Explanation

James Chadwick is known for his discovery of the neutron in 1932. 

Proton: Protons were discovered by Ernest Rutherford, before neutrons.

Electron: Electrons were discovered by J.J. Thomson, much earlier than protons or neutrons.

Positron: A positron is the antiparticle of an electron, and it was discovered by Carl Anderson in 1932. 

Explanation

Heavy water (D₂O) is used as a moderator because it slows neutrons without absorbing them much.

A moderator slows down fast neutrons in a nuclear reactor.

Slower neutrons help sustain the fission chain reaction.

Pakistan’s nuclear reactors commonly use heavy water as a moderator.

Explanation

The freezing point of heavy water is 3.82°C:

 Heavy water (D2O) Regular water

Freezing point 3.82°C 0°C |

Boiling point 101.42°C 100°C | 

Heavy water is made up of two deuterium atoms and one oxygen atom.

It contains more deuterium, a hydrogen isotope, than regular water, which contains hydrogen-1.

Heavy water is used in reaction mechanisms.

It is is prepared by electrolyzing water.

Explanation

Nuclear fission is the splitting a heavy nucleus into smaller nuclei. 

The word "fission" means splitting, so in nuclear fission, a large, unstable nucleus is split apart into smaller nuclei, releasing energy in the process. 

Explanation

Francium is extremely rare because it is highly radioactive and decays very quickly.

Only tiny trace amounts exist naturally in the Earth’s crust at any given time.