Unit 7: Formulas and Equations NaCl Aim: Writing a chemical formula using the compound neutrality rule. Bonding and Stability All things in nature contain energy and prefer to be at a lower energy state which results in a more stable substance. Atoms of elements will form chemical bonds to achieve stability. This can be done by 2 atoms sharing electrons or 2 atoms forming ions from a transfer of electrons. 1
1. What are the two types of ions that elements can form? *show transfer and charges* 1. What are the two types of ions that elements can form? cation Ca anion O Ca +2 O 2 Ca +2 O 2 formula= CaO #e lost= #e gained *show transfer and charges* 2
Ionic Compound: a compound formed from two or more ions attracting together after transferring electrons. Ex: MgI 2 Compound Neutrality Rule: the overall positive charge from the cations must equal the overall negative charge from the anions. Binary compound a metal and a nonmetal. Ex: NaCl 2 different types of atoms Classes of Compounds Ternary compound a metal and a polyatomic ion or 2 polyatomic ions. Ex: KNO 3 3 or more different types of atoms. Polyatomic Ion: two or more atoms bonded to form a large ion with an overall charge. Ex: SO 4 2 Jan 23 12:44 PM 3
A. Cesium oxide B.Potassium phosphate Writing Formulas 1. Write the ions and their charges. For nonmetal ions, always choose the top oxidation number (gives the stable octet). 2. Determine how many of each ion you need to balance the overall charge of the compound. 3. To write the formula, indicate how many of each ion you have with a SUBSCRIPT to the bottom right 4. If a polyatomic ion is present it ends in ate or ite and is on Table E. (exceptions: hydroxide, peroxide, cyanide, ammonium.) C. Iron (II) phosphate 5. If a roman numeral is written in the name after the metal ion, it tells you which charge the ion has. 6. When more than one polyatomic ion is present, parentheses must be used around the symbol. Writing Formulas D. Barium hydroxide E. Sodium sulfide 1. Write the ions and their charges. For nonmetal ions, always choose the top oxidation number (gives the stable octet). 2. Determine how many of each ion you need to balance the overall charge of the compound. 3. To write the formula, indicate how many of each ion you have with a SUBSCRIPT to the bottom right F. Chromium (VI) chloride4. If a polyatomic ion is present it ends in ate or ite and is on Table E. (exceptions: hydroxide, peroxide, cyanide, ammonium.) G. Aluminum sulfate H. Aluminum oxide 5. If a roman numeral is written in the name after the metal ion, it tells you which charge the ion has. 6. When more than one polyatomic ion is present, parentheses must be used around the symbol. 4
D. Barium hydroxide Formula Writing Practice E. Sodium sulfide F. Chromium (VI) chloride G. Aluminum sulfate H. Aluminum oxide Jan 11 8:24 AM D) Barium hydroxide Ba(OH) 2 Ba +2 OH - OH - +2 + (-2) = O E) Sodium sulfide Na 2 S Na + S -2 Na + +2 + (-2) = O G) Aluminum sulfate Al 2 (SO 4 ) 3 Al +3-2 -2 SO 4 SO 4 Al +3 SO 4-2 +6 + (-6) = O H) Aluminum oxide Al 2 O 3 Al +3 O -2 O -2 Al +3 O -2 +6 + (-6) = O F) Chromium (VI) chloride CrCl 6 Cr +6 Cl - Cl - Cl - Cl - Cl - Cl - +6 + (-6) = O Jan 11 8:24 AM 5
To write a formula: 1. Write the symbols and oxidation states. a. the first ion named is always a metal or polyatomic ion (Table E) b. the second ion named is in the P.T. or Table E (polyatomic ion). ends in "ide", look on the periodic table and find what element it sounds like, oxide= oxygen. Exceptions= hydroxide, cyanide and peroxide are on Table E. ends in "ate" or "ite" look on Table E. c. A roman numeral in the name tells you the oxidation state of the metal ion with more than one possible charge. A) Aluminum sulfide B) Iron (II) Phosphate Al +3 S 2 Al +3 S 2 Al 2 S 3 S 2 2 x 3= 6 you need a net positive and negative charge of 6 for the compound neutrality rule. Write the formula using subscripts to represent the number of each ion in the compound. Fe +2 3 PO 4 Fe +2 3 PO 4 Fe +2 Fe 3 (PO 4 ) 2 Make sure the number of cations and anions balance out so the total charge is zero. You want the lowest number of each ion possible. Multiple polyatomic ions in a formula but be represented in parentheses. Fe +2 +3 2 x 3= 6 you need a net positive and negative charge of 6 for the compound neutrality rule. 6