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Chem+P+Notes+11_10 - Lewis Dot Structures Chem 1’ Lecture...

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Unformatted text preview: Lewis Dot Structures Chem 1’ Lecture Handout Series #3: Figuring out Where the electrons go. (aka: Lone pairs, bonds and octets, oh my!) Once you know the possibilities for how the atoms are arranged, you then put the electrons 1n to the structures. ° Electrons that are not in bonds are grouped in pairs. These are called fll‘f’h—i ' The Octet Rule 18 extremely important. All atoms (except hydrogen) want . to have exactly EIGHT electrons in their outer shell These electrons need to be either only theirs, or shared with another atom ' Hydrogen 18 too small to follow the Octet Rule so it has its own mini-rule called the Duet Rule that says Hydrogen wants 2 electrons 1n its outer shell. Steps'for putting the electrons into alLewis Dot Structure: I ' 1. Figure out how many total valence electrons you have (from the atoms, and any charge the molecule has) 2. Calculate the number of total bonds the molecule has using the Number of Bonds Formula ,— J# g J J W/ ,, W/ X ,., figMfl): “Zita/é Jim/c [k 3. Put in enough bonds to connect all of the atom with single bonds. 4. It you have not used up all of your bonds yet, then put in the remaining bonds as double or triple bonds. VERY IMPORTANT: If there is more than ..-.—..._.....‘..............3 one possible way to do this, you have to draw them ALL, don’t just draw one and stop! We’ll learn soon how to decide which is best. 5. Now look at how many valence electrons you have and how many are already in the structure as bonds, and distribute the rest into the structure so that the octet rule is followed. 6. Check your work! You should have: 5 ' All of the atoms in the structure, - The correct number of electrons in the structure, ' Every atom should follow the octet or duet rule. Try some: Draw the structures for: 9 3 gwj’fi PH3 #”%’[% (:0 I630" i Use the arrangements we had before for 502'2 and now put all of the electrons In P’ ‘ F‘ j ‘ . Draw the Lewis Dot Structures for N3'1 and put all of the electrons in. There will be more than one way to do this. Draw them all! fir -. 2’3: KM 3 {/9446}; Lewis Dot Structures Chem P Lecture Handout Series #4: Formal Charges (aka: Excuse me atom, do you hold some charge? and aka: How to break the ties) What is Formal Charge? a . i TheFormulaforcalculatlngtheFormalChargeofm‘latomw 5me 0%?“ fed fl alt/g; # £32 I”, , 6 ‘5 ah ah Q79“ Inc.) 6 ‘5 I W“! Recopy the arrangements you had before for 802“? and now calculate the formal charges on each atom in each structure. «;@ °~r ‘ ’@ '"Z— The formal charge short—cut. The Formal Charge Bubble (thanks to Mark) Draw the Lewis Dot Structures for N3"1 that you calculated on the iast worksheet. Calculate the formal charge of each atom using the formal charge bubble. How to decide which structure is best when you have more than one option: Rule 1: You want the structure with the smallest Whole number formal charge. 80 Best is structures with all formal charges of zero. Next best is formal charges of +1 / 0/ -1. Next best is formal charges of +2/ +1 / O/-1 / —2. Rule 2: If you did rule 1 and you still have a tie, then You want the structure where the negative formal charges are on the most electronegative element and the positive charges are on the least electronegative I element. Rule 3:, If you did rules 1 and 2 and you sell have a tie, then You want the structure Where the formal charges are as far apart as possible. Which is the best structure for SO ‘2 usin these rules? 2 g (7; = mrlavs ‘j/f g :\n C€w~>flh éccw’bt awn-l)” % ”org, Which is the best structure for N3'1 using these rules? [l/L'l/l/I/l/ L’Uqu‘ffl $>0 m J‘Hr’de‘7Lr ...
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