The potential needed to remove electrons from an atom is known as ionization potential. The ionization potential is measured using a vapor filled gas diode. When the anode of the gas filled diode is at a positive potential with respect to the cathode, electrons move across the tube from the cathode to the anode. As the plate potential is increased beyond a particular value the plate current increases much more rapidly than it does below that critical value. This potential is equal to the ionization potential of the gas. Using this concept determine the ionization potential of mercury using a gas filled diode.
To find the ionization potential of mercury using a gas filled diode.
A gas filled (mercury vapour) diode, a D.C. power supply, a voltmeter, a millimeter and connecting wires.
The term ionization energy of an atom or molecule means the energy needed to remove electrons from an atom. Large atoms require low ionization energy while small atoms require high ionization energy. This quantity was formerly called ionization potential, i.e., the potential needed to remove electrons from an atom and was measured in volts (V). The name "ionization energy" is now strongly preferred. In atomic physics the ionization energy is measured using the unit "electron volt" (eV).
The ionization potential of mercury can be determined by introducing mercury vapor at a low pressure of 10mm to 50mm of mercury column in an evacuated tube fitted with a cathode and an anode. A mercury vapor filled gas diode is the most suitable
for the purpose. The cathode of the gas diode may be directly or indirectly heated. A hot cathode gas filled diode is known as phantom (or thyratron). A gas filled diode is symbolically represented as shown in figure 1. The dot in the tube
shows the presence of the gas or vapour. When the anode or plate of the gas filled diode is at a positive potential with respect to the cathode, electrons move across the tube from the cathode to the anode. This electronic current depends
upon two factors: (1) The number of electrons emitted per unit area from the cathode and its temperature.
(2) The effect of space charge region, i.e., the negatively charged region containing the electron cloud due to the accumulation
of electrons emitted by the cathode.
As the plate potential is increased, the plate current slowly increases. When the plate potential is increased beyond a critical value, the plate current also increases more rapidly. This is because
when the plate potential approaches critical value, the electrons arriving at the anode gain enough energy to knock out the electrons from the atom of the gas. These electrons are also attracted by the anode causing an increase in plate current.
The positive ions neutralize some of the space charge, which further helps to increase the kinetic energy of the thermal electrons. This value of plate potential is called the ionization potential of the gas. Circuit diagram to determine ionization
potential of mercury is given in figure 2.
Figure 1 Mercury filled gas diode.
If a graph is plotted between plate potential and plate current; the plate current at first increases slowly for a given increase in plate voltage and when the plate potential is equal to or greater than the ionization potential there is a
greater increase in plate current for the small increase in plate potential. The change in slope is, however, not very abrupt but there is a short curved portion within which the change in plate current goes on becoming more and more rapid.
To find the value of ionization potential the two straight portions AB and CD of the graph are produced to meet at a point E (Figure 3). If we draw a perpendicular EF on the X-axis, then OF represents the ionization potential as shown in figure
3.
1. Draw the diagram showing the scheme of connections as in figure 2 and make the connections accordingly.
2. Switch on the power supply and apply a suitable potential to the filament of the gas diode. The filament is heated in a short
time to become red hot.
3. Adjust the voltmeter reading to 1 volt and note the corresponding value of the current in the millimeter. Increase the plate potential by 1 volt and note the voltmeter reading as well as the millimeter reading.
Proceed till the plate potential is about 14 volts.
4. Taking the plate voltage along the x-axis and plate current along the y-axis plot a graph between plate current and plate voltage as shown in Fig 3. Draw the straight line AB between
the first few points and the straight line CD between the last few points and produce AB and DC to meet at E. Draw EF perpendicular to the X-axis, then point F on X-axis gives the value of the ionization potential of mercury.
1. A gas filled mercury vapor diode must be used.
2. The positive of the voltmeter as well as millimeter must be connected to the positive of the D.C. supply.
3. The plate potential should not exceed 15 volts.
4. To find the
exact position of ionization potential two straight lines joining the first few points and the last few points should be produced to meet. A smooth curve joining all the points should not be drawn.