Atom

The smallest part of any material that cannot be broken up by chemical means. Each atom has a center (the nucleus) which contains protons and neutrons. Electrons orbit around the nucleus. The atom is primarily empty space. If the nucleus of an atom were the size of the button on a baseball pitcher's cap, the electrons would be like dust particles revolving around the outside of the baseball stadium.

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Once considered the ultimate particle of an element, believed to be as indivisible as its name indicates. Discovery of radioactivity demonstrated the existence of subatomic particles, notably protons, neutrons, and electrons, the first two comprising most of the mass of the atomic nucleus. We now know that subatomic particles are further classified into hadrons, leptons, and quarks. [G. atomos, indivisible, uncut]

- activated a. an a. possessing more than normal energy as a result of input of energy. SEE ALSO: excited state. SYN: excited a..

- Bohr a. a concept or model of the a. in which the negatively charged electrons move in circular or elliptical orbits around the positively charged nucleus, energy being emitted or absorbed when electrons change from one orbit to another.

- excited a. SYN: activated a..

- ionized a. an a. that possesses an electrostatic charge as a result of loss or gain of electrons; e.g., H⁺, Ca²⁺, Cl⁻, O²⁻.

- labeled a. a radioactive a., or a stable but rare one, which by its presence in a molecule helps localization or measurement of that molecule. SYN: tagged a..

- nuclear a. a concept or model of the a. characterized by the presence of a small, massive nucleus at its center.

- quaternary carbon a. an a. of carbon to which four other carbon atoms are attached.

- radioactive a. an a. with an unstable nucleus, which emits particulate or electromagnetic radiation (radioactive emission) to achieve greater stability. See radionuclide, half-life, Becquerel.

- recoil a. the remainder of an a. from which a nuclear particle has been emitted or ejected at high velocity; the remainder recoils with a velocity inversely proportional to its mass.

- stripped a. an a. minus all its electrons; a nucleus.

- tagged a. SYN: labeled a..

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at·om 'at-əm n the smallest particle of an element that can exist either alone or in combination

atom·ic ə-'täm-ik adj

atom·i·cal·ly -i-k(ə-)lē adv

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at·om (atґəm) [Gr. atomos indivisible] any one of the ultimate particles of a molecule or of any matter. An atom is the smallest particle of an element that is capable of entering into a chemical reaction. The atom consists of a minute central nucleus, in which practically all of the mass of the atom is concentrated, and of surrounding electrons. The nucleus is positively charged; the amount of the charge corresponds to the atomic number of the atom. See Appendix 5. In a neutral atom the surrounding negative electrons are equal in number to the positive charges on the nucleus. The number and arrangement of these electrons determine all the properties of the atom except its atomic weight and its radioactivity. atomic adj