The syntax of the sentence "Spider is a dog" is very straightforward and simple, with a subject-verb-object structure.
In terms of atmosphere, this sentence alone does not provide much information to determine a specific atmosphere or tone. It is a basic statement of fact, with no added context or emotion conveyed through the syntax.
However, depending on the context in which the sentence is used, the syntax could contribute to a specific atmosphere or tone. For example, if the sentence is part of a longer passage describing a sad or tragic event involving Spider the dog, the simple and direct syntax could contribute to a somber or serious atmosphere. Conversely, if the sentence is used in a lighthearted or humorous context, the simple and straightforward syntax could contribute to a more playful or casual atmosphere.