13) Which emission causes the atomic number of a nuclide to decrease by 2 and its mass
number to decrease by 4?

Alpha decay.

Explanation:

When an element undergoes alpha decay, the resulting daughter nuclei will have a decrease of 4 in the mass number and a decrease of 2 in the atomic number.

In the attached photo, 235 92U undergo alpha decay to produce 231 90Th.

Comparing the mass number and atomic number of 235 92U with that of 231 90Th we can see that the mass of 231 90Th reduced by 4 and the atomic number reduce by 2 indicating that the parent nucleus is undergoing alpha decay.

Alpha emission

Explanation:

The alpha particle is akin to a helium nucleus hence it is often referred to as a helium nucleus. The alpha particle has four mass units and a charge if +2.

Alpha emission is one of the emissions undergone by radioactive nuclei. When an atom undergoes alpha emission, it is easy to notice because the mass number of the nucleus decreases by four while its atomic number decreases by two. The daughter nucleus is now found two places behind its parent in the periodic table.

An example of alpha emission is shown below;

23592U→42He+23190Th

Thorium is four mass units less than U-235 and two places behind uranium in the periodic table as explained above.