Which missing item would complete this beta decay reaction?

answer: the missing item in the given reaction is

explanation:

beta-minus decay is defined as the decay process in which beta particle is released. it can also be said that a neutron gets converted into a proton and an electron. the beta particle released does not carry any mass but carries a charge of -1 units.

the chemical equation representing beta decay process is:

for the given chemical reaction:

the missing item in the given reaction is

It is simple. you must know that  every nuclear reaction obeys conservation laws. this laws include:
1. conservation of charge
2. conservation of mass
3. conservation of spin
4. conservation of angular momentum(angular momentum is vector sum of orbital angular momentum and intrinsic angular momentum)
5. conservation of momentum
6. conservation of parity
7. conservation of spin
8. conservation of isotopic spin

you will learn 6, 7 and 8 in quantum physics. 

coming to your question, mass and charge should be conserved.
∴¹³¹₅₃>   ¹³¹₅₄xe +  ⁰₋₁β

Atomicity occurs to prevent updates of the database occurring only partially, which can cause greater problems than rejecting the whole series outright.some are monatomic, diatomic, triatomic etc because it shows the number of atoms. for example, monatomic means it contains only one atom.