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  Curved Arrows: One arrow per mobile pair Each curved arrow shows the movement of one electron pair. Therefore, the number of arrows should exactly equal the number of mobile pairs. This rule, # arrows = # mobile pairs, is easy to break if you aren't careful. Let me show you.
Too many arrows You may end up using too many arrows if you make a multiple arrow drawing in several steps. Look at the WRONG and RIGHT drawings below. They show the same reaction, but the WRONG drawing uses two arrows, while the RIGHT drawing uses only one. Notice there is only one mobile pair in this reaction (the CO bond) so there should be only one arrow.
Simple logic produces the WRONG drawing. When you compare the reactants and the product, you might think, "I've got to make an OH bond" and you draw an arrow. Then, you might think, "I've got to break a CO bond too." And there's your second arrow! I can't criticize this logic. The only problem is failing to look at the drawing afterwards to make sure it contains the right number of arrows. Check your work!
Too few arrows You may use too few arrows if you leave lone pairs out of your drawings. The WRONG drawing seems to move the bond pair where it is needed, but this is not enough. The RIGHT drawing shows that one lone pair is also mobile, so two arrows are needed.
You can't find mobile electron pairs if they aren't in your drawing. Get into the habit of drawing complete Lewis structures: all valence electrons, including lone pairs on key atoms, and formal charges.
Review problems Take some time to practice what you have learned on this page. The reactions below contain the wrong number of arrows (and the arrows are positioned incorrectly). Re-draw the reactants with correct arrows. Drawing suggestion: Draw complete formulas before checking your answers. Do not simply identify misdrawn arrows and do not these problems only in your head (even if you can). #1.
#2.
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