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atom

 [at´om]
the smallest particle of an element that has all the properties of the element. adj., adj atom´ic.ƒThere are two main parts of an atom: the nucleus and the electron cloud. The nucleus is made up of protons, which carry a positive electrical charge, and (except in hydrogen) neutrons, which contain one proton and one electron and carry no electrical charge. The electron cloud is made up of particles called electrons, which carry a negative electrical charge and move in orbits or “shells” around the nucleus. Different atoms have different numbers of protons, neutrons, and electrons in their makeup.

In a chemical change, atoms do not break up but act as individual units. The chemical behavior of an atom is controlled by the number and spatial arrangement of electrons in orbit around the nucleus. The atoms of radioactive elements are very unstable and are capable of emitting nuclear particles in a stream or “ray;” these particles are called radiations.

The atomic number of an element is the number of free protons (those not in neutrons) in the nucleus; it is equal to the net positive charge of the nucleus. The atomic weight is the weight of an atom of a substance as compared with the weight of an atom of carbon-12, which is taken as 12.
Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

at·om

(at'ŏm),
Formerly 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 making up most of the mass of the atomic nucleus. It is now known that subatomic particles are further classified into hadrons, leptons, and quarks.
[G. atomos, indivisible, uncut]
Farlex Partner Medical Dictionary © Farlex 2012

at·om

(at'ŏm)
Formerly 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 divisible into hadrons, leptons, and quarks.
[G. atomos, indivisible, uncut]
Medical Dictionary for the Health Professions and Nursing © Farlex 2012

atom

the smallest particle of matter possessing the properties of an element.
Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

at·om

(at'ŏm)
Formerly considered the ultimate particle of an element, discovery of radioactivity demonstrated the existence of subatomic particles, notably protons, neutrons, and electrons, the first two making up most of the mass of the atomic nucleus. Subatomic particles are now further classified into hadrons, leptons, and quarks.
[G. atomos, indivisible, uncut]
Medical Dictionary for the Dental Professions © Farlex 2012
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