Translation. Region: Russian Federation –
Source: International Atomic Energy Agency –
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What's what in the nuclear sphere?
Atoms are the fundamental building blocks of matter. Everything that surrounds us—air, water, rocks, plants, and animals—as well as ourselves and our bodies, is made up of atoms.
January 21, 2026
Emma Midgley, Public Information and Communications Bureau
An atom is the smallest particle of a chemical element, containing all of its chemical properties. It consists of protons, neutrons, and electrons.
Atoms are extremely tiny; they are the smallest particles of a chemical element that retain all of its chemical properties. The ancient Greeks believed them to be the smallest particles in existence—the word "atom" itself means "indivisible" in Greek. The thickness of a human hair contains approximately 500,000 carbon atoms.
A single strontium atom becomes visible because it absorbs laser light and then re-emits it. The distance between the electrodes in the photograph is 2 millimeters. Photo: David Nadlinger / University of Oxford
Atoms cannot be seen with the naked eye or even with a standard optical microscope, as they are too small to deflect visible light waves. However, atoms can be viewed with an electron microscope, which generates electron waves capable of interacting with atoms. In the photograph above, the atom became "visible" because it absorbed and then re-emitted laser light.
What do atoms look like? Scientists have changed their minds over the centuries.
What are atoms made of?
Every atom consists of three types of particles: protons, neutrons, and electrons. At the center of the atom is a dense nucleus containing protons and neutrons, yet it is significantly smaller than the atom as a whole. If the nucleus of an atom were the size of a dice, the atom itself would be the size of a sports stadium.
Protons have a positive electrical charge, while neutrons are electrically neutral. The nucleus does not decay due to nuclear forces of attraction. These forces bind protons and neutrons at distances close to the size of the nucleus. At such distances, the nuclear force is significantly stronger than the electrical repulsion between protons (otherwise, due to their equal charges, they would repel each other). At greater distances, the nuclear force quickly weakens to negligible strength.
The number of protons in an atom's nucleus determines what element it is. For example, an atom with one proton is hydrogen, while an atom with eight protons is oxygen.
The nucleus of an atom is surrounded by a cloud of electrons—negatively charged particles. The nucleus and electrons are bound together by the Coulomb force—a force in physics that determines the repulsion or attraction between similarly charged particles. However, if an electron receives sufficient energy, it can separate from the atom, turning the atom into a positively charged ion.
The atom in the center of the IAEA logo contains four electrons—it is a neutral, non-ionic beryllium atom.
What are ions?
Atoms with an equal number of negatively charged electrons and positively charged protons are neutral because their charges cancel each other out. If an atom gains or loses electrons, it becomes an ion.
The electric field of a neutral atom is weak, while an electrically charged or ionized atom has a strong electric field—because of this, it is strongly attracted to oppositely charged ions and molecules. Atoms can become ionized through collisions with other atoms, ions, and subatomic particles. They can also be ionized by exposure to gamma or X-ray radiation. Ionizing radiation is radiation with sufficient energy to separate an electron from an atom. Furthermore, exposure to such radiation can alter the chemical composition of a substance, which can lead, for example, to DNA damage in living tissue.
Most atoms on Earth are stable primarily because of the balanced composition of particles (neutrons and protons) in their nuclei.
However, in some types of unstable atoms, the number of protons and neutrons in their nuclei prevents them from holding together. This "decay" of the atom releases energy in the form of radiation (e.g., alpha particles, beta particles, gamma rays, or neutrons), which—under controlled conditions and with appropriate safety precautions—can be used for a variety of purposes.
Ernest Rutherford: Inventor of the First Nuclear Fission Device
In 1917, scientist Ernest Rutherford discovered that when beams of radioactive alpha particles collide with nitrogen gas, the nitrogen atom splits into oxygen and a hydrogen nucleus. This subatomic particle (the hydrogen nucleus) was later renamed the proton.
Rutherford's discovery led to the creation of the first particle accelerator, initially called an "atom smasher." This powerful device could use an electric field to accelerate charged particles to high energies along a specific trajectory and, using strong magnets, to create beams of single charged particles. When these fast-moving particles, whose speeds can approach the speed of light, hit a target, the atoms in the target were disintegrated.
Additionally, particle accelerators can be used to create radioactive material – atoms are bombarded with charged particles to turn them into other, unstable atoms, such as technetium-99m, used for medical imaging, or radioisotopes for targeted cancer therapy.
Today, particle accelerators are also used to sterilize medical equipment, study the origins of the universe (for example, at the Large Hadron Collider), analyze air samples, and refine materials and increase their resistance to damage. There are many types of particle accelerators, including ion implanters, electron beam accelerators, cyclotrons, synchrotrons, linear accelerators (linacs), and electrostatic accelerators.
Splitting an atom: nuclear fission reaction
In the 1930s, scientists discovered that when certain uranium atoms are bombarded with neutrons—uncharged subatomic particles—they can split into two fragments and emit a number of neutrons, releasing a huge amount of energy. This process is called fission.
Of all the naturally occurring elements on Earth, uranium has the highest atomic number, with a nucleus containing 92 protons. Uranium-235 fissiles more easily than other isotopes because its nucleus is relatively unstable and readily absorbs neutrons, causing it to split into two lighter atoms. However, only 0.7% of the uranium in the Earth's crust is of this fissile type.
The fission process can initiate a nuclear chain reaction. Each splitting of a uranium-235 atom releases an average of 2.5 neutrons. These neutrons, in turn, can split other fissile uranium nuclei, releasing even more neutrons. However, these "fast" neutrons initially propagate with too much energy, making them ineffective in initiating fission. The use of "moderators," such as water or graphite, can reduce the speed of neutrons. Upon collision with hydrogen or carbon atoms, neutrons lose most of their energy, becoming "thermal" or "slow" neutrons, which are much more likely to split other uranium nuclei.
Nuclear fission technology currently accounts for 10% of the world's carbon-free energy generation because the reaction does not produce carbon dioxide.
What happens to atoms during nuclear fusion?
Nuclear fusion is a process in which two light atomic nuclei combine to form a single heavier nucleus, releasing a massive amount of energy. This theory was first formulated in the 1920s.
Thermonuclear reactions occur in matter in a plasma state—a hot, charged gas composed of positive ions and freely moving electrons that has unique properties distinct from those of solids, liquids, or gases.
It is this reaction that provides the energy for the Sun and all other stars. To achieve fusion on the Sun, nuclei must collide at extremely high temperatures—around one hundred million degrees Celsius. This high temperature provides them with enough energy to overcome their mutual electrical repulsion. Once the nuclei overcome this repulsion and are very close to each other, the nuclear forces of attraction between them become stronger than the electrical repulsion, allowing them to fuse.
For this to occur, the nuclei must be confined to a confined space, which increases the likelihood of their collision. On the Sun, the conditions for thermonuclear fusion are created by the colossal pressure generated by its powerful gravity.
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