Why do some symbols have one letter some have two letters and some have three letters?

Answer 1

Elements are give symbols consisting of letters, and these are related to the names of the elements (in some language at some date). There are 26 letters in many alphabets, but more than four times that number of elements. So it is necessary to use some mechanism to distinguish among elements with names that start with the same letter. Adding numbers is done to designate the number of atoms in a compound, the isotope, and so on. The best solutions is to use pairs of letters for most elements.

Answer 2

Symbols in science, such as those representing elements, often have one- or two-letter symbols to make them easier to write and work with. Some elements have symbols derived from their Latin names, resulting in two-letter symbols. In cases where two elements have the same first letter, the second letter is used to differentiate them.

Answer 3

There are more than 26 elements. One letter symbols are more convenient, but there are only 26 of them, and there are over 100 elements.