Subscript One

In Word, type 2081 and press Alt+X. This inserts the subscript 1 as its own character; it does not resize an ordinary digit beside it.

What does subscript one mean in a formula?

Its reading depends on the character or number before it. In mathematics, a₁ can label an indexed item, such as the first member of a sequence. In a chemical formula, a lowered number counts atoms of the element or group immediately before it; the count 1 is normally left unwritten when there is only one.

The lowered-number convention in chemical formulas took shape in the early nineteenth century, as chemists sought compact ways to show proportions. Berzelius's 1826 examples put the numbers above the line; later chemical notation placed them below. This is the history of formula notation, not the origin of the standalone character.