What is one sugar molecule called?

It is called a sucrose molecule.

Lonely?

Monosaccharide

One sugar molecule is called a monosaccharide. Two monosaccharides together are called a disaccharide, and then a whole lot of them together forms a complex sugar ( or carbohydrates) called polysaccharides.

sucrose

They all have their own names: Ribose, Glucose, Galactose, Sucrose, Erythrose, etc.

Sugar.

hydrogen trioxide