Illustrated Glossary of Organic Chemistry

Sulfuric acid: A strong Bronsted acid, commonly used as a non-nucleophilic  proton source. pKa1 = -10; pKa2 = 2.0. The term can also refer to an aqueous solution of H2SO4. Concentrated sulfuric acid is pure (anhydrous) sulfuric acid. Oleum (also called fuming sulfuric acid) is a solution of sulfur trioxide dissolved in concentrated sulfuric acid.

Molecular Structure of Sulfuric Acid

Lewis structure   
Ball and spoke
Space filling model