When an a-particle escapes from an atomic nucleus, the number of protons (charge number) in the nucleus decreases by two, and the a-decay product turns out to be the nucleus of a chemical element with a charge number two units less than the initial one. In this case, the mass number of the new product core becomes four units less than the mass number of the original core.
Remember: The process of learning a person lasts a lifetime. The value of the same knowledge for different people may be different, it is determined by their individual characteristics and needs. Therefore, knowledge is always needed at any age and position.