Ions are electrically-charged atoms or groups of atoms. Atoms are made of protons, neutrons, and electrons. Protons have an electrical charge of +1 and a mass of one atomic mass unit. Neutrons have no charge and also have a mass of one atomic mass unit. The protons and neutrons are located in the dense center of the atom, called the nucleus. An element is identified by the number of protons that it has in its nucleus.
Electrons have an electrical charge of -1 and a mass of about 1/2000 of an atomic mass unit. In an electrically neutral atom, the number of electrons equals the number of protons. Because of their extremely low mass, electrons behave more like waves than particles. The electrons are located in different energy levels surrounding the nucleus. Each higher energy level is further away from the nucleus. Each energy level has a limit to the number of electrons in it, so when one energy level is filled, the next electron goes to the next higher energy level. The electrons in the highest energy level are the most important and are called the valence electrons.
To form ions, it is the electrons in the highest energy level that are gained or lost. When electrons are gained, the ion has a negative charge and is called an anion. When electrons are lost, the ion has a positive charge and is called a cation. This may seem backwards in terms of losing something becoming more positive and gaining something becoming more negative, but if you remember that electrons have a negative charge, what is being added or lost have a negative value. Generally, all metals (the leftmost two-thirds of the periodic table) lose electrons to form cations, whereas all non-metals gain electrons to form anions.
The number of electrons that are gained or lost depends on where the element is on the periodic table. The most stable elements are those where an energy level is complete. These are the noble gases, Group 18 (or VIIIA), found on the extreme right side of the periodic table. Because they are stable, these elements do not form ions, nor do they normally react with any other elements. The other elements gain or lose enough electrons to have an electron configuration like the noble gases. (Alternatively, they may share the outermost electrons to be like the noble gases.)
Thus, elements in Group 1 (or IA), such as sodium, Na, have only one valence electron, which is easily lost to form an ion with a +1 charge. Elements of Group 2 (or IIA), such as calcium, have two valence electrons which are lost to form cations with a +2 charge. Transition elements (Groups 3-12, or IB-VIIIB) usually only lose one, two, or rarely three electrons. Nonmetals in Group 17 (VIIA), such as chlorine, Cl, gain one electron to have an ion with a -1charge. The name of the anion formed from chlorine is called choride ion. Group 16 (VIA) elements, such as oxygen, gain two electrons for a -2 ion. All of these ions have electron configurations like the elements of Group 18 (VIIIA). However, since the number of protons does not equal the number of electrons, there is an imbalance in electrical charge.
Ionic compounds, or salts, have cations and anions in a ratio such that the compound has no charge. For example, sodium ion has a +1 charge and chloride ion has a -1 charge. Thus, a 1:1 ratio results in a compound with no overall charge: NaCl is the resulting formula. For calcium chloride, calcium ion has a +2 charge and chloride ion has a -1 charge (as it always does). Thus a 1:2 ratio gives a compound with no overall charge: CaCl2is the formula.
Most ionic compounds are crystalline and can dissolve in water. Water is a polar molecule. A polar molecule is one that has a partial plus charge on one side, and a partial minus charge on the other side. The water molecules align themselves so that the negative side attracts the positive ion of the ionic compound, and the positive side attracts the negative ion of the compound. The result is that water can separate and surround the ions. These ionic solutions are called electrolytes, since they can conduct electricity.