A. A radioactive

A.

We know that, when a nucleus undergoes decay, its mass number decreases by 4 and atomic number decreases by 2, whereas for decay, the mass number remains same and the atomic number decreases by 1, and decay does not effect any of them. So, for the above decay we can write as,

Hence,

For A₄,

Mass number 172

Atomic number 69

For A,

Mass number 180

Atomic number 74.

B. For decay,

The mass number A does not change and the atomic number goes up by 1 u. In decay, a neutron decays into proton, electron and antineutrino, the corresponding nuclear equation is,

N → p + e^-+

For decay,

The mass number A does not change and the atomic number reduces by 1 u. In decay, a proton decays into neutron, positron and neutrino, the corresponding nuclear equation is,

P → n + e⁺ +

It is important to take a note, a free neutron can decay to proton, the decay of proton to neutron is possible only inside the nucleus, since proton has smaller mass than neutron.