Читать книгу Foundations of Chemistry - Philippa B. Cranwell - Страница 59

Some isotopes are unstable and emit radiation; they are radioactive. Three types of emission are produced spontaneously from radioactive nuclei: alpha (α), beta (β), and gamma (γ). The exact species that is emitted is dependent upon the type of emission.

An alpha particle, α, has two protons and two neutrons – it is effectively the nucleus of a He atom. Compared to a helium atom, the alpha particle, α, has lost both of its electrons and therefore has a 2+ charge. We can represent it as: or . Alpha (α) emission occurs when an alpha particle or helium nucleus is lost from the radioisotope. This is usually restricted to elements with atomic numbers over 83 that have a large proton‐to‐neutron ratio. This emission causes a change in the composition of the nucleus of the element from which it was emitted, generating a new element. Alpha radiation is not very penetrating and can be stopped by either paper or skin.

Alpha (α) particles are emitted by . An equation can be written for this radioactive decay process:

or

In this case, because 2 protons are emitted from the nucleus, the atomic number of the new element formed is 82, which is the atomic number for lead (Pb). Therefore the element itself changes from polonium to lead. The atomic mass also alters because 2 protons and 2 neutrons have been lost. The new mass number (A) is 210 − 4 = 206, which tells us the new element has A = 206.

β⁻ (beta) emission arises when an electron () is released from the atom and a proton is added to the nucleus. This sounds confusing, as a nucleus does not contain electrons, but what actually happens is that a neutron splits to give a proton and an electron. The proton remains in the nucleus, and the electron is emitted. The atomic number of the atom formed therefore increases by one. β⁻ radiation that generates a proton is usually seen in an element that has a neutron‐rich nucleus. Beta radiation is more penetrating than alpha radiation and can be stopped by a sheet of aluminium foil.

An example of beta emission can be seen with the isotope:

The addition of a proton requires the atomic number to increase by one. In this case, potassium changed to calcium with an atomic number of 20.

Gamma (γ) radiation (or gamma rays) is penetrating electromagnetic radiation released from the nucleus of an atom. It consists of high‐energy photons. Gamma radiation is often released alongside alpha (α) or beta (β) radiation. Gamma radiation is the most strongly penetrating and ionising of the three types of radiation and can only be stopped by using a lead sheet.

Electromagnetic radiation is a continuous range of wavelengths and hence energies of radiation that includes visible light. A photon is the smallest discrete amount, or quantum, of electromagnetic radiation.