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

1 Answer
anor277
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)]^-#.

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