Ions

Niels Jonassen

The word ion (in Greek, ion) means wanderer. It denotes an entity, a particle, that will move under the action of an electric field. So, in principle, valence electrons in metals or holes in semiconductors could be considered ions. But in practice, the name ion is reserved for two species: electrolytic ions and gaseous ions.

If you have an aqueous solution of silver nitrate, the AgNO^₃ is dissociated as

Ag⁺ is called a silver ion and NO₃– a nitrate ion.

If an electric field is now applied to the liquid, the positive silver ions will move in the direction of the field toward the negative cathode, where they each will receive an electron, become neutralized, and plate out onto the electrode. This is the basis for electroplating.

A somewhat similar process takes place at the anode—but we are not going to discuss electrochemistry in detail. Rather, I will point out just a few facts about electrolytic ions. The silver and nitrate ions, as well as other electrolytic ions, have well-defined properties. All silver ions are identical, at least chemically speaking, and they never change their properties no matter what you do to them, as long as they remain ions.

If a given ion is exposed to an electric field with the strength E, it will move with a constant velocity v given by

where k is a constant representing the mobility of the ion. Again, a silver ion always has one positive charge and always the same mobility, at least when you consider a given isotope of silver. The same constancy is true for any other electrolytic ion.

Although ions may be formed in most gases, we will restrict ourselves here to discussion of those types of ions that may be formed and found in atmospheric air, the so-called air ions or atmospheric ions.

Figure 1. How air ions are formed.

The formation of an air ion starts with an electron being knocked off a neutral air molecule, as shown in Figure 1. The now positive molecule (oxygen or nitrogen) will rapidly attract a number of polar molecules (10–15), mostly water, and this cluster is called a positive air ion. The electron will probably attach to an oxygen molecule (nitrogen has no affinity for electrons), and this negative molecule will attract a number of water molecules (maybe 8–10), forming a cluster called a negative air ion. It is important to note that ions are always formed in pairs, and always the same number of positive and negative ions.

It takes a certain energy, about 34 eV (~5.4 x10^–18 J) to knock off the initial electron. This energy may be delivered by shortwave electromagnetic radiation (x-rays or gamma rays), or more often from a colliding particle.

Most of the ionization in the lower atmosphere is caused by airborne radioactive substances, primarily radon and its short-lived daughters. In most places of the world, ions are formed at a rate of 5–10 pairs per cm³ per second at sea level. With increasing altitude, cosmic radiation causes the ion production rate to increase. In areas with high radon exhalation from the soil (or building materials), the rate may be much higher.

It is primarily alpha-active materials that are responsible for the ionization. Each alpha particle (for instance, from a decaying radon atom) will, over its range of some centimeters, create approximately 150,000–200,000 ion pairs.

Although ionization from radioactive sources (often a polonium isotope) is used for technical purposes, and for certain applications it is to be preferred for any other method, the most common artificial method of producing ions is by field ionization.

It's somewhat ironic to realize that this method presupposes an ongoing, however weak, natural ionization. If a sufficiently strong electric field is established—for instance, between an electrode at a potential of some kilovolts and a ground—the electrons being freed by natural ionization may be accelerated to such velocities that they themselves can cause ionization, again creating pairs of (positive and negative) ions. It should be stressed that it does not take a high voltage, but high field strength, to cause ionization.

The breakdown field strength, as it is called, is somewhere around 3 MV/m between plane electrodes (in air at atmospheric pressure). If you have two metal plates at a distance of 1 cm, you need a voltage difference of about 30,000 V for ionization to take place in the space between the plates. If, however, one of the plates is replaced by a sharp metal point or a thin wire, the necessary voltage may be only a few kV. The explanation is that for a given voltage difference, the field strength in front of a point is much higher than between plane electrodes. Thus although the breakdown field strength is higher in front of a point, ionization is still established at lower voltages using sharp electrodes.

Now let's imagine an electrode, say a sharp metal point, kept at a positive potential of some kV with respect to ground, which may be represented by the walls of the room, as shown in Figure 2. In a small volume, perhaps a few cubic millimeters around the tip of the electrode, ion pairs are formed. The negative ions are attracted to the electrode, where they give off their charge and cease to exist as ions. The negative charge from the ions runs through the electrode to the voltage supply, making it look as though the electrode delivers a positive current to the air. The positive ions, formed in front of the electrode, are repelled by the electrode and move away. All in all, it appears that positive ions are emitted from the positive electrode.

Figure 2. Field inonization, caused by an electric field between an electrode and ground.

But this conclusion is completely wrong. The positive ions have never been in contact with the electrode. The electrode, often called an emitter, doesn't emit anything. Rather, it collects things (specifically, negative ions). Sadly, it's probably too late to change this linguistic malpractice.

Ions don't live forever. They may recombine with oppositely charged ions or, more likely, combine with aerosol particles in the air. The charged particles, sometimes called large ions, will also move in an electric field, although much more slowly than the air ions do.

This is the principle for the first technical electrostatic invention, the electro filter, without which we would have no means of effectively cleaning the smoke from coal- or oil-fired power plants and many other industrial installations.

Ions may also plate out onto surfaces, either by diffusion or aided by an electric field. And this is the basis for another important technical use of ionization.

Let's assume we have a charged insulator and we want to remove the charge.

Well, let's face it. It can't be done. There's no way by which a charge can be removed from an insulator. But don't panic. The charge in itself doesn't do any harm. It's the field from the charge we have to worry about. And the field may be used to neutralize itself.

If the charged insulator is exposed to an atmosphere containing ions of polarity opposite that of the charge, the field will attract ions, which will move toward the body and neutralize the charge. At least that's what appears to happen.

But a more strict formulation would be that the original (excess) charge is still there, and so is its field. The oppositely charged ions, attracted from the air, will deposit around the original charge, but not annihilate it. The resulting field, the sum of the fields from the opposite charges, will be zero, or at least very close to zero.

The use of air ionization for abating static electric effects is a slow method, compared to methods like the grounding of conductors or surface treatment with topical antistats. But it should be stressed that when we are talking about charged insulators, exposure to ionized air is the only method to remove the effects of the charge.

Soon after the discovery of atmospheric ions about a century ago, it was suggested that the ions might have an effect on people breathing the air containing the ions. Among the effects suggested was that air with an excess of negative ions would feel fresh, while an excess of positive ions would make the air stuffy.

This popular but still undemonstrated belief will be the subject of a subsequent column on static electricity and people.

Niels Jonassen, MS, DSc, retired from the Technical University of Denmark, where he conducted classes on static electricity. After retiring, he divided his time among the laboratory, his home, and Thailand, writing on static electricity topics and pursuing cooking classes. He passed away in 2006. .