AS level chemistry help- exam soon

Iodine exists as one isotope with mass number 127.
Chlorine exists as two isotopes with mass numbers 35 and 37.

How many molecular ion (ICl+3) peaks are there in the mass spectrum of ICl3?

A. 2
B. 3
C. 4
D. 5


The answer is C, but I don't know how. Could someone please explain? Thank you in advance

Each Chlorine atom either has a mass number of 35 or 37, and there are 3 chlorine atoms in ICl3. So find out all the different combinations of chlorine isotopes in ICL3, the different combinations of isotopes cause ICl3 to have many mass numbers. The peak in the mass spectrum is the mass/charge ratio, and the charge of the ion is 1, so different mass numbers of ICL+3 produce different peaks in its mass spectrum.

Combinations of ICL3:

- Iodine (127) + Chlorine (35) + Chlorine (35) + Chlorine (35) = mass 232
- Iodine (127) + Chlorine (37) + Chlorine (35) + Chlorine (35) = mass 234
- Iodine (127) + Chlorine (37) + Chlorine (37) + Chlorine (35) = mass 236
- Iodine (127) + Chlorine (37) + Chlorine (37) + Chlorine (37) = mass 238

There are only 4 different combinations of chlorine isotopes in ICL3, so there is only 4 mass peaks, this means that the correct answer is C.