Question

When a Cs-139 atom with 55 protons undergoes TWO successive beta minus decays, what is the daughter nucleus after the second decay? Cs-139 with 55 protons Xe-139 with 54 protons I-139 with 54 protons La-139 with 57 protons Ba-137 with 53 protons

Answer 1

In beta minus decay, a neutron decays into a proton, an electron, and an antineutrino. So, in one beta minus decay, daughter nuclei will have same mass number but atomic number will increase by one.

n → p + e− + ν

where, n is nuetron,

p is protons

e is electrons and,

ν is antineutrino.

A= Z+ N

where, A= mass number

Z= no. of protons

N= number of neutrons

So, in two successive beta decays, the number of protons will increase by 2 and number of neutrons will decrease by 2,  while mass number will remain same.

Hence, correct answer is option (d) La-139 with 57 protons.