Question

What does a subscript following an element in a chemical formula mean?

Answer 1

It usually specifies the standard state of the element......

Explanation:

Sometimes the state is fairly obvious....

`H_2(g) + 1/2O_2(g) rarr H_2O(l)`

..i.e dihydrogen and dioxygen are gases, whereas water is a liquid under standard conditions. And of course we could use the state symbols to represent phase changes.

`H_2O(l) rarr H_2O(s)+Delta`

`NaCl(s) + Delta rarr NaCl(l)`

And in aqueous solution, so-called `"aquated"` ions are represented....

`NaCl(s) stackrel(H_2O)rarrNa^+(aq) + Cl^(-)(aq)`

Where `Na^(+)(aq)` represents the aquated ion, probably `[Na(OH_2)_6]^+` and `Cl^(-)(aq)-=[Cl(H_2O)_(4-6)]^-`.