Final answer:
An argon atom with 21 neutrons is represented as the isotope Argon-39 or ^{39}Ar, since argon has 18 protons and the mass number is the sum of protons and neutrons.
Step-by-step explanation:
An argon atom with 21 neutrons is represented as an isotope by considering both its protons and neutrons. Argon has an atomic number of 18, which means each atom has 18 protons. To express this as an isotope, we add the number of protons (18) to the number of neutrons (21), giving us the mass number, which is 39. Therefore, the isotope can be represented as Argon-39 or ^{39}Ar.
An atom is considered an isotope when it has the same number of protons but a different number of neutrons compared to the standard atomic mass. In the case of an argon atom with 21 neutrons, this can be expressed as Ar-22. The standard atomic mass of argon is 39.95 amu, but the isotope Ar-22 would have a slightly different mass due to the additional neutron.