What is the structural formula of carbohydrates?

First, we have to define a bit more what Carbohydrates are.
Strictly speaking, the term means Hydrates of Carbon.

Trouble is, there are so many different ones, with a varying amount of Carbon atoms. The smallest amount of #C# -atoms is 3: the Trioses.

Carbohydrates, also known as Saccharides, are divided into four groups: Mono- Di-, Oligo- and PolySaccharides. The smaller ones (primarily the Mono- and Disaccharides) are commonly referred to as Sugars.

But the Oligo- and Polysaccharides are simply chains of the sugar monomers: any between 3 and 10 is generally referred to as Oligo- , anything from 10 upwards to MANY monomers (thousands and thousands, often branched) as PolySaccharides. Best known amongst the latter are Starch (aka Amylum ) and Cellulose...

BTW: The word Saccharide is derived from the Greek word for Sugar anyway, so what's in a name...

As we are dealing with hydrated Carbon, it follows that the generic, empirical formula for them is #C_x(H_2O)_y#. This certainly holds true for most Monosaccharides, though even there you can find exceptions: For instance, Ribose certainly conforms to this rule (#C_5H_10O_5#), but the central sugar in DNA doesn't: Deoxy -Ribose. (#C_5H_10O _color(red)(4)#)

As the di- oligo- and polysaccharides are simply polymers of the monosaccharides (and sometimes disaccharides), we'll concentrate on these...

The smallest ones are Trioses, and there are three: