Atomic Structure and Radiotherapy

An atom is the smallest unit of an element that retains its chemical properties. It contains a central nucleus made of protons and neutrons, surrounded by negatively charged electrons occupying energy levels. Most of the atom's mass is concentrated in the nucleus.

Protons have a positive charge (+1), neutrons have no charge (0), and electrons have a negative charge (-1). Protons and neutrons are found in the nucleus, while electrons occupy the surrounding regions. Protons and neutrons have similar masses; electrons are much lighter.

The atomic number is the number of protons in an atom's nucleus. It determines which element the atom is. For example, carbon always has 6 protons. In a neutral atom, the number of electrons equals the number of protons.

Mass number is the total number of protons and neutrons in the nucleus. Formula: A = protons + neutrons. Therefore, neutrons = A - Z. Example: carbon-14 has 6 protons and 8 neutrons because 14 - 6 = 8.

An element contains atoms with the same number of protons. A molecule consists of two or more chemically bonded atoms. A compound contains two or more different elements chemically bonded together, such as water (H2O).

A mixture contains two or more substances physically combined rather than chemically bonded. The substances retain their individual properties and can often be separated by physical methods. Air is an example of a mixture.

Isotopes are atoms of the same element with the same number of protons but different numbers of neutrons. They have the same atomic number but different mass numbers. Example: carbon-12 and carbon-14 are both carbon isotopes.

A radionuclide is an atom with an unstable nucleus that undergoes spontaneous radioactive decay. It releases particles or electromagnetic radiation as it moves towards greater stability. Not all isotopes are radioactive.

Electrons occupy specific energy levels called shells, labelled K, L, M and so on, starting nearest the nucleus. The K shell is the innermost. Electrons closer to the nucleus are generally more tightly bound and require more energy to remove.

Binding energy is the minimum energy needed to remove an electron from its shell. Inner-shell electrons have greater binding energies. These energies are important in photoelectric absorption and characteristic X-ray production.

Excitation occurs when an electron absorbs energy and moves to a higher energy level without leaving the atom. Ionisation occurs when enough energy is transferred to remove an electron completely, producing a positively charged ion and a free electron.

Understanding atomic structure explains how X-rays interact with tissue. Photons can eject bound electrons through photoelectric absorption or Compton scattering. Electron-shell transitions can also produce characteristic X-rays. These interactions contribute to radiation energy deposition.