In the novel It by Stephen King, a word which describes the tone in this excerpt best is nostalgic.
<em>Nostalgic</em> refers to the sentiments or affections which are related to the past, specifically for a place or periods relating to a happy personal association.
In the above excerpt line which depicts nostalgic affection is
“The energy you drew on so extravagantly when you were a kid, the energy you thought would never exhaust itself—that slipped away somewhere between eighteen and twenty-four,”
Richie gets nostalgic when he enters the room and feel the energy that he used to as a kid.
An expository article is usually about explaining something or talking about it. So if it's a true story and you write it like a life story then yes it can be. If it's just a fictional story you want to write then no.
