Final answer:
The isotope with 18 protons and a mass number of 42 is called argon-42, where argon is the element with 18 protons and the mass number indicates it has 24 neutrons.
Step-by-step explanation:
An isotope with 18 protons corresponds to the element argon on the periodic table, because the number of protons defines the element. The mass number of 42 indicates the sum of protons and neutrons in the isotope's nucleus.
Therefore, to identify the isotope, you first recognize it as argon due to the 18 protons, then use the mass number to find the number of neutrons by subtracting 18 (the number of protons) from 42, which gives us 24 neutrons.
By convention, isotopes are named after the element followed by the mass number. Consequently, the name of the isotope in question is argon-42.