A radioactive nucleus  undergoes a series of decays according to the schemeA→αA1→βA2→αA3→γA4If the mass number and atomic number of A are 180 and 72 respectively, then what are these number for A4?

Two α-particles reduce mass number by 8.∴New mass number = 180 — 8 = 172Emission of two α-particles reduces charge number by 4. Emission of β -particles increases charge number by 1.∴ New charge number = 72 — 4 + 1 = 69.