In a preface to the original edition of The Adventures of Tom Sawyer, author Mark Twain explains that he based the experiences of Tom Sawyer and Huck Finn on his own adventures and those of his boyhood friends and that he wrote the book for "the entertainment of boys and girls." We can keep this in mind as we assess the mood of the opening lines.

As the book opens, Tom's aunt Polly is calling the boy's name. She calls several times, and he doesn't answer. She pokes under the bed with a broom and then looks out the open door. While she is standing there, Tom tries to scurry past, but she catches him. It turns out that his hands and mouth are smeared with some jam that Aunt Polly told him not to touch. She is about to spank him with a switch when he diverts her attention and manages to run away.

In these opening lines, Twain sets the mood for the book and establishes the characters of Tom and of Aunt Polly. Tom is clever, fun-loving, troublemaking, and disobedient, while Aunt Polly is seemingly strict but is also amused by Tom's antics.

We see by these opening lines and this opening scene that the mood of the book is lighthearted, playful, and mischievous. Twain intends to cause boys and girls to sympathize with the protagonists and for adults to look back with nostalgia on the days of their youth.

What is the mood/atmosphere in the novel, The Adventures of Tom Sawyer.

There is no one way to describe the atmosphere or mood of the story. 

At the beginning of the novel, the mood is carefree and humorous. Tom is a mischievous boy who likes to have fun. He gets in trouble time and time again. The reader cannot but be amused at his behavior, such as when he says that David and Goliath are apostles, or when he falls in love with and engages Becky Thatcher, until Becky breaks up with him for his prior engagement with Amy Lawrence.  

As the novel progresses, there are elements of suspense. When Huck and Tom witness the murder of Dr. Robinson by Injun Joe, there is a sense of fear and suspense. The reader wonders what will happen next, especially as there are several "close calls" for Tom and Huck with Injun Joe. 

Finally, there is a mood of satisfaction as well, when justice finally comes to Injun Joe and when we see the maturation of Tom and the good fortune that befalls Huck. Things work out well in the end.