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Electrostatics and chemistry

Dear Reader,

Well it is time to start to educate the first years again, we are going to start with some of the chemical bonding in solids and liquids. Before you ask in the gas phase a typical molecule is all on its own. Normally a molecule in the gas phase spends most of its time not in contact with any other atoms, surfaces or molecules.

In the condensed (liquid and solid) phases many species form weak bonds to other species which do not involve full covalent bonds.

The most obvious one is ionic bonding, the reason why a salt such as sodium chloride (common salt) is a solid is the electrostatic effects which cause the sodium ions to attract the chloride ions. Maybe we should consider for a moment the wonders of ionic bonding.

Before we start the sensual journey into chemistry I think it is important that I share something with you. The basis of almost all chemistry is electrostatics, this explains ionic and covalent bonding together with the way that atoms and molecules interact with each other without forming full strength bonds.

Now for a moment I will digress off into electronic engineering, in high speed high gain valved electronics it is common to use a tetrode valve. The reason is that the capacitance between the anode and the grid tends to add a positive feedback loop into many circuits. In general positive feedback is very bad in amplifiers as it tends to lead to instability.

The best way to deal with this problem is to change from a triode to a tetrode. The tetrode has a cathode, an anode, the control grid and a second grid between the control grid and the anode. This extra grid is known as a screen grid. In DC terms it is normally at a voltage slightly lower than the anode and in RF terms it is connected to ground.

What this screen grid does is to act as electrostatic screen which reduces the influence of the anode on the control grid. As a result the valve has a lower capacitance between the anode and the first (control) grid. Because of this the valve is far more easy to use in high speed high gain circuits.

The high power triode valve (3CX2500) has a grid anode capacitance of 20 pF while tetrode valve with a similar power level (4CX3000) has a control grid to plate which is about 100 times smaller. For those of you who want to understand what the valve numbers mean, the first number is the number of electrodes, the C means ceramic (instead of glass), the X means an external anode and the number is the power which the anode can dissipate.

In a similar way the inner clouds of electrons in an atom tend to partly screen out the electrostatic effect of the positively charged nucleus. As an example we should consider a point charge which consists of nine protons together with some neutrons to hold them together.

If we make a graph of the electric potential against distance from the point charge then we will see that the closer we get to the point charge the higher the electric potential is, this is the upper blue line. If we add the effect of putting the positive charge in the centre of a cube (50 pm) which has an electron at each vertex. Then the effect of the positive charge is much weaker outside the atom.


Electric potential as a function of distance from the centre of the atom, with and without the effects of the electrons.



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