Most metals are very good conductors of electricity. In metals, electrical conduction arises by the movement of electrons, unlike the mechanism of conduction in aqueous solutions or fused melts of ionic compounds, where ions migrate during conduction.
The very property of water to convey the medicinal effect of a substance through retention and conveyance of electrical impulses is of great importance, not only for homeopathy but for the development of Science1. It is well known that conductance of electrical energy is a flow of electrons for metals, through ions in aqueous solution or water, and through holes in semiconductors.
It is said that the metallic bond which is responsible for holding/bonding atoms of a metal or metals (for alloys) together, helps movement of electrical energy through available electrons in its conduction band. In other words, delocalized electrons in the conduction band, help the flow of electrical energy. In the Molecular Orbital Theory the valence electrons are considered to be associated with all the nuclei concerned, as atomic orbitals combine to produce molecular orbitals. A brief discussion regarding conduction in conductors and semiconductors is presented here:
In electrical conductors (metals), either the valence band is only partly full, or the valence and conduction bands overlap. There is therefore no significant gap between filled and unfilled Molecular Orbitals (MO), and perturbation can occur readily.
For the Lithium molecule with configuration 1S2 2S1, if n atoms of Li are present in a Lithium crystal, then n 2S atomic orbitals combine to form n 2S molecular orbitals. Now the total valence electrons being n, they will find their place in the n/2 MO (as each MO can accommodate 2 electrons) and hence there will be n/2 filled MO and n/2 vacant MO Fig 1.
But in Beryllium with electronic configuration 1S2 2S2 , if there are n atoms of Be in a crystal combine they will combine to form n 2S MO. As the total number of valence electrons in the Be crystal is nX 2 it may be assumed that they all will be accommodated in the n2S MO (as each MO can accommodate 2 electrons). But this is not the fact, since in an isolated Be atom the 2S and 2p atomic orbitals are some 60 Kj mol-1 different in energy and in the same way as 2s AOs form a band of MOs, the 2p AOs (atomic orbitals) form a 2p band of MOs. The upper part of the 2s band overlaps with the lower part of the 2p band. Because of this overlap of the bands, some of the 2p band is occupied and some of the 2s band is empty Fig 2.
In both the cases it is found as if there is a single energy band with no forbidden gap (Fig 1A & Fig 2A). Consequently if any electron at the top of the filled portion of the band gains a little extra energy, say under the influence of electric field, it can easily move up into the empty part of the energy band. So now the electron being free to move in the crystal, may be termed a free electron.
Non-Metal conductor: In graphite only three of the valence electrons of each carbon atom are involved in forming sp2 hybrid bonds, the fourth electron forming a π bond. The π electrons in graphite are mobile, shared by a whole layer of carbon atoms and hence conduct electricity.
Insulators: The conditions for a crystalline substance to be an insulator at room temperature are
a) Its valence band must be full
b) The conduction band must be normally empty
c) The forbidden gap must be greater than about 1eV.
In diamond, which is colourless, each carbon atom utilizes sp3 hybrid orbitals to form four bonds. Thus each carbon atom is tetrahedrally bonded to four other carbon atoms and a three dimensional polymer is formed .
The energy band diagram (Fig 3) shows that it requires about 6eV of energy to move an electron from the valence band to the conduction band, whereas average energy possessed by an electron at room temperature i.e 300K is somewhat less than the required value. Therefore, diamond is a very good insulator at room temperature. An electric field cannot give this amount of energy to an electron in the solid (diamond).Thus no electron could acquire sufficient kinetic energy to cross the forbidden gap in the electric field. Therefore there will be no flow of electrical energy and the solid will be an insulator. But as average energy possessed by the electron rises with temperature, quite a good number of electrons cross the forbidden gap and thus conductivity of diamond increases with temperature.
Semi conductors : Actually a semiconductor has the energy band structure very similar to that of an insulator, with the only difference that in the semi conductor , the conduction and the valence band are separated by a narrow forbidden gap of about 1eV (fig 4). As in diamond, an energy gap separates the top of the filled valence energy band from a vacant higher band (conduction band). But the forbidden gap for Germanium and Silicon are 0.76eV and 1.1eV respectively. At low temperatures the conductivity of Silicon is very low. Since the energy gap is small even at room temperature, a small fraction of electrons in the valence band have sufficient kinetic energy owing to thermal agitation to cross the relatively small energy gap to enter the conduction band above it. The fractions are however sufficient to permit flow of limited amounts of electrical energy when an electric field is applied. Thus Silicon, having electrical conductivity lying between that of conductors and insulators, is considered a semiconductor.
The theory of conduction of electrical energy in water or aqueous solution got questioned when it was shown that solutions ionic or non-ionic conduct electrical energy through the orientation of water molecules2. But still the vital question remained how the orientation of water molecules could help such conduction of electrical energy. To help explain conductivity of electrical energy in water the following experiment is performed:
Experiment: An LED is connected to a 9 volt battery through a pair of graphite electrodes (Gr X & Gr Y) immersed in distilled water. Five Graphite electrodes Gr A, Gr B, Gr C, Gr D & Gr E are placed at different positions (about 3cm apart) in a beaker as shown in Fig 5.The emf of different pairs of electrodes (A,B,C,D &E) are measured before and after passing electrical energy through the circuit containing the LED. Results are recorded in the table below at 2minutes, 4mins & 6 mins:
Final emf on passing electrical energy(in mV)
Resultant emf (in mV)
|Pairs||(in mV)||At 2minutes||At 4 minutes||At 6 minutes||At 2mins||At 4 mins||At 6 mins|
Discussion: In this experiment when the current is allowed to pass through the circuit containing the LED, it is believed that the LED glows due to the passage of electrical energy through the ions present in distilled water. The ions being neutralized at the opposite electrodes while conveying current i.e, H3O+ ions move towards Cathode (negative electrode) while OH– move to the Anode (positive electrode). From the result shown in the table, two things are not clear:
i) Why the ions move to the relatively neutral electrodes ( A,B,C,D &E) to generate such emf ?
ii) Why and how the presence of ions in water generates different emf for different pairs of graphite electrodes?
Had the emf shown by different pairs of Graphite electrodes been due to ions in water, they should have been nearly the same, if not equal. It is also not clear why Graphite A behaves always as a positive electrode while Graphite E is never positive but always negative. A graph is plotted for emf generated by a pair of electrodes at 2 mins, 4mins & 6mins.
It shows that emf generated by the electrode pairs A to E is in the order A-E>A-D>A-C>A-B. That shows that negativity of the electrodes is in the order E>D>C>B>A. i.e, E is at relatively highest negative potential .
For the pairs B to E, emf generated is in the order B-E>B-D>B-C that again shows that negativity of the electrodes is in the order E>D>C>B. i.e, E is at relatively highest negative potential.
Similarly for the pairs C to E where C-E> D-E, again E is at relatively highest negative potential.
It therefore appears that Gr A being closest to Positive electrode of the circuit containing the LED is always positive and is always at relatively positive potential whereas Gr E being closest to the negative electrode of the circuit containing the LED is always at relatively negative potential, while the rest are intermediate in nature. So, B, C & D are negative with respect to A but are positive with respect to E.