Question

why is the symbol S for Sulphur but not Na for Sodium and Si for Silicon

Answer 1

because the symbols are based on first or first two letters of element.
AND NA IS SYMBOL FOR SODIUM BEACAUSE THE LATIN NAME OF SODIUM IS NATRIUM

Answer 2

sulphur's symbol is s because of sulphur first letter is s

and sodium's symbol is Na because of there latin name is (natrium ) and natrium frist two letter is na.
silicon frist two letter is si so silicon's symbol is si.
Hope it helps you