Chapter 8 AOxidation Numbers and Formulas Oxidations Numbers

Chapter 8 AOxidation Numbers and Formulas

Oxidations Numbers • Look at H 2 O – How many electrons does oxygen share? • 2 – This is it’s oxidation number (ON) • The number of electrons it has gained or lost • Oxidation numbers (ON) represent the number of electrons that an atom in a compound must gain or lose to return to its neutral state. • We use ON to keep track of electrons involved in bonding.

Oxidation Numbers • Can be negative or positive – Ions • negative means it gained electrons • positive means it lost electrons – Non-ions- same • Negative means it gained electrons • Positive means it lost electrons • Rules help determine ON

Oxidation Numbers • Rule 1 - The Freeelement Rule – Oxidation number of atoms in pure elements is zero • Ex: Fe, Na, Ar, He – Covalently bonded diatomic molecules always have a total ON of zero • Includes diatomic molecules • Rule 2 - The Ion Rule – Oxidation number of an ion is equal to the charge of the ion • Bromine gains an electron – becomes Br– ON is -1 • Mg loses 2 electrons – Becomes Mg 2+ – ON is +2

Oxidation Numbers • Rule 3 - The Zero-Sum Rule – Sum of all oxidation numbers of all atoms in a compound is 0 – Ionic bonds form between evenly balanced ions • Na+ + Cl- Na. Cl • Mg 2+ + 2 Cl- Mg. Cl 2 – Covalent bonds do the same thing – More electronegative atom is negative ON, less electronegative is positive ON (because it “give”/ “donates”) • Hydrogen has an ON of +1 • Oxygen has an ON of -2 • 2 H + O H 2 O – Element with highest electronegativity determines ON of other elements (negative or positive)

Oxidation Numbers • Rule 4 - The Specific Oxidation Number Rule – Certain elements/ groups almost always have same ON A. Alkali Metals (Group 1) always +1 B. Alkaline-Earth Metals (Group 2) always have +2 C. Hydrogen usually has +1 with nonmetals, but -1 with metals (forms metallic hydrides) D. Oxygen always has a -2 except when with Fluorine E. Halogens (Group 17) has -1 when bonded to metals, but not with nonmetals. EXCEPT Fluorine is always -1

Oxidation Numbers • Rule 5 - The Polyatomic Ion Rule – The oxidation numbers of all the atoms in a polyatomic ion add up to its charge – What is a polyatomic ion? • Covalently bonded groups of atoms that carry a charge – Example: OH • O: -2 (ON) • H: +1 (ON) • Total: -1

Polyatomic Ion with -1 charge Polyatomic Ion with -2 charge • acetate - C 2 H 3 O 2 • bicarbonate (or hydrogen carbonate) - HCO 3 • bisulfate (or hydrogen sulfate) - HSO 4 • hypochlorite - Cl. O • chlorate - Cl. O 3 • chlorite - Cl. O 2 • cyanate - OCN • cyanide - CN • dihydrogen phosphate - H 2 PO 4 • hydroxide - OH • nitrate - NO 3 • nitrite - NO 2 • perchlorate - Cl. O 4 • permanganate - Mn. O 4 • thiocyanate - SCN- • carbonate - CO 32 • chromate - Cr. O 42 • dichromate - Cr 2 O 72 • hydrogen phosphate - HPO 42 • peroxide - O 22 • sulfate - SO 42 • sulfite - SO 32 • thiosulfate - S 2 O 32 - Polyatomic Ion with -3 charge • borate - BO 33 • phosphate - PO 43 -

Rule Summary • Rule 1: free atoms = 0 • Rule 2: ion charge = ON • Rule 3: compound sum is 0 • Rule 4 A: Group 1 = +1 • Rule 4 B: Group 2 = +2 • Rule 4 C: H = +1 or -1 • Rule 4 D: O = -2 or -1 (with F) • Rule 4 E: Group 17 = -1 • Rule 5: sum of ONs in polyatomic ion = charge of polyatomic ion

Finding Oxidation Numbers • Na 2 O Rule 4 A- Na (sodium) always 2(Na) + (O) = 0 • H 2 SO 4 – Hydrogen has an ON of – Oxygen has an ON of 2(H) + S + 4(O) = ? ?

Ionic Compound Formulas • Barium and Iodine • Calcium and Oxygen

Atoms with Multiple Oxidation States • Some atoms more than 1 ON – N: +5, +4, +3, +2, +1, …. – C: +4, +2, -4 – H: +1, -1 • Table 8 -2 page 194 shows oxidation numbers

Polyatomic Ions • A polyatomic ion is a covalently bonded group of atoms with a charge – OH– NH 4+ – SO 42 -

Polyatomic Ions • What is the oxidation number of lead (Pb) in Pb(OH)2?

Polyatomic Ions • What is the formula of that compound that contains ammonium (NH 4+) ions and phosphate (PO 43 -) ions?

Polyatomic Ions • What is the oxidation state of Sulfur in a sulfate ion (SO 42 -)?