The tone of To Kill a Mockingbird shifts from chapter to chapter. The scenes with Jem, Scout and Dill and their antics concerning Boo Radley are mostly light-hearted and filled with childish humor. There are, of course, scenes of a more serious type, but even many of them are interspersed with examples of Harper Lee's comic touch.
The second half of the novel, which primarily centers on the trial of Tom Robinson, is much more serious, exploring the subjects of rape, racism and intolerance. The children are growing and coming to understand that the adult world which beckons them is not a perfect one. Some of the scenes are heart-rending, such as Tom's unjustified guilty verdict and Bob Ewell's depraved actions. But the overall tone is one of hope and promise for a better world in the future.
In literature, "mood" refers to how the reader feels while reading the story. The word "tone" describes the author's attitude toward the subject. Since Lee's To Kill a Mockingbird is told from a young girl's perspective, there are sure to be scenes of innocent humor along with glimpses of childhood fears. For example, readers may laugh at Scout's naive responses to her father and older brother. They may then empathize with her fears and concerns for the stories she hears, the events she witnesses, and the prejudices she faces.
On the other hand, it should be recognized that the narrator is not a child when she tells the story. In fact, Scout's older self narrates the story from her memories of the past. The tone of the story therefore shifts from one happy memory to another, and from one stressful event to another. Readers may feel the overall mood and tone are about life. There are ups and downs. There are struggles during times of joy. Whether we like it or not, dark times must be faced with courage and compassion, even though it seems the whole world is going crazy with hate. Atticus sums it up best:
You know what's going to happen as well as I do, Jack, and I hope and pray I can get Jem and Scout through it without bitterness, and most of all, without catching Maycomb's usual disease. . . I just hope that Jem and Scout come to me for their answers instead of listening to the town. I hope they trust me enough (88).
Atticus speaks of four major feelings in this passage: bitterness, prejudice, hope, and trust. Just as life is filled with both good and bad, so, too, is Lee's To Kill a Mockingbird. There are happy moments that make the reader laugh, and there are moments which concern the reader greatly. The tone, however, is also one of honesty. By honestly looking at the problems in Maycomb's society, discussions about the controversies found therein can be had. Therefore, even though life (as well as the book) has times of bitterness and disease, there are also times of happiness, hope, and trust.
Like the previous answer, the mood changes. The first part of the book is light and even funny at times. We see this side of the novel through the personalities, perceptions, and actions of Jem, Dill, and Scout. What is particularly amusing is the children's attempts to get Boo out of the house. Here is an example:
Jem gave a reasonable description of Boo: Boo was about six-and-a-half feet tall, judging from his tracks; he dined on raw squirrels and any cats he could catch, that’s why his hands were bloodstained—if you ate an animal raw, you could never wash the blood off.
As the novel progresses, the mood unalterably changes. When the trial of Tom Robinson is introduced, everything changes. The mood becomes somber and serious. This should not be unexpected as important themes are developed, such as rape, racism, courage, and justice.
In light of this, both Jem and Scout grow up and shed their childhood. They have to face their community and life with a new set of eyes where not all people are fair, not all people tell the truth, and not all people are loving. Evil also even touches them, as Bob Ewell attacks them.
With this said, there is also a note of hope. The children do not become bitter. They, like Atticus, will seek to do the right thing. In the end, Scout shows this as she sees Boo as a mockingbird, who needs to be protected.