In The Chrysalids, how does the setting contribute to the atmosphere, or mood, of the story?
I think we need to remember above all that the setting is in a community which is so righteous and zealous about protecting itself from any forms of deviance that these are destroyed or disposed of straight away. This religious extremism adds a real sense of horror to the story as we know the kind of fate that David and his group of fellow telepaths would suffer if they were caught. We are given many different examples of foreshadowing to indicate this, most poignantly in the dream that David has of his father slaughtering Sophie as he would do a deviant calf:
The sun's edge began to show above the horizon, and everyone started to sing a hymn. My father held Sophie with one arm just as he had held the struggling calf. He raised the other hand high, and as he swept it down the knife flashed in the light of the rising sun, just as it had flashed when he cut the calf's throat...
Note how this dream contributes to creating a setting of fear and violence. It reinforces how the Waknukians are opposed to everything that David and his telepathic friends stand for and their very existence. This in turn helps to create a mood that is not unlike a time bomb, as we wait for the discovery to occur and the violence and tragedy to be unleashed. Thus the setting helps establish the menacing and threatening mood that dominates the story.