, 0, 0, +
1
2
and
−
1
2
= 2 protons). Likewise the next six protons will be filled in level one. In this
fashion, protons can be filled in all other possible levels. Considering the arrangement
of six shells, the number nucleon distributed would follow as shown here: