Isotopes are atoms of the same elements with same number of protons and electrons but different number of neutrons.
the isotope we are asked to find has 71 protons and 71 neutrons. the atomic number is the number of protons. Atomic number is characteristic to that element. Since isotopes are of the same element then the number of protons determines what the element is, element with atomic number 71 is Lutetium - Lu.
then the mass number is sum of protons and neutrons. In Lu isotope - mass number is 71 + 104 = 175 is the mass number
therefore answer for the isotope is -1) ¹⁷⁵Lu
