Bellwork 2/17 Assign oxidation states to each atom in: Carbon in K 2 C 2 O 4 Phosphorus in PO 4 3- Chromium in Na 2 Cr 2 O 7

Polyatomic ions: Bellwork Write the formula: Potassium sulfite Lithium dichromate Hydrogen peroxide Write the name: FePO 4 Ca 3 (PO 4 ) 2 Al 2 (CrO 4 ) 3

OXIDATION NUMBERS It’s something we’ve already been doing but need to put a name on it!

This is what we’ll learn today: 1. How to assign oxidation numbers to elements in a compound (molecular and ionic). 2. Assign oxidation numbers to each element in the formula of a compound.

Oxidation numbers Numbers called oxidation numbers can be assigned to atoms in order to keep track of electrons in compounds… You’ve been doing this already and didn’t even know it! It’s how we’ve been balancing charges in compounds.

Based on electronegativity Oxidation numbers don’t have an exact physical meaning. In some cases they are arbitrary! But they are useful in naming compounds, writing formulas, and balancing equations. They are also helpful in studying certain types of reactions.

It’s a little different but kind of the same… Think of oxidation numbers like a “bookkeeping” device to keep track of electrons. 1. The atoms in a pure element have an oxidation number of zero. Pure sodium, pure oxygen (O 2 )… all = 0 2. Fluorine has an oxidation number of Oxygen usually has an oxidation number of -2. In compounds with F, such as OF 2, oxygen’s oxidation number is +2.

4. Hydrogen is +1 in all compounds containing elements that are more electronegative than it; it has an oxidation number of -1 in compounds with metals. 5. Monatomic ions will be equal to the charge of their ion. Na+, Ca 2+ and Cl- are +1, +2 and -1.

Still different, but similar… 6. The algebraic sum of the oxidation numbers in a neutral compound = zero. 7. The algebraic sum of the oxidation numbers in a polyatomic ion is equal to the charge of the ion.

What’s the difference between “charge” and oxidation number?

Can you see the difference between “charge” and oxidation number? Example: SO 4 2- has an overall ionic charge of -2. What are the oxidation numbers on all the atoms present? Oxygen = 4 x -2 = -8which means Sulfur = +6 oxidation number Algebraic sum = -2

Try a few yourself: Assign oxidation numbers to each atom in the following compounds or ions: UF 6 H 2 SO 4 ClO 3 -

Here’s how: Place the known oxidation numbers above the appropriate elements. Multiply by the number of atoms and place the totals by the corresponding elements. UF 6 F = -1 x 6 = -6 so U must be +6

H 2 SO 4 H= +1 x 2 = +2 O= -2 x 4 = -8 S must be +6.

ClO 3 - The overall charge should equal 1-. O= -2 x 3 = -6 Cl= +5 oxidation number in order to have the overall 1- charge.

As it turns out, many nonmetals can have more than one oxidation number. We’ll use this concept more in the next chapter when we balance equations. Sulfur can have +4 and +6 oxidation states… S and O can form SO 2 or SO 3.

Common Oxidation States of Nonmetals Numbers

Rules for Assigning Oxidation Numbers The oxidation number of any uncombined element is 0. The oxidation number of a monatomic ion equals the charge on the ion. The oxidation number of fluorine in a compound is always -1. Oxygen has an oxidation number of -2 unless it is combined with F (when it is +2), or it is in a peroxide (such as H 2 O 2 or Na 2 O 2 ), when it is -1. The oxidation state of hydrogen in most of its compounds is +1 unless it is combined with a metal, in which case it is -1. In compounds, the elements of groups 1 and 2 as well as aluminum have oxidation numbers of +1, +2, and +3 respectively. The sum of the oxidation numbers of all atoms in a neutral compound is 0. The sum of the oxidation numbers of all atoms in a polyatomic ion equals the charge of the ion.

Let’s practice a few more… I’ve got a hand out for you!

Target 2/16 Assign oxidation states to each atom in: Carbon in K 2 C 2 O 4 Phosphorus in PO 4 3- Chromium in Na 2 Cr 2 O 7

Target 2/19 1.What are the first 5 prefixes used for naming nonmetals? 2. Name these: SiO 2 SiCl 3 3. Write formulas for these: Phosphorus dichloride Diphosphorus trichloride

Target 2/17 Name these compounds which contain polyatomic ions: Na 2 ClO 2 Cu(CN) 2 Write the formula for these polyatomic ions: Sulfate and sulfite Phosphate Carbonate

2/11 Name the following compounds: AuBr 3 KOH Write the formula for: Iron (II) oxide Ammonium nitrate

2/17 Name these polyatomic ions: OH- NO 3 – NO 2 – Name these compounds containing polyatomic ions: Ca(NO 2 ) 2 CaOH

What’s the difference between a cation and an anion? Name these cations: Ba 2+ K +

2/6 Target questions… Write the formulas for and give the names of the compounds formed by: 1)Cr 2+ and F - 2)Ni 2+ and O 2- 3)Fe 3+ and O 2-