Question

In which of the following decays the atomic number decreases?

(a) α-decay
(b) β⁺-decay
(c) β⁻-decay
(d) γ-decay

Answer 1

(a) α-decay
(b) β⁺-decay

​In alpha particle decay, the unstable nucleus emits an alpha particle reducing its proton number (atomic number) Z as well as neutron number N by 2.

`""_Z^AX` → `""_(Z-2)^(A-4)Y` + `""_2^4He`

During  β⁻-decay​,  a neutron is converted to a proton​, an electron and an antineutrino. Thus, there is an increase in the  atomic number.

`""_Z^AX` → `""_(Z+1)^(A)Y` + `e^-` +`barnu`

During  β⁺-decay​,  a proton in the nucleus is converted to a neutron​, a positron and a neutrino in order to maintain the stability of the nucleus. Thus, there is a decrease in the atomic number. ​

`""_Z^AX` → `""_(Z-1)^(A)Y` + `β^+` +`nu`

When a nucleus is in higher excited state or has excess of energy, it comes to the lower state in order to become stable and release energy in the form of electromagnetic radiation called gamma ray. The element in the gamma decay doesn't change.
Therefore, alpha and beta plus decay suffer decrease in atomic number.