Question

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

Answer 1

The Latin name for Sodium is Natrium. The symbol came from Latin name, not an English name.
Same for Iron (Fe from Latin Ferrum)
But the symbol for sulphur and silicon came from English name.

S is the first letter of Sulphur
Si is the first two letter for silicon
Na is the first two letters of Natrium the Latin name for sodium.