Chapter 6: Chemical Names and Formulas

Molecules and Molecular Compounds Even though the atom is the smallest representative sample of an element, only the noble gas elements are normally found in nature as isolated atoms. All others form either molecules or ions. A molecule is an assembly of two or more atoms tightly bound together by a covalent bond created by two atoms sharing electrons. Diatomic atoms form diatomic molecules (remember 7… start at 7 form a 7 and hydrogen). Compounds that are composed of molecules that contain more than one type of element are molecular compounds. Most molecules are composed of nonmetals. Chemical formulas that indicate actual number and types of atoms in a molecules are called molecular formulas. Such as H 2 O, C 6 H 12 O 6, and C 2 H 4. Empirical formulas give only the relative number of atoms, they are basically the reduced formula. Such as H 2 O, CH 2 O, and CH 2.

Picturing Molecules The molecular formula of a substance describes the composition but doesn’t show how they come together. Structural formula: shows which atoms are attached to which. –Atoms are represented by their symbol and the bonds are represented by lines. Perspective Drawing: shows actual geometry to give some sense of three-dimensional shape. Ball-and-stick Models: shows atoms a spheres and bond as sticks. Accurately represents the angles at which the atoms are attached to one another within the molecules. Space-filling Model: shows what the molecule would look like if the atoms were scaled up to size.

Ions Some atoms can gain or lose electrons to try and get the same number of electrons as the nearest noble gas, when an electron is gained or lost from a neutral atom a charged particle occurs called an ion. An ion with a positive charge (lost an electron) is called a cation, where as an ion with a negative charge (gained an electron) is called an anion. In general, metals atoms tend to lose electrons to form cations and nonmetals tend to gain electrons to form anions. In addition to simple single atom ions, there are polyatomic ions, which consist of atoms joined as a molecule but they have a net positive or negative charge. Ionic charge can be predicted by determining how many electrons an atom has to lose to become like the nearest stably arranged noble gas. Do Practice Problems 16 and 17 on pg 145.

Ionic Compounds When a positive ion such as Na comes close to a negative ion such as Cl, their opposite charges are attracted and form an ionic compound connected by a ionic bond. Generally, they are combinations of metals and nonmetals such as Na and Cl. Ions in ionic compounds are arranged in three- dimensional structures. The formula for an ionic compound is always an empirical formula (most reduced form) because there is no discrete molecule of NaCl. Chemical compounds are always electrically neutral, so the empirical formula shows the ratio of the ions for this to be true. For example, Mg 2+ and N 3- would have to be Mg 3 N 2. Do Practice Problems #24, 25 on pg 151.

Naming Inorganic Compounds To obtain information about a particular substance you must know its chemical name and formula, the system used for this is chemical nomenclature. Some compounds also have common names in addition to their chemical nomenclature such as water. The rules for naming a compound is based on divisions of substances into categories. The major division is between inorganic and organic. Among the inorganic compounds the three basic divisions are ionic compounds, molecular compounds and acids. Aluminum chloride Sodium Nitrate Lithium Fluoride Potassium Oxide

Naming Positive Ions (Cations) Cations formed from metals atoms have the same name as the metal found on the periodic table. These are monatomic ions. –Mg 2+  Magnesium ion and K +  Potassium ion If a metal can form different cations, the positive charge is indicated by a roman numeral in parentheses following the name of the metal. These are usually transition metals. –Cu 2+  Copper (II) ion and Cu +  Copper (I) ion Cations formed from nonmetal atoms have name that end in –ium. These are polyatomic ions. –NH 4 -  Ammonium ion and H 3 O +  Hydronium ion

Naming Negative Ions (Anions) The names of monatomic anions are formed by replacing the ending of the name of the element with –ide. –O 2-  oxide ion and N 3-  nitride ion Polyatomic anions containing oxygen have names ending in -ate or -ite. –NO 3-  nitrate ion and NO 2-  nitrite ion Anions derived by adding H+ to an oxyanion are name by adding hydrogen or dihydrogen as a prefix as appropriate. –HCO 3 -  hydrogen carbonate ion and H2PO 4 -  dihydrogen phosphate ion

Naming Ionic Compounds Names of ionic compounds consist of the cation name followed by the anion name. Do Practice Problems # on pgs

Naming Acids You know a molecule is an acid because its cation is hydrogen. Acids containing anions whose names end in -ide are named by changing the -ide ending to -ic adding the prefix hydro- to this anion name and then following with the word acid. –HCl  Hydrochloric Acid Acids containing anions whose name end in -ate or -ite are named by changing the -ate to -ic and -ite to -ous and then adding the word acid. –HNO 3  Nitric Acid & HClO 2  Chlorous Acid Do Practice Problem #40 on pg 160.

Naming Binary Molecular Compounds The name of the element farther to the left in the periodic table is usually written first. Except Oxygen is written last with all except Flourine. If both elements are in the same group in the periodic table, the one having the higher atomic number is named first. The name of the second element is given and -ide ending. Greek prefixes are used to indicate the number of atoms of each element. Although mono is never used with the first element. –mono, di, tri, tetra, penta, hexa, hepta, octa, nona, deca N 2 O 5  Dinitrogen pentoxide Do Practice Problems # 37, 38 on pg 159.