Final answer:
Yttrium-91 has 39 protons and 52 neutrons. The atomic number of yttrium is 39, which indicates its number of protons. The neutrons are calculated by subtracting the atomic number from the mass number (91).
Step-by-step explanation:
The question is asking about the number of protons and neutrons in an isotope of Yttrium (Y), specifically Y-91. The number '91' refers to the mass number of the isotope, which is the sum of protons and neutrons. To find the number of protons in yttrium, we look at the periodic table and see that yttrium (Y) has an atomic number of 39, which means it has 39 protons. The number of neutrons can be found by subtracting the atomic number from the mass number, which in this case is 91 - 39 = 52 neutrons.