H and O are examples of CHEMICAL SYMBOLS. All the elements of the periodic table have a symbol representing their name which is shortened down to one or two letters. This way, it is easier to write as a symbol instead of writing the whole word. These examples you have listed are symbols of the elements Hydrogen (H) and Oxygen (O). Other examples of symbols are S for sulfur, Fe for Iron, and Al for aluminum.
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- Dotz
