see main article subatomic particles
Up until 1961, the subatomic particles were thought to consist of only protons, neutrons and electrons. However, protons and neutrons themselves are now known to consist of varieties of a still smaller particle called the quark, and the electron is considered a type of lepton. Therefore in modern atomic theory, the two basic constituents of matter are the lepton and the quark of which the above three particles of the atom are composed. All particles exhibit a wave-particle duality so that the electron is better understood as a wave when drawn about a nucleus. Each electron has its own wavefunction which is called an orbital.
see main article electron configuration
The chemical behavior of atoms is largely due to interactions between electrons. Electrons of an atom remain within certain, predictable electron configurations. Electrons fall into shells based on their relative energy level. Generally, the higher the energy level of a shell, the further away it is from the nucleus. The electrons in the outermost shell, called the valence electrons, have the greatest influence on chemical behavior. Core electrons (those not in the outer shell) play a role, but it is usually in terms of a secondary effect due to screening of the positive charge in the atomic nucleus.
An electron shell can hold up to 2n2 electrons, where n is the number of the shell. Whichever occupied shell is currently most outward is the valence shell, even if it only has one electron. In the most stable state, an atom's electrons will fill up its shells in order of increasing energy. Under some circumstances an electron may be excited to a higher energy level (that is, it absorbs energy from an external source and leaps to a higher shell), leaving a space in a lower shell, but at some point it will fall back to its previous level, emitting its excess energy as a photon.
Electron shells have distinctive shapes denoted by letters. In the illustration, the letters s, p, and d describe the shape of the atomic orbital. Electrons also have another property that describes their configuration called spin due to the fact that they rotate in space and so have angular momentum. The other intrinsic property of an electron is its magnetic moment along its spin axis.
The constituent protons and neutrons of the nucleus are collectively called nucleons. Nuclei can exist in states of different energy, but ordinary stable nuclei are always in the most bound state.
Nuclei can undergo transformations that affect their binding energies. When nuclei transformations take place spontaneously, this process is called radioactivity. The types of radiation discovered were labeled alpha decay, beta decay and gamma radiation.
Nuclear transformations also take place in nuclear reactions. In nuclear fusion, two light nuclei come together and merge into a single heavier nucleus. Fission is the division of a nucleus into two or more smaller nuclei.
see main article Standard Model
Unlike planets revolving around the sun, the electron is not held around the nucleus of the atom by gravity, but rather by electromagnetism. The atom is governed by the strong, weak, and electromagnetic fundamental forces.
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