Chapter 2A Chemical Formulas

Chemical Symbols Also called Chemical Formula Universal shorthand Each element has its unique symbol Symbol based on name: English Latin When elements are uncombined, most are written as monatomic elements No subscripts! H2 No little 2 7 are Diatomic Molecules

Diatomic Molecules 2 identical elements combined in a covalent bond together NEVER found by themselves in nature They are SO reactive that they are in a compound or a diatomic molecule 7 Elements in the shape of a SEVEN MUST always be written diatomically when written by themselves H2, N2, O2, F2, Cl2, Br2, I2 That means that it is the chemical symbol followed by a subscript 2

Chemical Formulas Use a combination of chemical symbols and numbers to show both quantitative and qualitative information Quantitative = Names the elements (symbols) Qualitative = Number of atoms of each element (subscript) Compounds are composed of combinations of elements CHEMICALLY COMBINED in definite proportions by weight. H2O means for every ONE oxygen there are TWO hydrogen

Examples: Which elements are there and how many of each? NaCl H2O H2SO4 N2O4 P4O10

Types of Formulas There are 3 types of formulas: 1. Molecular Formula Empirical Structural (We will talk about this one in a later chapter) 1. Molecular Formula Tells you the actual number of atoms of an element in a compound H2O2 2. Empirical Formula Smallest whole-number-ratio of atoms or ions in a compound HO 3. Structural Formula Indicates how the atoms are bonded to each other We will learn how to draw these in a later chapter

Examples: Change these molecular formulas into empirical formulas H2O2 C3H6 C6H12O6 P4O10 N2O4

Atoms vs. Ions: Review Atoms Ions Electrically neutral (o) because it has EQUAL numbers of protons (+) and electrons (-) Ions Not neutral: either positively or negatively charged If it loses electrons, it will have a (+ / -) charge and be (larger/smaller). If it gains electrons, it will have a (+ / -) charge and be (larger/smaller). Do you ever gain protons?

Polyatomic Ions Table E Group of atoms covalently bonded together, possessing a charge These ions ALWAYS stay as one unit and CANNOT be broken up So, if there is a polyatomic ion in a compound, it will stay as a polyatomic ion and should be treated as ONE entity When in a compound, there will be parentheses around the ion, unless there is only one ion (NH4)3PO4 To find the total number of each element, you would distribute the subscript (like you would in math) N = 1 x 3 = 3 Nitrogens H = 4 x 3 = 12 Hydrogens P = 1 Phosphorus O = 4 Oxygens

Examples: Identify the Polyatomic ion. How many of each element? KClO3 H2O2 H2SO4 KCr2O7 Pb3(PO4) 2 Al(OH) 3 (H3O) 2S B2(S2O3) 3

Coefficients The number in FRONT of a formula The number of units of the formula that are present Multiply the ENTIRE formula by that number to find the total number of each atom present 2NaCl There are 2 salt molecules or 2 Sodium and 2 Chlorine

Subscripts vs. Coefficients Written to the lower right of a chemical symbol Written smaller than chemical symbol Only applies to single atoms or to polyatomic ions Written to the left of the chemical symbol Written the same size as the chemical symbol Applies to every atom in molecule, including subscripts

Examples: How many of each element are there? 2Li2CO3 3MgCO3 4Ca3(PO4)2 5H2O

Hydrates When the crystal lattice of a solute binds with water BaCl2 * 2H2O FePO4 * 4H2O The water does NOT react! But, it does add mass to the compound Think about shipping concentrated orange juice vs. whole orange juice Anhydrate (anhydrous) is what it is called after it is heated and the water is removed (evaporated)

Open your book to p. 30 (1-15) 2 3 4 1 4 2 3 1

Writing Formulas All compounds must be electrically neutral. That means that it CANNOT have a charge. This is achieved by having a balance of positive and negative charges Based on Oxidation States Look at the periodic table p. 9 in your RT For Example: Na has an oxidation state of +1 Cl has an oxidation state of -1 If you put one of each of them together, the charges will cancel and the compound will be neutral. NaCl has a 1:1 ratio Positive ion (called a cation) is written first…this is usually a metal Negative ion (called an anion) is written second…this is usually a nonmetal For elements that do not have the same oxidation states, you have to do the “criss- cross method” to balance the charges

Criss-Cross Method Find the oxidation states of the elements Elements that have more than one oxidation state will have a roman numeral that corresponds to the oxidation state you should use. Iron (II) = +2 Iron (III) = +3 Lead (II) = +2 Lead (IV) = +4 If there is a polyatomic ion, it stays together and doesn’t change. You may have to put parentheses around it. Just look at the charge on the ion and use it in place of the oxidation state. Then write the oxidation state of one as the subscript of the other You just criss-cross them! Kris Kross

Example: Write the following formulas using the criss-cross method. Al+3 and F - Ca+2 and N-3 K+ and Cl- Mg +2 and I- Li+1 and OH-1 NH4+ and SO4-2 Cu+2 and PO4-3

Naming Compounds There are 2 types of compounds: Ionic Compounds Covalent Compounds Often made up of a metal and a nonmetal Electronegative difference is 1.7 or greater Often made up of only nonmetals Electronegative difference is less than 1.7

Ionic Compounds Binary Compounds Polyatomic Ions Stock System Containing only TWO elements Positively charged (usually metal) then negatively charged (usually nonmetal) particles Names of the elements End of second element is changed to -ide Polyatomic Ions If the polyatomic ion is negatively charged, just write both names If the polyatomic ion is positively charged, change the nonmetal to –ide just as before Stock System If a metal has more than one oxidation state, write a roman numeral after the name to show which oxidation state to use.

Examples: Name the following compounds. LiF CaBr2 K2S FeS MgO MnO CoI3 Mg(OH)2 Al(ClO3)3 (NH4)2O2

Covalent Compounds Remember, generally, they contain 2 nonmetals Molecular substance, not made up of ions So, look at electronegativity values Lower value goes first The name of the second element will still be changed to –ide Finally, you use a prefix to tell the reader how many of each element If there is only ONE of the first element, no need to use mono-

Examples: Name the following compounds. OF2 SF2 B2O3

Open your book to p. 32-33 (16-34)

STUDY!!!!!