What do the subscripts (lower figures) and superscripts (upper figures) represent?

Three different atoms of oxygen are 22. represented as ${}{8}^{16}O$,
${}
{8}^{17}O$ and
${}_{8}^{18}O$ .
(i) What do the subscripts (lower figures) and superscripts (upper figures) represent?
(ii) Which factors are responsible for the change in the superscripts, 16, 17 and 18, though the element is the same?

(i) The subscripts represent the atomic number of the given atoms while superscripts represent their mass number.
(ii) Due to the presence of different number of neutrons in the given atoms of oxygen, their mass number differs from each other because mass number is the sum of number of pfotons and neutrons.