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.
Obj 3 State the name, formula and charge of common polyatomic ions.
A polyatomic ion is a tightly (covalently) bound group of atoms that behave as a unit and have an overall charge. The polyatomic ions you need to know include:
ammoniumNH4+
acetateCH3COO- or C2H3O2-
chlorateClO3-
hydroxideOH-
nitrateNO3-
permanganateMnO4-
carbonateCO32-
chromateCrO42-
sulfateSO42-
phosphatePO43-
Obj 4 Name an ionic compound when given the formula of the compound
Ionic compounds are composed of positive and negative ions, and are thus usually composed of metalic cations and nonmetalic anions. When a metal and a nonmetal react, electrons are transferred from the metal to the nonmetal. The metal will form a positive ion (cation) and the non-metal will form a negative ion (anion). Polyatomic ions are also common components. Oppositely charged particles will attract one another to form an ionic compound in a proportion that will result in an overall neutral charge. The ions are arranged in orderly three-dimensional patterns making most ionic compounds crystalline solids. A salt crystal (NaCl), for example, is not made of just one sodium ion and one chloride ion but is a matrix of sodium and chloride ions in a one to one ratio giving it a neutral charge and a cubic shape.
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!
Obj 5 name a binary molecular compound when given the formula of a compound
Molecular compounds are usually composed of two or more nonmetals which share electrons to form what are called covalent bonds, which are very strong. Ions are not formed. Many molecular compounds exist as gases or liquids at room temperature. Water and carbon dioxide are common examples. Unlike ionic compounds, molecules are made of exact numbers of atoms. For example a molecule of water is made of two hydrogen atoms and one oxygen atom. Some molecules are soluble in water (sugar) but many are not (oil). Because ions are not present, molecular solutions do not conduct electricity even when dissolved or melted.
Chemical formulas of molecular compounds. Molecular compounds are represented in chemical formulas as molecular formulas which show the number and kinds of atoms present in a molecule. A hydrogen peroxide molecule is made of 2 hydrogen atoms and 2 oxygen atoms. It's molecular formula is H2O2. (Note: molecular formulas represent the actual number of atoms in a particle and are not reduced to the lowest ratio, as are ionic formula units.) Glucose, another molecule, has the formula C6H12O6.
Naming molecular compounds. The following prefixes are used to indicate the number of atoms of each type in a molecular compound: mono- (for 1), di- (for 2), tri- (for 3), tetra- (for 4), penta- (for 5), hexa- (for 6), hepta- (for 7), octa- (for 8), nona- (for 9) and deca- (for 10). The atoms are listed in the order they occur in the molecular formula. Note:
- change the ending of the last element of binary molecular compounds to –ide (like you do for binary ionic compounds);
- if the first element listed is represented by only one atom then do not use a prefix (example CO2 = carbon dioxide, not monocarbon dioxide);
- if the element begins with a vowel, the vowel at the end of the prefix is often dropped (monoxide, not monooxide).