atoms
An illustration of the helium atom, depicting the nucleus (pink) and the electron cloud distribution (black). The nucleus (upper right) in helium-4 is in reality spherically symmetric and closely resembles the electron cloud, although for more complicated nuclei this is not always the case.
Standard Model of Elementary Particles
In particle physics, an elementary particle refers to a particle of which other, larger particles are composed. For example, atoms are made up of smaller particles known as electrons, protons, and neutrons. The proton and neutron, in turn, are composed of more elementary particles known as quarks. One of the outstanding problems of particle physics is to find the most elementary particles - or the so-called fundamental particles - which make up all the other particles found in Nature, and are not themselves made up of smaller particles.
Atomic, molecular, and optical physics is the study of matter-matter and light-matter interactions on the scale of single atoms or structures containing a few atoms. The three areas are grouped together because of their interrelationships, the similarity of methods used, and the commonality of the energy scales that are relevant.
Atomic physics is distinct from nuclear physics, despite their association in the public consciousness. Atomic physics is unconcerned with the nuclear processes studied in nuclear physics, although properties of the nucleus can be important in atomic physics (e.g., hyperfine splitting).
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