Jöns Jacob Berzelius suggests the use of…

Jöns Jacob Berzelius suggests the use of letters as symbols for the chemical elements in 1814.

In order to aid his experiments, the Swedish chemist has developed a system of chemical notation in which the elements are given simple written labels—such as O for oxygen, or Fe for iron—with proportions noted by numbers.

This is the same basic system used today, the only difference being that instead of the subscript number used today (e.g., H₂O), Berzelius uses a superscript (H²O).