Atomic and nuclear physics both describe the physics of the very small. When working with such small objects, your intuition built from your understanding of classical mechanics often fails. This is the realm of quantum mechanics, short range nuclear forces, electromagnetic radiation and the standard model of particle physics.

Atomic physics is the branch of physics that deals with the structure of the atom, associated energy states, and the interaction of the atom with particles and fields. In contrast, nuclear physics focuses specifically on the goings-on inside the atomic nucleus, which is described in more detail in the next section.

There are several items of study in particle physics. First and foremost is the structure of the atom itself. Atoms consist of a tightly bound nucleus, which contains protons and neutrons, and a diffuse electron cloud.

Given that the nucleus is usually on the order of 10^-15 to 10^-14 m in diameter, and atoms themselves are on the order of 10^-10 m in diameter (and the size of the electrons is negligible), it turns out that atoms are mostly empty space. Of course they don’t seem like they are and all of the matter made of atoms certainly feels like substance.

The reason atoms do not seem like they are mostly empty space is that you are also made of atoms, and all atoms interact with electromagnetic energy. Even though your hand, which is made up of mostly empty-space atoms, presses against a table, also made up of mostly empty space, it does not pass through the table because of the electromagnetic forces between the atoms as they come in contact.

The neutrino, a particle that does not interact with the electromagnetic force, is able to pass through most atomic material virtually undetected, however. In fact, 100 trillion neutrinos pass through your body every second!

Atoms are classified by atomic number in the periodic table. The atomic number is the number of protons the atom contains in its nucleus. This number defines the element.

While a given element will always have the same number of protons, it can contain different numbers of neutrons. Different isotopes of an element contain differing numbers of neutrons. Some isotopes are more stable than others (meaning less likely to spontaneously decay into something else), and this stability typically depends on the number of neutrons, which is why, for most elements, the majority of the atoms tend to be of one specific isotope.

The number of electrons an atom contains determines whether it is ionized, or charged. A neutral atom contains the same number of electrons as protons, but sometimes atoms can gain or lose electrons and become charged. How easily an atom gains or loses electrons depends on its electron orbital structure.

The hydrogen atom is the simplest atom, containing only one proton in its nucleus. The three most stable isotopes of hydrogen are protium (containing no neutrons), deuterium (containing one neutron) and tritium (containing two neutrons) with protium being the most abundant.

Different models of the atom have been proposed over the years, leading to the current model. Early work was done by Ernest Rutherford, Niels Bohr and others.

As mentioned, atoms interact with the electromagnetic force. The protons in an atom carry positive charge and the electrons carry negative charge. The electrons in the atom can absorb electromagnetic radiation and achieve a higher energy state as a result, or emit radiation and move to a lower energy state.

One key property of this absorbing and emitting of radiation is that atoms absorb and emit radiation only at very specific quantized values. And for each different type of atom, those specific values are different.

A hot gas of atomic material will emit radiation at very specific wavelengths. If light coming from this gas is passed through a spectroscope, which spreads the light out in a spectrum by wavelength (like a rainbow), distinct emission lines will appear. The set of emission lines coming from the gas can be read almost like a barcode telling you exactly what atoms are in the gas.

Similarly, if a continuous spectrum of light is incident on a cool gas, and the light that passes through that gas is then passed through a spectroscope, you would see a continuous spectrum with dark gaps at the specific wavelengths that the gas absorbed. This absorption spectrum will look like the inverse of the emission spectrum, the dark lines appearing where the bright lines were for the same gas. As such, it can also be read like a barcode telling you the composition of the gas. Astronomers use this all the time to determine the composition of material in space.

Nuclear physics focuses on the atomic nucleus, nuclear reactions and the interaction of the nucleus with other particles. It explores radioactive decay, nuclear fusion and nuclear fission, and binding energy, among other topics.

The nucleus contains a tightly bound clump of protons and neutrons. However, these are not fundamental particles. Protons and neutrons are made of still smaller particles called quarks.

Quarks are particles with fractional charge, and somewhat silly names. They come in six so-called flavors: up, down, top, bottom, strange and charm. A neutron is made up of two down quarks and an up quark, and a proton is made of two up quarks and a down quark. The quarks in each nucleon are tightly bound by the strong nuclear force.

The strong nuclear force is mediated by particles called gluons. Are you sensing a theme? The scientists had a lot of fun naming these particles! Gluons, of course, “glue” the quarks together. The strong nuclear force acts at only a very short range – at a distance comparable to the diameter of the average-sized nucleus.

Every isolated neutron has a mass of 1.6749275 × 10^-27 kg, and every isolated proton has a mass of 1.6726219 × 10^-27 kg; however, when bound together in an atomic nucleus, the atomic mass is not the sum of its constituent parts due to something called binding energy.

By becoming tightly bound, the nucleons achieve a lower energy state as a result of some of the total mass they had as individual particles being converted into energy. This mass difference that is converted to energy is called the binding energy of the nucleus. The relationship that describes how much energy corresponds to a given amount of mass is Einstein's famous E = mc² equation where m is the mass, c is the speed of light and E is the energy.

A related concept is the binding energy per nucleon, which is the total binding energy of a nucleus averaged over its constituent parts. The binding energy per nucleon is a good indicator of how stable a nucleus is. A low binding energy per nucleon indicates that a more favorable state of lower total energy might exist for that particular nucleus, meaning it will likely want to either split apart or fuse with another nucleus under the proper conditions.

In general, nuclei lighter than iron nuclei tend to achieve lower energy states, and higher binding energy per nucleon, by fusing with other nuclei, while nuclei that are heavier than iron tend to achieve lower energy states by breaking apart into lighter nuclei. The processes by which these changes occur are described in the next section.

The main focus of nuclear physics is on studying fission, fusion and decay of atomic nuclei. These processes are all driven by a fundamental notion that all particles prefer lower energy states.

Fission occurs when a heavy nucleus breaks apart into smaller nuclei. Very heavy nuclei are more prone to doing this because they have a smaller binding energy per nucleon. As you might recall, there are a few forces governing what is going on in an atomic nucleus. The strong nuclear force tightly binds the nucleons together, but it is a very short-range force. So for very large nuclei, it is less effective.

The positively charged protons in the nucleus also repel each other via the electromagnetic force. This repulsion must be overcome by the strong nuclear force and can also be mediated by having enough neutrons around. But the larger the nucleus, the less favorable the force balance is for stability.

Hence larger nuclei tend to want to break apart either via radioactive decay processes, or via fission reactions such as those that occur in nuclear reactors or fission bombs.

Fusion occurs when two lighter nuclei achieve a more favorable energy state by combining into a heavier nucleus. However, in order for fission to occur, the nuclei in question must get close enough to each other so that the strong nuclear force can take over. This means that they must be moving fast enough so that they can overcome electric repulsion.

Nuclei move around quickly in extreme temperatures, so this condition is often required. This is how nuclear fusion is able to take place in the extremely hot core of the sun. To this day, scientists are still trying to find a way to make cold fusion occur – that is, fusion at lower temperatures. Since energy is released in the fusion process and does not leave radioactive waste like fission reactors tend to do, it would be an incredible energy resource if achieved.

Radioactive decay is a common means by which nuclei undergo changes to become more stable. There are three main types of decay: alpha decay, beta decay and gamma decay.

In alpha decay, a radioactive nucleus releases an alpha particle (a helium-4 nucleus) and becomes more stable as a result. Beta decay comes in a few varieties, but in essence results from either a neutron becoming a proton or a proton becoming a neutron and releasing a β^- or β⁺ particle (an electron or a positron). Gamma decay occurs when an nucleus in an excited state releases energy in the form of gamma rays, but maintains its overall number of neutrons and protons.

The study of nuclear physics extends into the larger field of particle physics, which aims to understand the workings of all fundamental particles. The standard model classifies particles into fermions and bosons, and then further classifies fermions into quarks and leptons, and bosons into gauge and scalar bosons.

Bosons do not obey number conservation laws, but fermions do. There is also a law of conservation for both lepton and quark numbers in addition to other conserved quantities. Interactions of the fundamental particles are mediated by the energy-carrying bosons.

Applications of nuclear and atomic physics are abundant. Nuclear reactors in nuclear powerplants create clean energy by harnessing the energy released during fission processes. Nuclear medicine makes use of radioactive isotopes for imaging. Astrophysicists use spectroscopy to determine composition of distant nebulae. Magnetic resonance imaging allows doctors to create detailed images of their patients’ insides. Even X-ray technology makes use of nuclear physics.