If you're looking for his pity, you're going to have to present a much stronger case.
First, you have to convince him that you deserve that grade. As an instructor, he wants to evaluate his students fairly. You have to convince him that this hasn't been the case for you. One argument you can make is that you could've, should've, would've done better on the exam had it not been for some unforeseen, fucked up and mostly unfair circumstances that happened out of the blue the week before the exam. For example, an emofag of a friend of yours has attempted suicided and you spent the week the week trying to console him. Basically, you're a better student than the grade reflects, but a situation has handicapped you and prevented you from showing this.
Ask him for a second chance. Ask for additional an assignment, or the possibility to re-take the exam.
Second, convince him that you do need this grade. Don't start crying, but convince him that it's really important to you. You're not merely saying what you're saying because you want, but rather you absolutely need it. You have so much at stake at that grade, your parents count on you so much etc... Whatever your story is, it's going to have to be good and very special. Be emotional.
Third, don't do it over email, idiot. Any schmuck can compose an email to all of his professors and see who bites. That's too easy. Seek him out in person (call the school and find out his hours) and then present your case. You can act desperate, but not pushy! Be cohesive and respectful, don't forget that he doesn't owe you shit.
So make a good story, practice it and stick to it. And save your participation argument (your most solid one) for last.
All I can hear, I me mine
I me mine, I me mine