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: $\text{ } \quad \frac{9}{4} Be$
This represents 4 protons and 5 neutrons.

A beryllium atom can become a positive ion by losing one of its electrons. This loss occurs because the atom gains energy in the form of heat or light, which allows an electron to escape. Since the atom now has more protons (4) than electrons (3), it becomes positively charged.

During nuclear fusion, two light nuclei, such as those of hydrogen or helium, collide at extremely high temperatures and pressures. They combine to form a heavier nucleus, releasing a significant amount of energy in the form of light and heat. This process is responsible for the energy produced by the Sun.

In nuclear fission, a heavy nucleus, like that of uranium-235, absorbs a neutron and becomes unstable. This instability causes the nucleus to split into two smaller nuclei, along with additional neutrons and a large amount of energy. To control this reaction in a nuclear reactor, control rods made of materials like boron or cadmium are used. These rods can absorb free neutrons, thereby slowing down the fission process and preventing a runaway reaction. The released energy is utilized to heat water, which produces steam to drive turbines for electricity generation.