Question

why is the symbol S for Sulphur but Na for Sodium and Si for Silicon. EXPLAIN!

Answer 1

The symbols for the elements are derived from their Latin and Greek synonyms to avoid confusion due to English words. 
For Sodium the symbol is Na derived from Natrium, because the symbol S was already taken for Sulphur so Si is the symbol for Silicon

Answer 2

hey mate here is ur ans

a symbol is the short hand reperesentation of an element

♡Here all the three elements start with S
so.... it is taken as such that S is For sulphur as s is the first letter of it
♡sodium's latin name is natrium so we take it as Na, the first two letters of the latin name
we cant write only N as it will be Nitrogen
♡silicon is Si the first two letters of the name of the element