Elements of Modern Physics
140
The situation becomes ambiguous if some of the subshells have approximately
the same energy, of interpenetrate, and would require a more careful analysis.
Chemical Properties
The chemical properties of elements are related to the forces between the
atoms. In this connection it is seen that (i) the atoms with slightly filled outer
shells, are characterized by a small ionization energy. They can easily lose the
electrons in the outer shell, and form positive ions. These elements are chemically
active. (ii) Atoms with only a small number of vacancies in the outer shell (i.e.
with almost-filled outer shell), though their net charge is zero, can easily
accommodate electrons in the unfilled shell since the screening of the nucleus is
mainly due to the inner shells. For example, Cl can bind an additional electron
with a binding energy of 3.80 eV (even the hydrogen atom can bind an additional
electron with a binding energy of 0.75 eV). With the acceptance of electrons,
these elements form negative ions. They are chemically active. (iii) Since the
chemical activity is related to the number of electrons in the outer shell, there
exist groups of atoms with similar outer shells but with different inner shells,
which have similar chemical properties. This is the origin of the periodic table
(see Table 5.2) where the atoms are arranged in such a way that the atoms in
vertical columns have similar outer shells and hence similar chemical properties.
Examples of these are (i) Noble gases, helium, neon, argon, krypton, xenon and
radon. They have closed outer shells. Their ionization potential is large, and they
cannot easily form positive ions. Nor can they bind an additional electron which
has to go into the next shell and hence would hardly experience any attraction.
These atoms are therefore chemically quite inactive. (ii) Alkali metals, lithium,
sodium, potassium, rubidium, caesium and francium. They have only one
s-wave electron in the outer shell. Since the electron has a small binding energy,
it can be easily lost. These atoms are positive monovalent, and are chemically
active. (iii) Halogens, fluorine, chlorine, bromine, iodine and astatine with five
electrons in the outer p-shell. They have an affinity for an additional electron
which would complete the shell. The atoms are negative monvalent and are
chemically active. (iv) Especially interesting are the groups of ten elements that
correspond to progressive filling of an nd subshell but have a complete (n + 1)s
subshell.
140
The situation becomes ambiguous if some of the subshells have approximately
the same energy, of interpenetrate, and would require a more careful analysis.
Chemical Properties
The chemical properties of elements are related to the forces between the
atoms. In this connection it is seen that (i) the atoms with slightly filled outer
shells, are characterized by a small ionization energy. They can easily lose the
electrons in the outer shell, and form positive ions. These elements are chemically
active. (ii) Atoms with only a small number of vacancies in the outer shell (i.e.
with almost-filled outer shell), though their net charge is zero, can easily
accommodate electrons in the unfilled shell since the screening of the nucleus is
mainly due to the inner shells. For example, Cl can bind an additional electron
with a binding energy of 3.80 eV (even the hydrogen atom can bind an additional
electron with a binding energy of 0.75 eV). With the acceptance of electrons,
these elements form negative ions. They are chemically active. (iii) Since the
chemical activity is related to the number of electrons in the outer shell, there
exist groups of atoms with similar outer shells but with different inner shells,
which have similar chemical properties. This is the origin of the periodic table
(see Table 5.2) where the atoms are arranged in such a way that the atoms in
vertical columns have similar outer shells and hence similar chemical properties.
Examples of these are (i) Noble gases, helium, neon, argon, krypton, xenon and
radon. They have closed outer shells. Their ionization potential is large, and they
cannot easily form positive ions. Nor can they bind an additional electron which
has to go into the next shell and hence would hardly experience any attraction.
These atoms are therefore chemically quite inactive. (ii) Alkali metals, lithium,
sodium, potassium, rubidium, caesium and francium. They have only one
s-wave electron in the outer shell. Since the electron has a small binding energy,
it can be easily lost. These atoms are positive monovalent, and are chemically
active. (iii) Halogens, fluorine, chlorine, bromine, iodine and astatine with five
electrons in the outer p-shell. They have an affinity for an additional electron
which would complete the shell. The atoms are negative monvalent and are
chemically active. (iv) Especially interesting are the groups of ten elements that
correspond to progressive filling of an nd subshell but have a complete (n + 1)s
subshell.