Answer :
In alpha decay, a helium nucleus is ejected from the nucleus. This removes 2 protons and 2 neutrons from the mother nucleus, thus reducing the mass number by 4 and the atomic number by 2. The daughter isotope is thorium 234, atomic number 90.
When the nucleus emits beta particles it loses electrons. The atomic mass of the mother nucleus is the same as that of the daughter nucleus since the mass of electrons is very small compared to the mass of protons and neutrons. The atomic number of the daughter nucleus is one higher than that of the mother nucleus.
In a nuclear decay reaction also called radioactive decay an unstable nucleus emits radiation and transforms into the nucleus of one or more other elements. The resulting daughter nucleus has lower mass and energy than the decaying parent nucleus. The atomic number and mass number of the daughter nucleus must be calculated by subtracting the atomic number and mass number.
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