Alpha emission is a type of radioactive decay in which an alpha particle is emitted from the nucleus. An alpha particle consists of two protons and two neutrons, which means that its mass number is 4.
During alpha emission, the mass number of the original nucleus decreases by 4, as the alpha particle is being emitted. Therefore, to calculate the mass number of the nucleus after alpha emission, we subtract 4 from the original mass number.
If the original nucleus has a mass number of 155, the mass number after alpha emission would be:
Mass number after alpha emission = Original mass number - 4
Mass number after alpha emission = 155 - 4
Mass number after alpha emission = 151
Therefore, the mass number of the nucleus after alpha emission is 151.