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148
7.
THE OUTER PARTS OF THE ATOM
ergs, the atom will be ionized by the ejection of one of its
electrons.
If the energy is greater than this amount, the atom’s
electron will be ejected and, in addition, possess kinetic energy
equal to 108Ve —
ergs.
'
The minimum ionizing potential V,—, expressed in volts, gives a
measure of the energy required to remove the most loosely bound
electron from the atom and hence is numerically equal to
…
the
energy of this electron in
normal state in the atom.
For exam;
ple, it requires 3.88 electron volts to remove an electron from the , '
cesium atom, whereas, the value for the energy of an electron in
its normal state in the helium atom is 24.5 electron volts.
Values
for various elements are listed in Table 2 at the end of
book.
It may be seen that they range between the values quoted
-
for cesium and helium, that they are periodic functions of the
,
atomic number of the elements, that they are higher for the rare
gases, lower for the alkali metals and that they usually decrease in
any column of the periodic table with increasing atomic number.
The probability of ionization,8 ie, the number of positive
ions created per collision, rises very rapidly from a zero value just
at the ioni2ing potential. It reaches a maximum value at 210
electron volts ”for helium, 340 for‘neon, 140 for argon,
145 for
.
hydrogen, 175 for nitrogen and 135 electron volts for mercury
vapor.
For. electrons of greater than the maximum energy, the
ioniiation probability decreases slowly.
,
7—6.
Energy
Potentials.—In 1914, F ranck
and Hertz used a three—electrode. tube somewhat like that in
'
gure7—7 but so constructed that the grid was much closer tothe
-_
plate.
‘
The potential of the plate Was made positive With respect
,
but slightly negative, say 0.5 volt, With respect to
the grid. .
greater than 0.5 volt were
_
tq,_reach the plate against this small retarding potential whereas
were returned to the grid. As the gr1dwas
made increasingly positive from zero, the current to the
foundto vary inthe
represented in figure
Theform
Ofthls
be
in the following manner. ‘ AS: the
grldbecomes mcreas1ngly positive, an increasing number
ShOWHbYÏheregwnOï‘AtlÂ; hoWever; some of the fasterelec
tr0nShaVeacquiredsuic1entenergy tb”èollide mai…m/1ymththe
148
7.
THE OUTER PARTS OF THE ATOM
ergs, the atom will be ionized by the ejection of one of its
electrons.
If the energy is greater than this amount, the atom’s
electron will be ejected and, in addition, possess kinetic energy
equal to 108Ve —
ergs.
'
The minimum ionizing potential V,—, expressed in volts, gives a
measure of the energy required to remove the most loosely bound
electron from the atom and hence is numerically equal to
…
the
energy of this electron in
normal state in the atom.
For exam;
ple, it requires 3.88 electron volts to remove an electron from the , '
cesium atom, whereas, the value for the energy of an electron in
its normal state in the helium atom is 24.5 electron volts.
Values
for various elements are listed in Table 2 at the end of
book.
It may be seen that they range between the values quoted
-
for cesium and helium, that they are periodic functions of the
,
atomic number of the elements, that they are higher for the rare
gases, lower for the alkali metals and that they usually decrease in
any column of the periodic table with increasing atomic number.
The probability of ionization,8 ie, the number of positive
ions created per collision, rises very rapidly from a zero value just
at the ioni2ing potential. It reaches a maximum value at 210
electron volts ”for helium, 340 for‘neon, 140 for argon,
145 for
.
hydrogen, 175 for nitrogen and 135 electron volts for mercury
vapor.
For. electrons of greater than the maximum energy, the
ioniiation probability decreases slowly.
,
7—6.
Energy
Potentials.—In 1914, F ranck
and Hertz used a three—electrode. tube somewhat like that in
'
gure7—7 but so constructed that the grid was much closer tothe
-_
plate.
‘
The potential of the plate Was made positive With respect
,
but slightly negative, say 0.5 volt, With respect to
the grid. .
greater than 0.5 volt were
_
tq,_reach the plate against this small retarding potential whereas
were returned to the grid. As the gr1dwas
made increasingly positive from zero, the current to the
foundto vary inthe
represented in figure
Theform
Ofthls
be
in the following manner. ‘ AS: the
grldbecomes mcreas1ngly positive, an increasing number
ShOWHbYÏheregwnOï‘AtlÂ; hoWever; some of the fasterelec
tr0nShaVeacquiredsuic1entenergy tb”èollide mai…m/1ymththe