1. first nuclear reactor
atomic mass
2.
fusion
3. decaying nuclei
curie
4. number of neutrons and protons
becquerel
5. builds heavier elements
fermi
6. discovered radioactivity
fission
7. unit of radiation
neutron

The correct matches are as follows:

1. first nuclear reactor 
Fermi - an italian physicist who made the first nuclear reactor
2. 1/0 η
 atomic mass 
3. decaying nuclei 
fission - nuclear reaction from a heavy elements to lighter elements
4. number of neutrons and protons 
neutron 
5. builds heavier elements 
fusion - nuclear reaction from two or more elements to a heavier element
6. discovered radioactivity 
Curie
7. unit of radiation 
 Becquerel