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Chemical Formulas Writing Binary Compounds Tim Bass Updated 11-22-01 (c) 2001 Tim Bass 1 Chemical Formulas A shorthand method of representing the makeup of chemicals using chemical symbols and oxidation numbers. Positive Ions always go 1st in the formula. Negative Ions always go last. Ionic Bonded chemical formulas are always written as empirical formulas. Empirical Formula: The smallest whole number ratio of elements in a compound. Updated 11-22-01 (c) 2001 Tim Bass 2 Golden Rule For Formulas: The sum of the oxidation #s = zero!! +2 + -2 = 0 Mg+2 Cl-1 2 +2 + -2 = 0 K+1 2 Updated 11-22-01 Sum of the oxidation numbers always equal zero!!!! O-2 (c) 2001 Tim Bass 3 Charged Particles Ion: a particle with an electrical charge. Cation: a positively charged ion. Anion: a negatively charged ion. Cation always goes 1st in the formula. Anion always goes last in the formula. Updated 11-22-01 (c) 2001 Tim Bass 4 Oxidation Number Oxidation Number: The apparent charge on an atom. The charge on an ion. Also called the valence. Example: Ca+2 Cl-1 Calcium: Chlorine: 20P 17p 20N 18N 18 e18eUpdated 11-22-01 (c) 2001 Tim Bass 5 Noble Gas The greater the stability the less active a substance becomes. The noble gasses are the least active substances (therefore are very stable and low energy). When the outer shells of an element are full the atom is at a much lower energy level. Updated 11-22-01 (c) 2001 Tim Bass 6 Shorthand Notation 10Ne: 18Ar: 36Kr: 1s2 2s2 2p6 • (8 valence electrons) 1s2 2s2 2p6 3s2 3p6 • (8 valence electrons) 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 • (8 valence electrons) All noble gases (except He) have 8 valence electrons. Updated 11-22-01 (c) 2001 Tim Bass 7 Noble Gas (Octet) Law Noble Gas Law: When forming compounds, elements tend to gain or lose electrons until they have an electron configuration similar to one of the noble gases. Metals tend to lose electrons (become cations) Nonmetals tend to gain electrons (become anions) Updated 11-22-01 (c) 2001 Tim Bass 8 Metals and Nonmetals Nonmetals Metals Updated 11-22-01 (c) 2001 Tim Bass 9 Oxidation Examples Na + Cl - Na+1Cl-1 +2+2 -1 -1 Mg ClCl Mg++ Cl 2ClMg Mg 2 Al + + 3Cl Al Al+3+3Cl Cl-13-1 Al Cl Updated 11-22-01 (c) 2001 Tim Bass 10 Oxidation Numbers The sum of the oxidation numbers in a chemical formula must equal zero. This is the golden rule of writing formulas for inorganic compounds! There is only one category that does not follow this rule. They follow the rule also, but the rules change a little to apply to them. Nonmetals bonded to nonmetals . Updated 11-22-01 (c) 2001 Tim Bass 11 Updated 11-22-01 (c) 2001 Tim Bass 12 Subscripts Subscript: Small numbers to the lower right of a symbol. Represent the number of atoms of that element in the compound. Affects only the element immediately in front of the subscript. Coefficient: Large number in front of a chemical formula. Affects everything following the coefficient. Updated 11-22-01 (c) 2001 Tim Bass 13 H2O Hydrogen: Oxidation # X Subscript +1 X 2 = +2 Oxygen: Oxidation # X Subscript -2 X 1 = -2 Sum of oxidation numbers = 0 (+2) + (-2) = 0 Updated 11-22-01 (c) 2001 Tim Bass 14 A Shortcut Make the oxidation of one element the subscript of the other element. Updated 11-22-01 (c) 2001 Tim Bass 15 A Shortcut There is one problem! +2 -2 CaO C a2 O2 These subscripts will reduce. Remember: Ionic bonded compounds are always written as an empirical formula. Updated 11-22-01 (c) 2001 Tim Bass 16 Polyatomic Ion An ion made up of more than one atom that behaves like one atom. Updated 11-22-01 (c) 2001 Tim Bass 17 Polyatomic Ion Never change subscripts within a polyatomic ion. (Changes the polyatomic ion.) If more than one polyatomic ion is needed, put parenthesis around the ion and add the subscript needed outside the parenthesis. NH4OH (NH4)2O Al(MnO4)3 Updated 11-22-01 Ammonium Hydroxide Ammonium Oxide Aluminum Permanganate (c) 2001 Tim Bass 18 Polyatomic Ion The sum of the oxidation numbers in an ion is equal to the charge on the ion. You must learn the names, formulas, and charges of the most common ions. MnO4-1 OH-1 CN-1 NH4+1 Updated 11-22-01 Permanganate Hydroxide Cyanide Ammonium (c) 2001 Tim Bass 19 Warnings 1. Once the sum of the oxidation #s = 0, do not change the formula. 2. Parenthesis are only for polyatomic ions, and only if they are needed two times or more in a chemical formula. 3. Always make sure that the final chemical formula is an empirical formula! Updated 11-22-01 (c) 2001 Tim Bass 20 Chemical Formulas End Updated 11-22-01 (c) 2001 Tim Bass 21