# How to find an isotope given mass number and neutrons?

1. Dec 16, 2017

### axer

1. The problem statement, all variables and given/known data
An isotope of which element has a mass number of 111 and has 5 more neutrons
A- Antimonu
C- Iodine
D- Tin
E- Xenon

2. Relevant equations

3. The attempt at a solution
I dont know where to start

2. Dec 16, 2017

### .Scott

It means 5 more neutrons than protons.

3. Dec 16, 2017

### axer

Thank you its C lol Thanks got it.

4. Dec 16, 2017

### epenguin

You have reported the question leaving out some words apparently - five more neutrons than what?
You give no reasoning but I think your answer has to be wrong on any possible guess of what the question is. I'm afraid #2 is wrong too. And worst of all that the question is wrong as well (which we are getting quite often recently) - none of the options can be a correct answer.

5. Dec 18, 2017

### .Scott

The atomic numbers of the possible answers are 51 (Antimony, Sb), 48 (Cd), 53 (I), 50 (Sn), and 54 (Xe). Antimony-111 has a half-life of 75 seconds. Cadmium-111 is stable. Iodine-111 has a half life of 2.5 seconds. Tin-111 has a half-life of 35 minutes. Xenon-111 has a half-life of 0.74 seconds. So all of the possible answers have a possible mass number of 111.
But only one has 5 more neutrons than protons: Iodine-111, with 53 Protons and 58 Neutrons.

6. Dec 19, 2017

### epenguin

Sorry, Scott was better than me at guessing what the question was. The most natural reading of the incomplete question was “five more neutrons than the naturally occurring most common isotope“.

You will probably never meet iodine 111 again.