We call these letters symbols. And an element is a pure substance made from only one type of atom. And there are quite a lot of elements. There are something like 100, and each has a unique symbol ...

Atom. Chem, Atom. Symbol Wt. Symbol Wt. Strontium Sr 87.63 Calcium Ca 40.07 Vanadium Va 51.0 Magnesium Mg 24.32 Rubidium Rb 85.45 Potassium K 39.10 Zinc Zn 65.37 Sodium Na 23.0 Copper Cu 63.57 ...