Essential Concepts: Chemical Names and Formulas

Obj 1 Name ions.

If an atom loses or gains electrons, it will no longer be neutral and can be called an ion (a charged particle).

Cations are atoms or groups of atoms with positive charges. In the presence of an atom with a high attraction for electrons (a nonmetal), a metal will tend to lose electrons and form a cation (a positively charged ion). For example, when it reacts with a nonmetal, magnesium will lose 2 electrons and then have a +2 charge. The symbol for the magnesium cation is Mg2+. It is called a "magnesium ion."

Anions are atoms or groups of atoms with negative charges. In the presence of an atom with "loose" electrons (a metal), a nonmetal will pick up additional electrons to form an anion (a negativly charged ion). The ending of the names of atoms changes when they become anions. The ending -ide replaces the ending of the uncharged atom's name. When oxygen becomes an anion it is called an "oxide ion."

Polyatomic ions are groups of atoms strongly bonded that act as a unit (some are cations and some are anions). The names, charges, and formulas of ten common polyatomic ions have been given in prior units and should be memorized. The polyatomic ions you need to know include:

ammonium NH4+

acetate C2H3O2-

chlorate ClO3-

hydroxide OH-

nitrate NO3-

permanganate MnO4-

carbonate CO32-

chromate CrO42-

sulfate SO42-

phosphate PO43-

Obj 3 Name an ionic compound when given the formula of the compound

Chemical formulas of ionic compounds. Ionic compounds are represented in chemical formulas as formula units: the lowest whole number ratio of ions in an electrically balanced ionic compound. Thus although sodium chloride is made of many sodium and chloride ions, it is represented by the formula unit NaCl, indicating a one to one ratio of ions. A compound of magnesium and chloride ions would have the formula MgCl2, indicating that for the compound to be neutral two chloride ions are present for every magnesium ion. Note the positive ion is always written first in a formula unit.

Naming ionic compounds. To name an ionic compound, list the positive ion first, then the negative ion. If the compound is made of only two elements (termed binary) then change the ending of the negative ion to –ide. (Don't change the endings of polyatomic ions.) MgCl2 = magnesium chloride, Li2S = lithium sulfide; NaNO3 = sodium nitrate.

For elements that can form more than one type of ion (mostly the transition metals) you must indicate which ion is represented. The "stock" method of naming ionic compounds that contain an element that can form more than one ion uses Roman numerals to indicate the charge of the ion in the compound. For instance, copper can form ions with either a +1 or +2 charge. The name of the compound consisting of the +1 form of copper, as in Cu2O, would be copper(I) oxide; the name of the +2 form, as in CuO, would be copper(II) oxide. By doing this, the names are unambiguous and the correct formula can be derived from the name. Note: the Roman numerals are only used in the compound's name, never in its formula!