CH2O)n
Wood is mostly cellulose, hemicellulose, and lignin. These are carbohydrates. Carbo (C) hydrate (H2O)
So what's the “n”? It's a big number. Really big. Something in the millions. But it's different for every fiber of that wood.
Oh there are other elements in there, too. Calcium and sodium and nitrogen and chlorine just to name a few. But the bulk of what you see in a piece of wood is just three elements: C, H, and O.