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DANIEL CELL and salt bridge

EXPERIMENT 2.4 Daniell cell and salt bridge
Experimental part
Materials and procedure for the experiment:
 Cu and Zn metals strips;
 CuSO4 and ZnSO4 as 1M solutions;
 Agar Agar
Few different salt bridges are made after the following recipe:
 3 g of agar-agar are boiled in 250 ml deionised water.
The entire volume is divided in four and to every quarter is added:
1) NaOH 0,1M and higher concentrations - aprox. 5 M
2) CaCl2 about 0.1 g and CaCl2 about 2 g
In this way four different salt bridges are made. Four tubes of PVC (or another material) are curved in a U shape and filled with up presented solution. At the ends of the tube some glass or cotton wool is fitted in order to have a good contact between bridge and solution. The tubes are leaved to cool down and after the gel apparition, the bridge is ready.
The 4th bridge is a repetition of 3rd bridge, but instead of glass wool at the ends of the tube, 2 pieces of graphite recovered from some old carbon zinc battery are used.
The experiment is designed to check the movement of the ions from bridge to solution.
First, the diluted CaCl2 bridge is used. The potential registered is about 1,1 V. In solution there is no confirmation of a precipitate apparition after 2 hour of working with multimeter connected to it and electrodes in short circuit
Another cell is formed with concentrated CaCl2 solution sequestered into agar agar. The potential registered is about 1,1 V. In solution there is no confirmation of a precipitate apparition after 2 hours of cell working.
The third cell is formed with a NaOH sequestered into agar agar as salt bridge. Before bridge insertion and after 2 hours of cell working the pH is measured with pH paper or with a pH meter. Before bridge insertion the pH of ZnSO4 was approx 5 and the pH of CuSO4 was about 9. After two hours the same values were registered.
The 4th cell is formed with a bridge having a “mixed” conduction. The graphite is an electronic conductor and the gel is an ionic conductor (according to actual classification). The contact between bridge and CuSO4 and ZnSO4 solutions is made only by graphite portion of the bridge. The potential registered is about 0,8 V.
A “formal Daniel cell” is formed using a Zn electrode immersed in a ZnSO4 1M solution and on the other hand a Cu electrode immersed in a CuSO4 solution. The solutions are connected with one of up presented salt bridge. CaCl2 was preferred instead of KCl due to the simplicity of Ca and Cl ions detection in an analytical way.
Before cell formation the mass of Cu and Zn electrodes was measured with an analytical balance. Once the cell formed, the difference of potential is measured and this is 1,070 V. Further, in order to speed up the processes from electrodes, the Zn and Cu electrodes are connected by a metallic wire (short circuit) as in fig. 2.6.
After 1 day of Daniel cell working, the metallic wire is disconnected and the difference of potential is again measured. The value found for the “Daniel cell” is the same 1,07 V, like before short circuit.

Daniel Cell

Figure 2.6 Daniel cell with salt bridge

 The electrodes are again connected in short circuit and after 3 days, the procedure of measuring the difference of potential repeated. Strange enough, the same value 1,07 V was counted. Again the electrodes are connected in short circuit and after one week the procedure of difference of potential measuring is repeated measuring the same value for the cell potential.

There is no increase of Cu of electrode mass but there is a slow and uniform decrease of Zn mass.
Experimental interpretation
The purpose of experiment is to analyse the specific mechanism of salt bridge working.
If an ion circulation is necessary for cell working, the calcium and chloride ions should have a directional and clear movement toward a specific compartment. Calcium species must move toward copper compartment and chloride species must move toward zinc compartment.
From common chemistry it is known that calcium reacts with sulphate and a white precipitate must appear.
In the first case with diluted concentration of CaCl2 used for bridge construction it is difficult to arrive to a product solubility of CaSO4 and in this time the salt bridge should be exhausted.
In case of concentrated CaCl2 bridge, in the CuSO4 baker should appear a precipitate of CaSO4. The quantity of Cl and Ca gained by every solution can be determined analytically. After hours of cell working a small quantity of Cl should be put in evidence using AgNO3 as reagent.
Despite these considerations, CaSO4 precipitate does not appear spontaneously in the copper compartment. In the same time, silver nitrate does not precipitate chloride into zinc compartment. There is a small and insignificant transport of CaCl2 from the bridge in solution, due to the Fick diffusion and not due to an electrochemical transport.
In the case of salt bridge based on NaOH, the pH of the solution should appear basic in a Zn compartment, and Zn(OH)2 should precipitate too, which is not the case in reality.
The purpose of this material is to demonstrate that a Daniel cell with a salt bridge use a complete different mechanism for working by comparison with original Daniel cell. In the modern representation of Daniell cell (found in every book of physics and chemistry), there is no expected reaction between components.
Let’s analyze what does not fit in actual explanation of Daniel cell modelling. In fig. 2.7, a simple Daniel cell similar with any other picture from a low level school is presented. The salt bridge is represented by 4 KCl species, for a simple description of phenomena.
In order to work properly, Daniell cell require a compensation of ions from the salt bridge as described in fig. 2.8. Chloride ions compensate the Zn cations and potassium ions compensate sulphate anions in the opposite compartment. After a time it should exist a depletion of KCl in the bridge (in the picture are remaining 2 molecule instead of 4).
If this principle of cell working is correct, making a salt with a small quantity of salt, depletion and an exhausting of salt bridge should be counted. It can be imagined an experiment where the salt bridge is dissected and the depletion of salt concentration is analysed. But as is described further there are other and simpler possibilities to check the mechanism of cell working. If the quantity of salt (CaCl2 in our case) in the bridge is varied, the current must depend on salt concentration. This means at low quantity of salt it will mean a low current, and a high quantity will mean a higher current. The experimental measured currents show an insignificant dependency on the concentration of salt inside salt bridge.

Daniel Cell1

Figure 2.7 Daniel cell with salt bridge

Daniel Cell2

Figure 2.8 Daniel cell mechanisms

Let’s estimate the time of life for a salt bridge Daniel cell in short circuit mode and other strange consequences. It was used as salt in the bridge a NaOH solution captured in an agar agar gel, as described in below experiments.
The resistance of salt bridge is about 16 kΩ and the resistance of solution about 4 KΩ. Let’s exaggerate and consider that internal resistance of battery is 50 KΩ. In this condition considering the external circuit of resistance zero the short-circuit current is:
I =U/(R+r) =1,01/50000 = 21 microA
A little bit deeper into actual physic:

I = Q/t = ne/t 
where n = number of charges in our case electrons; e the charge of electron; t – time of working in short-circuit
The actual Daniel battery is working in short circuit already from 8 days which means 691200 s. The number of electrons which has already passed by external conductor is:
n = It/e = 0,9 ×10 exp(20) 
These numbers of electrons are coming from the Zn dissolution reaction:
Zn = Zn2+ + 2e-
which means a number of 0,4536×10exp(20) atoms of Zn are released in solution.
For this number of Zn cations a double number of hydroxyls ions must come from salt bridge.
Mass of Zn passed in solution is:
0,4536×10exp(20)×65,37/(6,023×1023) =4.92 mg
Let’s calculate the size of hydroxyl quantity present in the cell. For the salt bridge 5 ml of 0,1 M NaOH were diluted to 75 ml agar agar solution. After that the tube was filled with 20 ml of this solution.
The quantity of NaOH in 5 ml solution is: 20 ×10exp(-3) g
In the tube there is a quantity is 5×10-3 g
From this mass the mass of hydroxyl ions is:
5×17/40 = 2.125 mg
The stoechiometry of reaction:
Zn2+ + 2HO-  =Zn(OH)2
65,37 mg 2× 17 mg
4.92…………..y
y = 2,55 mg
So in order to neutralise the 4,92 mg of Zn2+ there are necessary 2,55 mg OH-. But the total quantity of hydroxyl available in the entire bridge is only 2,125 mg.
Considering that all this quantity is flowing into Zn compartment, there will remain still an excess of Zn cations already moving into solution and searching for anions able to maintain the neutrality of solution. Of course, in the Cu compartment there is excess of anions moving into solution and searching for cations able to maintain the neutrality.
The cell is still working at full potential after 8 days and this means there is a transfer of cations and anions from one compartment to another through salt bridge.
In the same time if all hydroxyl from the bridge is passed into solution the Zn compartment should have an alkaline pH. This is not confirmed by direct measurements - both compartment are at pH about 5, in acid region and not alkaline.
How is possible for formed Zn(OH)2 to have a week acid pH?
Further analysis must regard the formation of Zn(OH)2 precipitate. The product of solubility of Zn(OH)2 is KSP = 3 ×10exp(-17) so it is very easy to be observed visually a increasing of hydroxyl concentration. It reality the Zn(OH)2 precipitate does not appear even in other repetitions of experiment the concentration of NaOH was increased to 4M.
In some contradictory discussions with other physicists it was argued that only sulphate migrate through the cell in order to have a chemical reaction at Zn electrode. The case is analysed on the site and is too fantastic to be reloaded here.
The problem with the Daniel cell having a salt bridge in its constitution regards the absence of expected chemical reaction. In this later case a secondary reaction between Zn and water in one compartment generates the measured electric current. Therefore there is a tremendous difference between salt Bridge Daniel cell and original Daniell cell, invented in 1836 by John Frederic Daniell regarding the output power.
The reaction between Zn and water is slow and consequently in case of salt bridge the output power is smaller. From simple elementary school manuals to the high scientific treatise, the Daniel cell is presented with a salt bridge ….but this cell has never worked and will never work.

 

Content

CONTENT

Chapter I

Chemistry Review

 

Chapter II

Atoms, molecule and ions

 

Chapter III

Electron structure of atoms

 

Chapter IV

Periodic properties of elements

 

Chapter V

Chemical bounds

 

Chapter VI

Structure of inorganic compounds

 

Chapter VII

Structure of organic compounds

 

Chapter VIII

Reaction kinetics

 

Chapter IX

Reaction dynamic - collision theory, activatetd state theory

 

Chapter X

Electrochemical processes

 

Chapter XI

Organic reaction mechanisms

 

 

 

Concentration cells

Concentration cell and foundation of electrochemistry
The presentation in the book is more extensive. Here you will find only the ,,article" submmited to ChemCommunication and considered not worth printing because ,,a series of stand alone experiments", made with scarce amount of money, cannot be published in a scientific journal. Part of the discussion is presented at the end of the material.  


Although the concentration cells represent a theoretical and practical model for the conversion of chemical energy in electric energy, the actual explanation of cathode and anode events is far away from experimental reality. The experiments show no transfer of mass between electrodes and the migration of ionic species in solution is not consistent with theoretical frame. A new interpretation for electrode phenomena has to be proposed with a new insight for conversion of chemical energy into electrical energy.
A concentration cell is a galvanic cell with half-cells of identical composition but differing concentrations. The most common example is represented by two copper electrodes inserted in two solutions of copper sulphate at different concentrations. The connection between half-cells is insured by a salt bridge or by an osmotic membrane and at copper electrodes a certain voltage difference is measured.
In case of osmotic membrane, there is the possibility for ions in solution to pass through it, in order to equilibrate the concentrations, so after a certain interval of time both solutions arrive at same concentration and the difference of potential becomes null. In case of a salt bridge, this furnishes ions to solutions and the same events happen. In concentrated half-cell, copper is reduced and the remaining anions get cations from the bridge; correspondently in half diluted cell, copper is oxidized and pass in solution and the bridge furnishes anions.
As consequence, the dilution of the cathode half-cell is achieved by reducing Cu2+ to Cu metal and plating that metal onto the Cu electrode. In the anode half-cell, the Cu anode is oxidized to Cu2+ and, thus, dissolved into the solution, making the anode cell more concentrated.
In order to calculate the cell potential for up specified concentrations (0.01M and 1M) at 25oC, Nernst Equation is used:
E = E° - (0.059/n) log Q = +0.058 V
The concentration cells are an interesting case of cells, where no net chemical reactions occurs. The numbers of copper and sulphate ions in system does not change; it is the distribution of these species in the cell that provides the driving force. From thermodynamic point of view, two solutions with different concentrations will diffuse one into another in order to arrive at a uniform concentration. Putting copper electrodes into the solutions offers an alternative method of achieving the same end. One electrode can release a Cu2+ ion into the dilute solution; the electrons so produced then travel through the wires to the other electrode, and a Cu2+ ion is removed from the concentrated solution, reduced to a copper atom and plated out on the electrode. Both these events are simultaneously.

Experiment 1
The purpose of this experiment is to measure the mass transfer in case of a diffusion concentration cell.
Materials
• 1 M copper sulphate solution,
• 0.01 M copper sulphate solution
• two PVC containers ( as in figure 1- section), with copper electrodes, passing taps and the possibility of connection through a membrane filter or a thicker layer.
In the experiment the containers were 2 litres volume and the filter membrane used was a nylon 0,22 micrometer pore size. One container was filled with 1 M solution and the other with a 0.01M of CuSO4. The taps of system were opened (fig. 1) and the potential was measured in order to check the actual prediction (Nernst equation); further the system was shortcircuited and left for a week in order to equilibrate the concentrations in both compartments. After this time both electrodes were weighed with a precision balance and used again for another week, filling the containers with fresh solutions. At the end of second week again both electrodes are again weighed and the procedure of forming a new concentration cell was repeated. The entire procedure was repeated for the third week again.

concentration01

Figure 1. Diffusion concentration cell details

During these 3 weeks of concentrations equilibration there was no transfer of mass between copper electrodes. On the electrode immersed in concentrated CuSO4 solution, in time the blue CuSO4 adhere to the electrode and the mass of this electrode is increased a little bit (about 1 g). If the electrode is washed with pure water, the final mass of electrode is the same like initial one.
The increase of electrode mass found in the diluted solution is about 0.3 g. Again after simple washing the mass of this electrode remain invariant. The increase of mass electrodes is caused by CuSO4 which adhere to the metal proportional with its concentration in solution and not due to other processes.
In the experiment, the masses of both electrodes were measured in order to avoid any experimental errors. A mass transport of Cu should give us a correlation between an eventually decrease of an electrode mass and increase of another one.

Experiment interpretation
What are the predictions of actual physical chemistry for up presented experiment? To be more precise what should be the theoretical mass of transported copper?
If anode generates a Cu cation, another sulphate anion must pass the membrane in order to have the neutrality of solution and simultaneously a copper atom is deposited on the cathode. The final concentration of both solutions is about 0.5M (the initial concentration of anode compartment is neglected). In order to have a 0.5 M sulphate concentration in anode compartment, the quantity of copper released in 2 litres of solution must be equal with 1 mole, which means 63,5 g. On the other hand the mass of cathode should appear increased with 63,5 g.
As initial mass of electrodes was 25 g, normally after one week one electrode must be dissolved into solution and the mass of second electrode should be increased with correspondent quantity.
As the solution was changed once a week for three times, theoretically, the total quantity of copper displaced should be about 190 g. It is impossible to not see such a mass transfer even with naked eyes.
The experimental reality is completely different. Except the CuSO4 material which adheres to electrodes, no increased mass of copper is observed for one electrode and no decrease mass for the other electrode.
The up presented calculus was made in considering the concentration cell working with a yield of 100 %. Of course there is a Fick diffusion between compartments which diminishes the copper transfer. In case of a concentration cell working with a yield of 0.1 % (which is absurd because the cells are recognized to have a better yield then another thermodynamic processes), at a single concentration equilibration should appear a mass transfer of 0.1*63.5 =6,35 g.

Experiment 2

The purpose of this experiment is to analyse how a salt bridge concentration cell works.
Materials:
• 1 M copper sulphate solution,
• 0.01 M copper sulphate solution
• two small containers with copper electrodes,
• one salt bridge made by saturating a piece of cotton in a CaCl2 solution and another one made with agar agar and CaCl2. The use of CaCl2 in the salt bridge instead of KCl is motivated only by the simpler analytical possibilities of Ca determination.
Two concentrations cells are build up using the up presented salt bridges and their potentials are measured. After this operation, the Cu electrodes are put in short circuit with a simple metallic conductor in order to hurry up the electrochemical processes as in fig. 2.

concentration02

Figure 2. Salt bridge concentration cell details
The potential difference between electrodes were measured after 3, 5 and finally after 8 days of continuous cells shortcircuit.
In case of agar agar-CaCl2 bridge, the potential difference remained the same like the initial one (E’ = 0.048) after this intervals of time. The measurements of Cl and Ca concentrations in both compartments,indicate a level of their concentration under the chemical limit of detection (no precipitation with AgNO3 and H2SO4, few ppm detected with more specific methods).

In case of cotton salt bridge at the same time interval there is a diminishing of cell potential with approx. 0.002 V after 3 days and with approx. 0.007 V after another 5 days; after 8 days the variation of potential was about 0.01 V (initial measured value was 0.046 V). Both Ca and Cl species were detected in the anode and cathode compartments by precipitation with AgNO3 and H2SO4. A normal analytical procedure was followed for measuring their concentrations and it was found that:

Anode [Ca]=[Cl]/2=1.74 M/L
Cathode [Ca] =[Cl]/2=1.67 M/L
The absolute values of Ca and Cl species at anode are a little bit higher than at cathode region (probably due to the small different level of immersion of bridge into solution).
Again there is no transfer of mass between electrodes (both electrodes after washing with clean water have the same mass as initial ones).

Experiment interpretation
As far the masses of electrodes remain constant, the same considerations presented at first experiment are to be repeated here.
More important for the foundation of chemistry is the analysis of physical and chemical phenomenon which should take place at cathode and anode. Why copper ions should be reduced in case of concentrated solution and copper should be oxidized in diluted solution? Why there are not opposite phenomenon, more precisely Cu2+ from diluted solution reduced and in case of concentrated solution Cu metal oxidized?
Are directions of chemical reactions dictated by gradient concentration in the last time in chemistry?
From low level chemistry it is well known that an oxidation process takes place in presence of a reducing agent.
For example Cu metal can be oxidized by an oxidizing agent like oxygen or nitric acid. On the other hand a reduction of Cu2+ needs a reducing agent like hydrogen, etc. Equations for the reactions are
2 Cu(s ) + O2(g ) --> 2 CuO(s )
CuO(s ) + H2(g ) --> Cu(s ) + H2O(g )
By contrary to this well known comportment of copper, a simple diffusion of ions (without any chemical processes according to actual interpretation) in case of a salt bridge cell is able to produce an oxidation of Cu to one electrode and reduction of Cu2+ to other electrode. This actual explanation is completely inaccurate and in contradiction with experimental results.

Conclusions
The actual accepted explanation is not able to give a coherent explanation of ,,cathode and anode” mass transfer in case of concentration cells. Further there is no coherent explanation for the oxidation and reduction phenomenon dictated by a solution gradient. A new explanation for electrical energy generation has to be proposed.

 

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Electrod potential and isotopes

ISOTOPE CHANGE AND ELECTROCHEMISTRY

In the chemistry book I proposed the experiment below using radioisotopes. In the meantime analytical techniques for measuring the stable isotopic ratio of metals have become cheap and convenient, so the experiment was performed using transitional stable isotopes as in part 2 (basics of chemical physics book). 

Experimental part
Some special and quite expensive materials are necessary for this experiment. Zirconium element presents more stable radioisotopes with longer life time (95Zr, 96Zr, 88Zr) besides the two stable isotopes (91Zr and 92Zr).
A metal electrode constituted from a Zr radioisotope is dipped into a salt solution of stable isotopes, for example Zr(NO3)2. Other metals found after hydrogen in the electrochemical series can be used for the purpose of experiment. The experiment has the purpose to test the isotopic change of an element between metal electrode and its solution species. For this purpose, the radioisotope (95Zr) metal piece is immersed into a non radioisotope salt solution Zr(NO3) and for a certain interval of time it is allowed for this isotopic change to take place as in fig 2.9. After that, the radioactivity of solution and remnant radioactivity of metal plate are measured.
Other radioisotopes elements or the use of stable isotopes can serve for the purpose of experiment.
At the moment of book printing the experiment was not performed, so only the prediction of proposed theory by comparison with actual one are presented.

isotope01
Figure 2.9 Isotopic experiment
Experiment interpretation
Having a metal electrode deeped into a solution that contains ions of that metal, a potential difference between the metal and the solution appears according to actual interpretation due to the following equilibrium:

                           M  = Mn+ + ne-

The entire electrochemistry is build up on the concept of this continous exchange (atoms are going out from metal network into solution and other atoms from solution are deposited on the metal surface. Of course these things are happening at atomci level so at macroscopic level ,,we measure" a certain electric potential for this primary electrode. The potential appears because for different metals we have different tendencies to release electrons and of course to switch some atoms from metallic network with atoms from solution. 
Consequently, when the metal strip contain only one isotope (radioisotope) and the solution of its salt contain non radioactive isotope or isotopes, after a period of time there will be a process of isotopic change between metal and solution.
Actual orthodox theory admits as real this isotopic change between metal and its salt solution. In proposed theory there will be no isotopic change between metal and its solution.
A new perspective is offered in proposed theory for the specific comportment of a metal and its salt solution component.
When no reaction takes place between metal strip and solvent (usually water), no isotopic change takes place at the interface solid solution.
When metal piece react with solvent (water), there is a mass transfer between metal piece and solution, so the isotopic pattern of solution is changed. But there is no change of isotopic pattern for remaining metal part, because no metals atoms are deposited back on the metal piece.

 

COVALENT BOND

COVALENT BOND

Background and actual explanation

Covalent bonds are formed as a result of the sharing of one or more electrons. In classical covalent bond, each atom donates half of the electrons to be shared. According to actual theories, this sharing of electrons is as a result of the electronegativity (electron attracting ability) of the bonded atoms. As long as the electronegativity difference is no greater than 1,7 the atoms can only share the bonding electrons.
Being in impossibility to explain coordinative complex and also the structure of a lot of common compounds, new variants of covalent bound theory are proposed. In the Valence Bond (VB) theory – one of must representative in quantum mechanic - an atom rearranges its atomic orbital prior to the bond formation. The equation that serves as a mathematical model for electrons movement inside atoms is known as the Schrodinger equation:
covalent101
where: m is the mass of the electron, E is its total energy, V is its potential energy, and h is Planck’s constant. The solution of Schrodinger equation, which gives the electron density distribution are called orbital.
The simplest types of orbital s and p are presented in fig. 1.
covalent102
Figure 1. s and p orbital shapes

The s orbital has a spherical symmetry and p orbital has a bilobar form with a node in the middle.
In the simplest case, a covalent bound should be formed by superposition of two atomic orbital as in fig. 2
covalent103
Figure 2. Hypothetical covalent bound between s and p orbital

Instead of using the atomic orbital directly, mixtures of them (hybrids) are formed. This mixing process is termed hybridization and as result are obtained spatially-directed hybrid orbital.
We will describe a simple hybridization for s and p orbital. In this case we can have three basic types of hybridization: sp3, sp2 and sp. These terms specifically refer to the hybridization of the atom and indicate the number of p orbital used to form hybrids.
In sp3 hybridization all three p orbital are mixed with the s orbital to generate four new hybrids (all will form σ type bonds or hold lone electron pairs).
covalent104
Figure 3 sp3 hybridization

If two p orbital are utilized in making hybrids with the s orbital, we get three new hybrid orbital that will form σ type bonds (or hold lone electron pairs), and the "unused" p may participate in π type bonding. We call such arrangement sp2 hybridization.
If only one p orbital is mixed with the s orbital, in sp hybridization, we produce two hybrids that will participate in σ type bonding (or hold a lone electron pair). In this case, the remaining two p orbital may be a part of two perpendicular π systems.

An atom will adjust its hybridization in such a way as to form the strongest possible bonds and keep all its bonding and lone-pair electrons in as low-energy hybrids as possible, and as far from each other as possible (to minimize electron-electron repulsions). 
In the simplest example hydrogen molecule formation:
covalent105
Hydrogen atoms need two electrons in their outer level to reach the noble gas structure of helium. The covalent bond, formed by sharing one electron from every hydrogen atom, holds the two atoms together because the pair of electrons is attracted to both nuclei.
In order to explain the form of a molecules quantum mechanic propose a new theory called ,,electron pair repulsion theory”. According to this, the shape of a molecule or ion is governed by the arrangement of the electron pairs on the last shell around the central atom; this arrangement is made in such manner to produce the minimum amount of repulsion between them.
In case of two pairs of electrons (like BeCl2) around central atom the molecule is linear because an angle of 180º insure a minimum interaction between electrons pairs.
covalent106
In case of three electron pairs around the central atom (BF3 or BCl3) the molecules adopt a trigonal planar shape with a bond angle of 120º:

covalent107
In case of four electron pairs around the central atom (CH4) we have a tetrahedral arrangement. A tetrahedron is a regular triangularly-based pyramid. The carbon atom would be at the centre and hydrogen at the four corners. All the bond angles are 109.5°.
covalent108
For five pairs around central atom (PF5), the shape is a trigonal bipyramid. Three of the fluorine are in a plane at 120° to each other; the other two are at right angles to this plane. The trigonal bipyramid therefore has two different bond angles - 120° and 90°.
covalent109
In case of six electron pairs around the central atom (SF6) the structure is an octahedral.

covalent110

Molecular orbital (MO) theory is an alternative way of describing molecular structure and electron density. The fundamental premise of MO theory is that the orbitals used to describe the molecule are not necessarily associated with particular bonds between the atoms but can encompass all the atoms of the molecule. Molecular orbitals consist of combinations of atomic orbitals. In simple molecular orbital (MO) theory, a number of atomic orbitals will combine to form the same number of molecular orbitals. For example, "n" atomic orbitals will combine to form "n" molecular orbitals.

The properties of the molecule are described by the sum of the contributions of all orbitals having electrons. 
In localized bonding the number of atomic orbitals that overlap is two (each containing one electron), so that two molecular orbitals are generated. One of these, called a bonding orbital, has a lower energy than the original atomic orbital, and the other, called an antibonding orbital, has a higher energy. As orbital of lower energy fill first, in case of one electron sharing between atoms, both electrons go into the new molecular bonding orbital, since any orbital can hold two electrons. In this case the antibonding orbital remains empty in the ground state.
Let’s consider the simplest molecule - H2. Each H atom has an electron in a 1s orbital. When they come together, these two 1s orbitals overlap as in fig. 4 and form the bound orbital.
In the same time a higher electron density between atoms is counted.
The other anti-bonding orbital is empty and has a shape that would lead to electrons spending more time away from the region between the two atoms. Because of this, this orbital is considered an.
covalent112
Figure 4. Bonding and antibonding molecular orbital for hydrogen molecule

Proposed model of covalent bound

In proposed theory a covalent bound implies only a coupling of magnetic moments of individual atoms (more precise the electron magnetic moments of last shell electrons) in order to obtain a greater stability. The electrons remain and orbit around proper nucleus, and consequently there is no sharing of electrons between atoms. When a covalent bound is broken the coupling between these magnetic moments is lost and of course every atom remains with his electrons. The situation is quite different in quantum theories, because when a covalent bond is broken the electrons are probabilistically distributed back to atoms so an electron form one atom can arrive to the other atom participating at bound.
According to new interpretation, every atom of hydrogen possesses an electron magnetic moment due to the electron movement. The magnetic moment of nucleus is lower so it is not important in this case. The electron magnetic moment is formed by combination of orbital and spin magnetic moment using known rules of vectors. The covalent bond means that both atoms attract reciprocally due to the magnetic interaction between their magnetic moments. The simplest interaction between two magnetic moments of different electron from different atoms is showed in fig 5. The magnetic moments are pointed parallel but with opposite directions.
Every atom has own electron and the electron orbit only around his nucleus and the orbits of electrons are situated in parallel planes (fig. 5). There is a dynamical equilibrium regarding a minimum distance between atoms, when the electrostatic repulsion force became stronger and a maximum distance between atoms when the coupling between magnetic moments force the atoms to move one to another. There is also an electrostatic push due to the electron reciprocal interaction and a nuclear push due to the nucleus reciprocal interaction. These interactions regulate the distance between atoms, because when distance becomes lower due to the attractive magnetic interaction, the electrostatic repulsion increase and the equilibrium is maintained.

cov006
Figure 5 Hydrogen covalent bond formations

The hydrogen molecules formed due to the opposite orientation of electrons magnetic moments has a lower energy comparative with the state of single atoms of hydrogen. The energy interaction between hydrogen atoms is given by:
cov007(1.1)

where B1 represent the intensity of magnetic field created by μ1 at level of secondary atom orbit (r2) and B2 represent the intensity of magnetic field created by μ2 at level of first atom orbit (r1).
cos θ1 and cos θ2 represent the angle between μ1 and B2, respectively μ2 and B1 and due to the symmetry of hydrogen molecule θ1=θ2.
So in a first approximation, one electron is moving in the magnetic field created by the other electron from the other atom and reciprocally.
The orientation of B1 and B2 is antiparallel with orientation of μ1, respective μ2 (for the ecuatorial plane). This is due to the orientation of B tangent to the line of magnetic field created by μ1, respective μ2. In fig 6 is presented, as example, the magnetic moment produced by electron moving in the x-y plane with nucleus in the origin of system. The magnetic moment is along the z axis, the line of magnetic field go from North Pole and enter into the South Pole. The vector B is tangent to the magnetic line field, and at ecuatorial plane (orbit electron plane) and in other direction then N and S poles, B is generally antiparallel with μ.
Due to the orientation of electrons orbits, in case of covalent bound, the same antiparallel orientation is valid also for the μ1 and B2, respectively B1 and μ2.
The energy of magnetic interaction between two electrons became:
cov010 (1.2)
θ1 = θ2 = 0 that means cos θ1 = cos θ2 =1
The value of B created by a magnetic moment at distance r is given, according to electrodynamics, by:

 cov011(1.3)
where: B is the strength of the field;
r is the distance from the center
λ is the magnetic latitude (90°-θ) where θ = magnetic colatitudes, measured in radians or degrees from the dipole axis (Magnetic colatitudes is 0 along the dipole's axis and 90° in the plane perpendicular to its axis.);
M is the dipole moment, measured in ampere square-meters, which equals joules per tesla;
μ0 is the permeability of free space, measured in henrys per meter.
For our case, l= 0, M = m, so the field created by first electron at second electron level is
 cov012(1.4)
cov013

Figure 6 Antiparallel orientations of B and m for the same magnetic moment at xy plane of electron orbit

And for second electron at first electron orbit we have:
cov014(1.5)
The magnetic interaction became:
cov015(1.6)
where and and μ0 is the permeability of free space, measured in henrys per meter.
For hydrogen electrons due to the symmetry of atom arrangement we have as equal value for electron magnetic moments, so we can write:
cov018(3.7)
The major and fundamental difference between quantum theory and proposed theory is that after forming of hydrogen molecules, every atom of hydrogen has only one electron around nucleus. The hydrogen atom doesn’t have a doublet structure according to new theory. There is no difference in atomic structure between atom of alone hydrogen atom and hydrogen atom in molecule. The only difference is the coupling of magnetic moment of hydrogen with another magnetic moment and this coupling insure a lower energy in case of molecule.
As comparison, quantum mechanic is incapable to explain why two opposite spin are lowering the energy of system. In the same time there is a contradiction in actual theory when the electrons are filled on subshell in atomic structure and when a covalent bound is formed. More precisely, the electrons fill a subshell first with one electron in every orbital with parallel spins and after that the existing electrons complete the orbital occupation with opposite electron spin. So if the coupled spin state is more stable, at occupation of subshell should be occupied complete an orbital and after hat another orbital. 
For other elements, when we have a single electron in the last shell the situation is simple because for the inner shells, magnetic moments suffer an internal compensation. What’s happened when we have more electrons on the last shell?

Normally in the ground state electrons form pairs with opposite spin in order to maintain a low level of energy. But at interaction with other reactants a process of decoupling of pairs of electrons can happened. Depending on the condition of reaction, on the structure of element, on the stability of formed compound it is possible to have a partial decoupling or a total decoupling of electrons from last shell. As example: chloride having 7 electrons on the last shell, can participate:
• with one electron in chemical combination like in ground state,
• with 3 electrons, that means a decoupling of one pair of electrons plus the initial decoupled electron;
• with 5 electrons, that means a decoupling of two pairs of electrons plus the initial decoupled electron;
• with 7 electrons, that means a decoupling of three pairs of electrons plus the initial decoupled electron.
When a single electron on the last shell is presented and we have a single element bound, the orientation of electron magnetic moment is not so important. Of course the molecule formed is linear. When the number of electron magnetic moments is greater, the situation it is a little bit complicated but solvable and easy to understand. The magnetic moments of electrons are treated classical this means, the energy is minimum when the spread of magnetic moment is maximum. As consequence the magnetic moments, and of course the formed bounds, will have such orientation in order to insure a minimum interaction.
In case of two electrons on the last shell, this means two magnetic moments, and consequently two covalent bounds, the molecule is linear, the angle between bounds is 180º in case of two simple bound.
In case of three magnetic moments (three covalent simple bounds) a trigonal planar arrangement is preferred or a pyramidal trigonal structure in case of central atom with one lonely electron pair.
In case of four magnetic moments (four covalent simple bounds) the molecule will have a tetrahedral arrangement.
For five and six magnetic moment (five or six simple covalent bounds), a trigonal bipyramid and an octahedral structure are preferred.
In case of seven magnetic moments, due to the sterical interaction, it is imperative that minimum one covalent bound to be double due to the geometry of molecules.
Chloride with his electron structure can form up to seven covalent bounds. Don’t be scared with counting of number of electrons around chloride nucleus. Even we have seven covalent bound we will have only seven electrons on the last shell. But, sometimes the structure forms needs the necessity of an eighth bound, and in this case chloride catches another electron, and will form eight covalent bounds. We will see this situation for example at anion perchlorat structure.
This is the situation when only simple bounds are formed between atoms. But what is possible to predict using our model when a double or triple bond is formed?
READ more in the book .....
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