Question

Why is the symbol S for sulphur , but Na for sodium and Si for silicon

Answer 1

According to the theory of atoms 
first letter is taken as its symbol.
but for some elements latin names are taken as their symbols.
and some elements have same starting letter so their first and second letter are taken as their symbols.

hope this helps you

Answer 2

Hey friend, Harish here.

Here is your answer:

We know that,

Soduim, Sulphur & Silicon starts with the same letter S. As two different elements cannot have the same symbol, they different name by different method.

1) Sulphur  starts with S, So they kept as " S ".

2) Sodium also starts with S. But as sulphur got that symbol already they made it different. 

Sodium's Latin name is Natrium, So they made its symbol as " Na ".  

3) Silicon starts with Si, So they made it's name as Si.
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Hope my answer is helpful to you