Particles

Molecules

Molecules are particles that are formed from chains of atoms. The constitution of molecules can vary, and they can have very complex structures. Molecules do not have an electrical charge. Examples of molecules include:

Atom

Atoms, the basic blocks of chemistry, are the smaller units of matter that can have a neutral charge. Atoms of different chemical elements have different numbers of neutrons, protons, and electrons, which gives them their distinctive properties.

Protons and neutrons are the particles that exist in the atom's nucleus. Electrons move around the nucleus.

Figure 1. An atom, with protons and neutrons displayed in green and the orbiting electrons in red. Source: Manuel R. Camacho, StudySmarter.

Atoms are classified by their atomic number, which identifies the number of protons found in the nucleus. Different elements have different numbers of protons. See the following examples of atoms belonging to different elements, including their symbols and atomic numbers:

Isotopes

When an element has different numbers of neutrons and protons, it is called an isotope. Isotopes occur in nature but can also be produced in laboratories. Hydrogen has three natural isotopes, as listed below:

• Protium, which has one electron and one proton.
• Deuterium, which has one electron, one proton, and one neutron.
• Tritium, which has one electron, one proton, and two neutrons.

Figure 2. The isotopes of protium, deuterium, and tritium (from left to right). Source: Manuel R. Camacho, StudySmarter.

Classical subatomic particles

In classical atomic theory, atoms are composed of three subatomic particles: electrons, protons, and neutrons, the first two of which have an electrical charge, while neutrons are neutral. The classical theory also states that there is another particle, the photon, which has no mass and is responsible for electromagnetic waves, as in light. In summary, we have:

• Protons, which are present in the atom's nucleus and have a positive charge.
• Electrons, which move around the atom's nucleus and have a negative charge.
• Neutrons, which, like protons, are present in the atom's nucleus and do not have any charge.
• Photons, which are responsible for carrying energy in the electromagnetic spectrum, including radio waves, gamma rays, UV light, visible light, etc.

Figure 3. What we perceive as sunlight is, in fact, particles called 'photons'. Source: Andrey Larin, Flickr (CC0 1.0).

New particles in the 20th century

In the first quarter of the 20th century, only a few classical atomic particles were known, but discoveries predicted electrons with a positive charge. These predictions, made by the British scientist Paul Dirac, were confirmed by American physicist Carl Anderson after studying comics rays. The positively charged electron, also known as a 'positron', was the first proof of antimatter.

The new discoveries led to a classification of particles into large groups of families known as leptons, hadrons, and bosons.

Leptons, hadrons, and atomic forces

On a subatomic scale, we now know of two other forces in addition to the gravitational and electromagnetic ones. The first of these forces acting on an atomic scale is the strong nuclear force, which is responsible for keeping particles together. The second additional force is the weak nuclear force.

Leptons, which include electrons, neutrinos, taus, and muons, are not affected by the strong force, while hadrons, which comprise protons, neutrons and mesons, are affected by it.

Bosons

Bosons are responsible for three of the four fundamental forces of the universe: electromagnetism, the weak nuclear force, and the strong nuclear force. The particle responsible for gravity has not been found and exists only in theory (graviton). In the boson family, we find the photon, the gluon, the Z boson, the W boson, and the Higgs boson.

Particles resulting from radioactive processes

There are particles that are created as part of radioactive decay where an atom emits particles as it breaks. These particles include alpha particles and beta particles:

• Alpha particles: composed of two neutrons and two protons, they form the helium nucleus.
• Beta particles: composed of high-velocity electrons, they can be positive or negative (antimatter).