1. Chemical Formula: s hows the kinds and numbers of atoms in the smallest representative unit of the substance (aka formula unit) H2OH2OH2O2H2O2 What’s the difference between these two formulas? When you change the subscripts the formula no longer represents that compound – it is something else.

2. CharacteristicCovalent compoundIonic compound Representative unit Molecule (atoms sharing electrons) Formula unit (balance of oppositely charged ions) Type of elementsNonmetallic Metallic combined with nonmetallic Physical stateSolid, liquid or gasSolid Important differences between covalent and ionic compounds Covalent compounds- Composed of two or more nonmetals Ionic compounds- composed of a metal and a nonmetal - total positive charge equals total negative charge

3. Molecules A molecule is the smallest electrically neutral unit of a substance that still has the properties of the substance 1. Monatomic consist as a single atom (Ex: Noble gases) 2. Diatomic molecule (Ex: O 2 ) “BrINCl HOF” 3. Triatomic molecule ( Ex: O 3 ) He

4. Chemical Formulas How to read formulas: H 2 O = 2 hydrogen atoms & 1 oxygen atom Ca(OH) 2 = 1 calcium atom, 2 oxygen atoms & 2 hydrogen atoms Practice: MgCl 2 H 2 SO 4 Al 2 (SO 4 ) 3 Mg = Cl = H = S = O = Al = S = O = 1 21 2 2 1 42 1 4 2 3 12

5. Oxidation Numbers – how many electrons an atom would gain or lose to follow the octet rule (also called its charges) +1 +2 +3 -2-3

6. Charges of Transition Metals These transition metals have more than one charge: Cr 2+ or 3+ Cu + or 2+ Mn 2+ or 3+ Sn 2+ or 4+ Fe 2+ or 3+ Pb 2+ or 4+ Co 2+ or 3+ Ni +2 or +3 Hg +1 or +2 Au +1 or +3 Add to the periodic table in your Reference Section:

7. Writing Formulas for Binary Ionic Compounds Rules: 1.Write symbol and charge for both cation and anion. 2.Crisscross charges, drop the charge and write as subscripts. 3.CHECK!! The formula must be in the lowest possible ratio. The cation is always written first!

8. Example Problems Write formulas for these compounds: calcium iodide Ca I +2- CaI 2 21

9. O -2 Tin (IV) oxide Sn +4 Sn 2 O 4 SnO 2 4 2