Ionization is the process in which one or more electrons are removed from an atom or molecule. The charged particle that results is called an ion. As an example, consider an atom of oxygen. An oxygen atom consists of a nucleus containing eight protons and eight neutrons. Each proton carries a single positive electrical charge, and each neutron is electrically neutral. The oxygen atom also contains eight electrons, each carrying a single negative electrical charge. With eight positive charges and eight negative charges, an oxygen atom is neutral.
Some of the electrons in an atom can be removed rather easily. If an oxygen atom should lose one electron, for example, it would then have eight positive charges and only seven negative charges. Overall the atom would have an electric charge of +1. If two electrons were to be removed, it would have a charge of +2.
Under some circumstances, the oxygen atom could also gain electrons. If it gained one electron, it would then have eight positive charges and nine negative charges, or an overall charge of −1. The charged atoms of oxygen in all of the above cases are no longer called atoms. They are called ions. If they carry a positive charge, they are known as cations, and if they carry a negative charge they are known as anions.
Molecules can also be ionized. Molecules are collections of atoms held together by shared pairs of electrons. If an electron in a molecule is removed, the portion of the molecule that remains becomes charged, just as is the case with atoms. For example, if a molecule of nitrogen loses an electron, it becomes a cation with a charge of +1.
Electrons in an atom are attracted to the atomic nucleus by electrical forces. An electron is negatively charged; the nucleus is positively charged. Since opposite charges attract each other, an electron tends to stay with its atomic nucleus.
In order to remove an electron from an atom, then, energy must be provided to overcome the force of attraction of the nucleus. That energy is called ionization energy.
The energy needed to remove an electron differs from atom to atom. Consider the difference between hydrogen and sodium. Hydrogen has only one electron, located fairly close to its nucleus. A good deal of energy is needed to overcome the attraction of the hydrogen nucleus for its electron. Sodium has 11 electrons, one of which is at a relatively great distance from the nucleus. The force of attraction by the nucleus for that outermost electron is small, compared to the force in a hydrogen atom. The outermost sodium electron can be removed with a relatively small amount of energy.
This comparison can be confirmed by looking at the first ionization energy for both hydrogen and sodium. The first ionization energy is the amount of energy required to remove the first electron from an atom. For hydrogen, that number is 1,312 kilojoules per mole, and for sodium it is 495.9 kilojoules per mole. (A mole is a unit used to represent a certain number of particles, usually atoms or molecules.)
Similar measurements can be made for removing the second electron, third electron, fourth electron, and so on, from an atom. These measurements are known as the second ionization energy, third ionization energy, fourth ionization energy, and so on.
Crystal: A solid composed of positively and negatively charged ions.
Dissociation: The process by which ions are set free from a crystal, usually in water solution.
Ion: A molecule or atom that has lost one or more electrons and is, therefore, electrically charged.
Ionization energy: The amount of energy required to completely remove an electron from an atom or molecule, thereby creating a positively charged ion.
Molecule: The smallest particle of which a compound consists, made of two or more atoms bonded to each other by shared pairs of electrons.
Ionization in solution. The term ionization also has a second meaning when used in discussions of solutions. The way substances behave electrically in water solution is often very different from the way they behave as solids or gases. As an example, consider the compound known as acetic acid. Acetic acid is a liquid that does not conduct an electric current. Yet, when acetic acid is added to water, the solution that is formed does conduct an electric current.
In order for a solution to conduct an electric current, ions must be present. Pure acetic acid is made of molecules. It contains no ions. If you pass an electric current into acetic acid, nothing will happen because no ions are present. An important change takes place, however, when acetic acid is added to water. Water molecules have the ability to tear acetic acid molecules apart, breaking them down into hydrogen ions and acetate ions. Now that ions are present, the water solution of acetic acid can conduct an electric current. This process is known as ionization because ions are produced from a substance (acetic acid) that did not contain them originally.
Dissociation. A similar story about the conductivity and nonconductivity of sodium chloride could be told. If the two ends of a battery are attached to a large crystal of sodium chloride, no electric current will flow. One might guess that this result indicates that no ions are present in sodium chloride. However, that is not the case.
Indeed, a crystal of sodium chloride is made up entirely of ions, positively charged sodium ions and negatively charged chloride ions. The problem is, however, that these ions are held together very tightly by electrical forces. Sodium ions are bound tightly to chloride ions, and vice versa.
The situation changes, however, when sodium chloride is added to water. Water molecules are able to tear apart sodium ions and chloride ions in much the same way they tear apart acetic acid molecules. Once the sodium ions and chloride ions are no longer bound tightly to each other, they are free to roam through the salt/water solution.
The name given to this change is dissociation. The term means that ions already existed in the sodium chloride crystal before it was put into water. Water did not create the ions, it only set them free. It is this difference between creating ions and setting them free that distinguishes ionization from dissociation.