atom

atom (at´om)

The once 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 divisible ino 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.excited a;
Bohr's 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. 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.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 a.'s 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. labeled a

 

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