(a) The diagram represents an atom of beryllium (Be) - Edexcel - GCSE Physics - Question 5 - 2012 - Paper 1

The correct symbol for the nucleus of beryllium is D, which is represented as $^9_4 Be$.

A beryllium atom can become a positive ion by losing one or more electrons. When it loses one electron, it gains a positive charge of +1, turning it into a positively charged ion. This process of gaining or losing electrons is essential for the formation of ions.

During nuclear fusion, two light nuclei, typically hydrogen isotopes, combine or fuse together to form a heavier nucleus, such as helium. This process releases a significant amount of energy, as a small fraction of the mass is converted into energy, according to Einstein’s equation $E=mc^2$.

In the fission process, a heavy nucleus, such as uranium-235, absorbs a neutron and becomes unstable. This instability causes the nucleus to split into two smaller nuclei, along with the release of more neutrons and a large amount of energy. The released neutrons can then cause further fission reactions in nearby uranium-235 nuclei, creating a chain reaction.

To control this reaction in a nuclear reactor, control rods made of materials like boron or cadmium are inserted into the reactor core. These rods absorb neutrons, thereby slowing down the reaction rate. The reactor's power output can be adjusted by raising or lowering the control rods to maintain a steady and safe rate of fission.