Question

Three different atoms of oxygen are represented as:
${}_8{}^{16}\mathrm{O},{}_8{}^{17}\mathrm{O}$ and ​${}_8{}^{18}\mathrm{O}$
(a) What do the subscripts (lower figures) and superscripts (upper figures) represent?
(b) What factor is responsible for the change in the superscripts 16, 17 and 18, though the element is the same?
(c) What is the usual name for such atoms of an element?
(d) Give the nuclear composition of ${}_8{}^{18}\mathrm{O}$​.

Answer 1

(a) Subscripts represent the atomic number while superscripts represent the atomic mass of the atom.

(b) The number of neutrons is different for all the three given atoms, so the value of atomic mass is different too. That's why superscripts are different though the element is the same.

(c) The atoms which have the same atomic number but a different mass number are known as isotopes.

(d) Nuclear composition of ${}_8{}^{18}\mathrm{O}$:
Number of electrons = 8
Number of protons = 8
Number of neutrons = Atomic mass - Number of protons
Number of neutrons = 18 - 8
Number of neutrons = 10