Question

The formulae of compound silver iodide is SI.

Answer 1

Hint: Silver is commercially written as Ag and iodide as I. Silver combines with iodide and forms silver iodide. The S in the chemical symbol is used for sulphur.

Complete answer:
The given symbol of the chemical compound is SI but S is the chemical symbol of sulphur and I is the chemical symbol for iodine hence the given chemical symbol is of sulphur iodide not for silver chloride.
It may be confusing for many people, but in chemistry mostly we have chemical symbols which starts with the initials of their name. For example: the chemical symbol for hydrogen is H, the chemical symbol for lithium is Li, the chemical symbol for carbon is C.
But there are also exceptions, for example the chemical symbol of silver is Ag, the chemical symbol of gold is Au etc.
The chemical symbol of an element usually consists of one or two letters from the Latin alphabet and the first letter is always capital. There are many sources from which the symbols are derived, it includes Greek name of an element or German name etc. For example: the symbol for lead is Pb and it is derived from the Latin name of lead with is plumbum, the symbol for tungsten is W and it is derived from the German name of tungsten which is Wolfram etc.

The formula of compound silver iodide is AgI.

Note: Sometimes a 3 letter symbol is also assigned to a new element which is a temporary symbol for the element. For example: the chemical symbol for unniloctium was Uno which was later changed to Hs after the discovery of hassium.