The 5th Wave makes extensive use of onomatopoeia throughout the novel to help shape the mood and draw the reader into the story.
"It fell between his legs with a sharp cling." (11)
The onomatopoeia here describes the sound that the Crucifix soldier's gun makes when he lets it fall to the ground. It is an ominous sound that demonstrates the precarious nature of the soldier's situation and life, and almost calls to mind the sound of a death knell.
"A thuwhump! when it broke the sound barrier, an ear-piercing shriek as it rocketed to earth, the ground shuddering under our feet when it plowed into a fallow cornfield." (27)
This illustrates the sound that a sound wave from the Eye makes when it detonates, killing all the adults in Camp Ashpit and stunning Cassie, who is far away enough from it to survive the blast. It helps the reader imagine what it would be like to face a sound wave that can kill.