Final answer:
A beta particle is missing in the provided nuclear equation. Beta decay involves the conversion of a neutron into a proton and the emission of a beta particle, which increases the atomic number by one while the mass number remains unaffected.
Step-by-step explanation:
The missing particle in the nuclear equation given would be a beta particle. The original equation indicates a reduction in the neutron count while the element transitions from one with 74 protons to one with 75 protons. This is indicative of a beta decay process. In beta decay, a neutron (n) is converted into a proton (p) and a beta particle (_β) is emitted. The beta particle, which is a high-energy electron, carries away a negative charge, effectively increasing the atomic number of the original atom.
Since the mass number remains the same (as the mass of the electron is negligible in comparison to protons and neutrons) and the atomic number increases by one unit, the emission of a beta particle (_β) best fits the provided nuclear equation.