Alpha Decay

Alpha decay, as treated in NCERT, is the emission of an alpha particle, which is a helium-4 nucleus containing two protons and two neutrons. Conservation of nucleon number and charge requires the daughter to lose exactly what the alpha carries away, so the mass number drops by 4 and the atomic number drops by 2, written ${}_Z^{A}X\to {}_{Z-2}^{A-4}Y+{}_2^4\text{He}$. The option with mass down by 2 and atomic number down by 4 is wrong because it swaps the two changes. The option with unchanged mass and atomic number up by 1 describes beta-minus decay, not alpha decay. The option with atomic number increasing by 2 is wrong because emitting positive charge must lower, not raise, the nuclear charge. As stated in NCERT Class 12, Chapter 13 (Nuclei), alpha emission moves the daughter two places lower in the periodic table. A consistency check: the sums of mass numbers ($A=A-4+4$) and of charges ($Z=Z-2+2$) both balance, confirming the decay conserves nucleon number and charge.