Task: Sentence Ordering
Input (English): Steps: (1) The nuclei of some atoms have too few or too many neutrons (2) The atoms become unstable (3) The atoms split (4) In alpha decay, two protons and two neutrons are released from the atom (5) In beta decay, a neutron changes into a proton and an electron (6) The electron is released  
Where does the following step fit into the given list of steps: The proton joins the nucleus of the atom 
Output (English):  After step 5.