What isotope has 18 protons and 22 neutrons? Enter the name of the element followed by a hyphen and the mass number (e.g., uranium-234). Which element does X represent in the following expression: 55 25 X? Enter the chemical symbol of the element.
An element has a distinct no of protons
so 18 specifies it must be an isotope of Ar.
The mass no, is
18 + 22
The isotope is therefore Ar-40