It's pretty basic. Other elements are much more complicated.

The periodic table, in which the elements are very cleverly arranged in order of various things, such as electronegativity. It is more complicated that it first appears. Google it.

first being the American system of weights and second is complicated spelling.

They are usually written simply as they are noted as on the Periodic Table. There are several more complicated rules concerning how compounds are written.

Quantum mechanics. It's fairly complicated and you'd need at least some calculus to understand the details, but if you really want to know I recommend Levine's textbook Quantum Chemistry.