Ions are charged particles formed by the transfer of electrons from one element or combination of elements to another element or combination of elements. When atoms of reactive metals such as the elemtns in Group IA (group 1) and the elements toward the bottom of Group IIA (group 2) in the periodic table combine with atoms of nonmetals to form compounds, enough electrons are transferred to give each atom the same number of electrons as the noble gas with the closest atomic number. Because the total negative charge of the electrons in an atom equals the total positive charge on the nucleus, the loss of electrons leaves an ion with a net positive charge. For example, consider the formation of a cacium ion from a calcium atom. The atomic number of calcium is 20, and a calcium atom has 20 electrons outside its nucleus.
The noble gas that has an atomic number closest to 20, the atomic number of calcium, is argon, with atomic number 18. Therefore, to get the same number of electrons as an argon atom, a calcium atom must lose two electrons. Because atoms are electrically neutral, loss of two electrons leaves a calcium ion with a 2+ charge; the charge of an ion is written as a right superscript:
Ca → Ca2+
Notice that in the formula for an ion, the charge is written with the number on the left followed by the sign of the charge. Positively charged ions, such as Ca2+, are called cations. Metals usually form cations.
Nonmetals usually gain electrons when they form ions. Consider the fluorine atom, atomic number 9. The noble gas that has an atomic number closest to 9 is neon, with atomic number 10. To get the same number of electrons as a neon atom, a fluorine atom must gain one electron. Because atoms are electrically neutral, a gain of one electron gives a fluoride ion a 1− charge. The number 1 is usually not written in the superscript:
F → F−
Negatively charged ions such as F− are called anions. Nonmetals usually form anions.
Although a fluoride ion has the same number of electrons as a neon atom, a fluoride ion is different from a neon atom. Fluorine has an atomic number of 9, and a fluoride ion has nine protons in its nucleus; the atomic number of neon is 10, and a neon atom has ten protons in its nucleus. The number of protons in the nucleus determines the identity of an element. In addition, a fluoride ion has a 1− charge; a neon atom has no net charge.
Regarding compounds of reactive metals with nonmetals, there are no molecules. The unuits of the compound are positive and negative ions. Compounds composed of ions are called ionic compounds. Compounds do not have net electric charges—that is, compounds are electrically neutral. Because compounds do not have net electric charges, in compounds, ions must be combined in such a way that the sum of the positive charges equals the sum of the negative charges. Charges must cancel so that the net charge is zero. For example, the compound sodium chloride, which is composed of sodium ions, Na+, and chloride ions, Cl−, must have one Na+ for each Cl−. The formula for the compound sodium chloride is NaCl. Note that cations are always written to the left of anions in formulas and named first. The name of a cation is the same as the name of the element from which the cation is formed. The ending of the name of the element from which a monatomic anion is formed is changed to -ide.
Sodium ions are a little smaller than sodium atoms; chloride ions are about twice as large as chlorine atoms. Both sodium ions and chloride ions, however, are so small that a grain of salt contains three billion billion sodium ions and an equal number of chloride ions.
Each sodium ion of sodium chloride is surrounded by six chloride ions, and each chloride ion is surrounded by six sodium ions. The ions are held in place by the attraction between positive and negative charges, which is referred to as an electrostatic force. Because the attraction between positive and negative charges is strong, solids that are composed of ions such as sodium chloride are hard and have high melting points. The formula for sodium chloride, NaCl, gives the simplest ratio of sodium ions and chloride ions, that is, one sodium ion for each chloride ion.
An ion, pronounced EYE uhn or pronounced EYE ahn, is an atom or a group of atoms that has an electric charge. Atoms and molecules become charged if they gain or lose electrons. Each atom has a cloud of negatively charged electrons around a small, heavy nucleus. The nucleus contains positively charged protons. If the number of electrons around the nucleus equals the number of protons inside the nucleus, the atom is neutral. The process of removing electrons from atoms or molecules to produce positive ions is called ionization. The electrons removed may then join other atoms or groups of atoms, causing them to become negative ions. The amount of electric charge an ion has is determined by the number of electrons gained or lost by the atom or molecule.
Many common substances contain ions. For example, table salt consists of equal numbers of positively charged sodium ions and negatively charged chloride ions. In forming table salt from the elements sodium and chlorine, each sodium atom loses an electron and becomes a positive sodium ion. Likewise, each chlorine atom gains an electron and becomes a negative chloride ion. Seawater contains many kinds of ions. The most common ones, in order of their amount, are chloride, sodium, sulfate, and magnesium. The earth's atmosphere also contains ions. They are concentrated in a layer called the ionosphere.
Ions in Solids usually fit together in a regular, repeating, three-dimensional arrangement. Such a substance is called an ionic crystal. For example, sodium ions alternate with chloride ions in a crystal of table salt. Ions in an ionic crystal are held together by electrostatic attraction, the attraction between opposite charges.
Ions in Liquids can migrate throughout the liquid. In a solution, each ion attracts one or more molecules of the solvent (dissolving liquid). Ionic crystals, such as sodium chloride, usually dissolve only in solvents that contain polar molecules. Polar molecules have a positive end and a negative end. Each ion on the surface of the crystal attracts the oppositely charged end of polar molecules. This attraction weakens the attraction between ions in the crystal. The ions then break away from the crystal and enter the solution, combined with polar molecules. Ions combined with polar molecules of the solvent are said to be solvated.
Ions in Gases are too far apart at normal pressures to attract each other strongly. As a result, single ions in a gas may drift for a long time before they combine with other ions. A mixture consisting of ionized gas and electrons is called a plasma.
Many particles in space are ions. Some of these ions are trapped by the earth's magnetic field and make up part of the Van Allen belt.
Polyatomic Ions
Besides simple monatomic ions (formed from one atom) such as Na+ and Cl−, a number of polyatomic ions also exist. A polyatomic ion is a charged particle formed from more than one atom.
Behavior of Ions
All ionic solids and liquids, and most ionized gases, are electrically neutral. The total charge of all their positive ions equals the total charge of all their negative ions. This general rule also applies to all other kinds of matter, and is called the principle of electroneutrality.
Like neutral atoms and molecules, ions in a liquid or a gas are constantly moving. Each one changes its direction of motion billions of times each second because of collisions and the forces exerted on it by other particles. After each change in direction, an ion usually is no more likely to be moving in one direction than in any other. Such random motion is called Brownian motion. When two oppositely charged electrodes are placed in a liquid or gas, each ion loses part of its random motion and starts to drift toward one of the electrodes. Negative ions drift toward the anode (negative electrode) and are called anions. Positive ions drift toward the cathode (positive electrode) and are called cations. The movement of the charges carried by the drifting ions makes up an electric current.
The ability of a solution to conduct electric current depends on the concentration of ions in the solution. For example, drinking water drawn from a typical municipal treatment plant in the United States contains few ions and therefore is a poor conductor of current. But seawater, with significant amounts of dissolved sodium chloride, magnesium sulfate, and other ionic compounds, is a good conductor.
Producing Ions
Any process that can add or remove electrons from an atom or a molecule can produce ions. Radiation and chemical reactions are such processes. Radiation can increase the energy of the electrons in an atom or a group of atoms. If this energy is increased enough, one or more electrons can overcome the attraction of the nucleus and escape from the atom. The atom then becomes a positive ion. Radiation that can produce ions includes light, X rays, gamma rays, atomic nuclei, subnuclear particles, and electrons.
High-energy radiation absorbed by plant or animal tissues produces unnatural ions in the tissues. These ions become involved in potentially harmful chemical reactions. In human beings and other animals, the symptoms of these reactions are called radiation sickness. The amount of radiation absorbed by tissues is measured in units called roentgens or radiation units (rads). One roentgen or rad corresponds to the formation of about 2 billion pairs of ions in the tissues. Death is likely to occur if a person's body absorbs 500 roentgens of radiation over a short period.
Ions are formed in a chemical reaction if molecules split into electrically charged parts. For example, molecules of hydrogen chloride (HCl) split when added to water. They form positive hydrogen ions and negative chloride ions.