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

1 Answer
Aug 15, 2017

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)]^-.