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I. Formulas a) Chemical Formula: represents the type and number of element(s) in a chemical compound. • The type of element is represented by its chemical symbol. • The number of atoms of the element in the compound is represented by its subscript. NOTE: Li3PO4 Lithium = Li 3 atoms Phosphorus = P 1 atoms Oxygen = O 4 atoms Subscripts are the numbers found to the lower right of a given chemical symbol. If a subscript is not present assume that it is a number one. Types of Formulas 1. Molecular Formulas • Represents the type and number of elements in a covalent compound. • The subscripts do not need to be reduced in a molecular formula. Ex: C2H4 C6H12O6 Ethene Glucose 2. Empirical Formulas • Represents the lowest whole number ratio of atoms in a compound. • For ionic compounds, the formula is always empirical. This is due to the fact that one must reduce the subscripts in an ionic compound to the lowest whole number ratio. Ex: Calcium Sulfide Ca S Ca+2 S-2 Crisscross Method Ca2 S2 Disregard the signs. Ca S Reduce • For covalent compounds (molecules), reducing its molecular formula to an empirical formula only shows the ratio of elements within the compound. Ex: C6H12O6 C1H2O1 Ratio: C:H:O = 1:2:1 Molecular Formula Empirical Formula H2O Molecular and Empirical Formula 3. Structural Formulas • Structural formulas indicate the type, number, and arrangement of atoms within a compound. O H O=C=O H + [Na] [Cl] Glucose II. Formula Writing a) Ionic compounds: Given the name of the compound: 1. Determine if the compound is binary or ternary. 2. If the compound is binary, write out the symbols for the two elements present: metal then non-metal. * Elements with lower electronegativity values are usually listed first in a chemical formula. 3. Find and write out the oxidation number for the metal and non-metal. 4. Perform the crisscross method to determine the subscripts. * Eliminate the signs from the subscripts and if necessary reduce the subscripts to the lower whole number. 5. If the compound is ternary, refers to Reference Table E. 6. Write out the symbol for the metal and the polyatomic ion. 7. Place parenthesis around the polyatomic ion. 8. Write in the oxidation number for the metal and the ionic charge of the polyatomic ion. 9. Perform the crisscross method to determine the subscripts. * Eliminate the signs from the subscripts and if necessary reduce the subscripts to the lower whole number. Naming Ionic Compounds with a Transition Metals Elements that can have more than one possible charge MUST have a Roman Numeral to indicate the charge on the individual ion. Example #1 : Copper (II) Sulfate CuSO4 Copper’s oxidation number is +2. Sulfate’s charge is -2. The Roman Numeral “II” indicates the oxidation number of the transition metal. Example #2: Iron (III) Oxide Fe2O3 Iron’s oxidation number is +3. Oxygen’s oxidation number is -2. The Roman Numeral “III” indicates the oxidation number of the transition metal. Examples of Older Names of Cations formed from Transition Metals Complete the names of the following binary compounds with variable metal ions: FeBr2 iron (_____) bromide CuCl copper (_____) chloride SnO2 ___(_____ ) ______________ Fe2O3 ________________________ Hg2S ________________________ Complete the formulas of the following ternary compounds with variable metal ions: 1. copper(II) nitrate 2. Iron (III) hydroxide 3. Tin(IV) hydroxide b) Covalent Compounds (molecular substances) Prefix System (binary compounds) 1. Less electronegative non-metal comes first. 2. Add prefixes to indicate # of atoms. Omit mono- prefix on the FIRST element. Mono- is OPTIONAL on the SECOND element. 3. Change the ending of the second element to -ide. Greek Numerical Prefixes 1 mono 2 di 3 tri 4 tetra 5 penta 6 hexa 7 hepta 8 octa 9 nona 10 deca Given the names of the following molecules, determine their formulas. Dinitrogen monoxide Potassium sulfide Dichlorine heptoxide Iodine monochloride Given the formulas of the following molecules, determine their names. BaI2 P4S3 I2O5 CS2 B2Cl4