1 A double bond forms when two atoms share two pairs of electrons... CH 2 O example is formaldehyde, CH 2 O. Lewis structure is... H C O H C O H

2 N 2 A triple bond forms when two atoms share three pairs of electrons. Example is N 2. N N one triple bond; 2 unshared pairs of e -. N

3 How to go about making Lewis Structures 1. Find total number of electrons needed: add up the number of valence electrons for atoms in the formula.

4 2. Write atoms that are bonded next to each other. 3. At the minimum, connect adjacent atoms with 2 e - (one pair). 4. Every atom except H should form a number of bonds = (8-#valence e - ).

5 5. Every atom should have a total of 8 electrons around it (except H has only 2 max). The extra ones needed to make 8 can be unshared. 6. Make double or triple bonds if there aren’t enough e - to go around for each atom to have only single bonds.

6 Permissible multiple bonds are: C=C, C=N, C=O, C=S N=N, N=O, O=O (Oxygen gas) and S=O. Also C C and C N.

7 Polyatomic Ions have covalent bonds internally: Consider: hydroxide, HO -. How many electrons? 8 = one e - six e - - one e -

8 There can be but one O-H bond: O H H has outer shell filled. Remaining e - are assigned to O: O H So why the - charge? and where?

9 Notice oxygen has one bond = 2 shared electrons, one of which “belongs” to O. There are 6 unshared electrons around Oxygen. Altogether, O has use of 7 electrons, but there are only 6 protons in its nucleus. So HO - is an anion.

10 Formal charge compares the number of valence electrons an atom has when it is bonded to the number of valence electrons the isolated atom has. We count unshared electrons as “belonging” to one atom; one-half the shared electrons in bond “belong” to each atom.

11 Example, using hydroxide ion: : O H.. There are 2 total shared e- at H, so ½ (2) or 1 belongs to H. As an isolated atom hydrogen is H· Since H has the same # e - in both cases, there is 0 formal charge.

12 But oxygen in hydroxide has 6 unshared electrons and a total of 2 shared e -, for a total of 6 + ½(2) = 7. Since oxygen atom has only 6 valence electrons (see slide 7), there is one more negative particle around O in hydroxide. O has a formal charge of -1.

13 Polyatomic Ions to know + NH 4 ammonium ion - OHhydroxide NO 3 - nitrate HCO 3 - bicarbonate CO 3 -2 carbonate PO 4 -3 phosphate

14 Polyatomic ions can form ionic bonds, ionic compounds: Na + HO - (NaOH) is sodium hydroxide. H bond to O is covalent; HO - group has ionic bond to Na +

15 Naming... Na 2 CO 3 is sodium carbonate; (NH 4 ) 2 CO 3 is ammonium carbonate; NaNO 3 is sodium nitrate; NH 4 NO 3 is ammonium nitrate

16 The non-metals bonded in a polyatomic ion act as a unit. So (NH 4 ) 2 CO 3 has two NH 4 + ions and one CO 3 2- ion in it. The ( ) keeps this clear. Without ( ) the formula looks like NH 42 CO 3, which is impossible!