In what chapter does David Strorm run from his father in the book The Chrysalids?

 David runs away in Chapter 12.

David’s father Joseph Strorm is a passionately religious man.  He rules his domain with an iron fist.

He was still the largest landowner, he still continued to preach frequently on Sundays and to explain with practical clarity the laws and views held in heaven upon a variety of matters and practices, and, upon the appointed days, he administered the laws temporal, as a magistrate. (Ch. 2)

David is not as staunchly religious as his father.  He does not really believe in the True Image.  When he meets and befriends a girl with extra toes, he does not consider it sacrilege.  His father would never allow it.  David understands what it is to be different.  He is a telepath, and he and the other telepaths live in constant fear that they will be discovered. 

The beginning of the end is when David’s friend with the extra toes, Sophie, is discovered.  David tries to protect her, and holds out as long as he can against his father’s beating.  Sophie is caught, sterilized, and sent to the badlands in the Fringes.  This is where all of Waknuk’s outcasts are expelled to. 

Somewhere in the back of his mind, David must have always known he would someday be expelled.  His father would not protect him.  Even his own uncle was expelled.  Family means nothing when it comes to the True Image.  As soon as it is discovered that David is telepathic, that is it.  He and the other telepaths have to run for their lives to the Fringes. 

It happens when David’s sister Petra calls the others to her with psychic screaming after her pony is attacked.  The telepaths gather, and when a man finds them together he doesn’t buy their explanation that they heard her screaming.  David tries to explain to Petra using words that she has to be careful.  Uncle Axel, who knows about David, warns him that people have been asking questions. 

Soon, two of their own are captured—Sally and Katherine.  They have to flee before they are all caught.  Michael explains how dangerous the situation has become. 

Frankly, David, you're going to need your wits, too. There's hell to pay over this. They must have found out much more about you than we ever guessed, though they aren't on to Mark or Rachel or me yet.  But it's got them very worried indeed. They're going to send posses after you. (Ch. 12)

Things get only worse from there.  Waknuk is dangerous because they are being hunted, but the Fringes are dangerous too, because they are ruled by David’s uncle, whom he calls Spider.  He was the one expelled years before.  He is not happy about the new heat David's band brings on him.