An atom is the smallest unit of an element that retains its chemical properties. It is made of protons, neutrons, and electrons. (Image: M. Magnaye)
Atoms are the building blocks of matter. Everything around us - from air and water, to rocks, plants and animals - as well as everything within our bodies, is made up of atoms.
They are very small, the smallest units of an element that retain the element's chemical properties. The Ancient Greeks believed they were the smallest particles in existence, and the word 'atom' is derived from 'indivisible' in Greek. A single strand of human hair is as thick as 500 000 carbon atoms stacked on top of each other.
This single atom of the metal strontium is visible in this photograph because it has absorbed and re-emitted the light of a laser. The electrodes in the picture are two millimetres apart. (Photo: David Nadlinger/Oxford University)
Atoms cannot be seen with the naked eye, or even under a standard microscope. An atom is too small to deflect visible light waves, meaning it will not show up under light-focusing microscopes. Atoms can be viewed under an electron microscope, which generate electron waves that can interact with atoms. In the picture above, the atom is 'visible' because it has absorbed and re-emitted the light of a laser.
What do atoms look like? Scientists have changed their minds over the centuries. (Infographic: M. Magnaye)
What are Atoms Made Of?
Each atom consists of three types of particles: protons, neutrons and electrons. At the centre of an atom is a dense nucleus, which contains protons and neutrons, and is much smaller than the entire atom. If the nucleus of the atom were the size of a marble, the atom would be the size of a sports stadium .
Protons have a positive electrical charge, while neutrons are neutral. The nucleus stays together due to the 'nuclear force'. T his force binds the protons and neutrons together at distances close to the size of the nucleus. The nuclear force at this distance is much stronger than the electrical repulsion between the protons (as they have equal charges, they would otherwise repel each other). At larger distances this nuclear force rapidly becomes insignificantly small.
The number of protons in an atom's nucleus determines which element it is. For example, an atom with one proton is hydrogen, while an atom with eight protons is oxygen.
Surrounding the nucleus is a cloud of electrons - negatively charged particles. The atomic nucleus and the electrons are bound together by Coulomb force interactions - the forces in physics that describe the repulsion or attraction between these charged particles. However, when an electron gains energy, it can separate from the atom, causing the atom to become a positively charged ion.
The atom at the centre of the IAEA's logo has four electrons - meaning it is Beryllium if it is neutral and not ionized. (Infographic: M. Magnaye)
What are Ions?
Atoms with the same number of negatively charged electrons and positively charged protons are neutral, as the charges cancel each other out. If an atom gains or loses electrons it becomes an ion.
(Infographic: M. Magnaye)
While the electric field of a neutral atom is weak, an electrically charged or ionized atom has a strong electrical field, making it strongly attracted to oppositely-charged ions and molecules. Atoms can be ionized by collisions with other atoms, ions and subatomic particles. They can also be ionized by exposure to gamma or X ray radiation. Ionizing radiation refers to radiation that has enough energy to break an electron away from an atom. It can also chemically alter material, for example damaging DNA in living tissue.
(Infographic: M. Magnaye)
Most atoms on Earth are stable, mainly thanks to a balanced composition of particles (neutrons and protons) in their nucleus.
However, in some types of unstable atoms, the composition of the number of protons and neutrons in their nucleus does not allow them to hold those particles together. In this case, the atom 'decays', and releases energy in the form of radiation (for example alpha particles, beta particles, gamma rays or neutrons), which, when safely harnessed and used, can produce various benefits.
