What was discovered by chemists after mendeleevs periodic table was accepted

The image shows a process that releases nuclear energy which statement best identifies the process shown the process must be fusion because energy is released the process must be fusion because of have your nucleus formed a smaller nuclei the process must be fission because a large nucleus breaks into smaller nuclei the process must be fission because neutrons are formed

Which element is least likely to undergo a chemical reaction

How many neutrons does an element have if its atomic number is 50 and its mass number is 166