When thorium (90 protons) ejects a beta particle, the resulting nucleus has question 18 options: 87 protons. 88 protons. 89 protons. 90 protons. none of the above

none of the above

Explanation:

The actual answer is '91 protons'. In fact, the beta decay of the thorium-234 is the following:

where inside the nucleus of Thorium (90 protons), a neutron turns into an electron (the beta particle) + a proton. Therefore, the resulting nucleus (which is Protoactinium) has a total of 90+1 = 91 protons.

So, the correct answer would be '91 protons'.