Explanation:
According to question
Element : An atom with a unique number of protons.
A pure substance which consist of only one kind of atoms
Photon : A bundle or quantum of light.
Photon is elementary particle. light is moved over a space by photon.
Isotopes : Atoms with same atomic number but different mass number
Isotopes are two atoms having same atomic number but different mass number.
Nuclear fission: Splitting of an atom into two or more parts.
When a large atomic nucleus splits into smaller nuclei it is called nuclear fission.
