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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.

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]

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]

atom

the smallest particle of matter possessing the properties of an element.

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]
References in periodicals archive ?
Science historians are likely to say that Nagaoka abandoned his "Saturnian" model of the atom (the 1904 Nagaoka paper) (3) and turned to spectroscopy in order to understand the structure of the atom. This is wrong.
Wherever he went he received civic receptions, and halls were packed to overflowing to hear him give illustrated talks on the structure of the atom. Rutherford declared that he had always been very proud of being a New Zealander.
Discoveries made and theories formulated at the turn of the century set off a sequence of research that challenged existing ideas from the structure of the atom to the structure of the universe.
The topics include the structure of the atom, radioactive decay, nuclear reactions, radioactivity at work, applications of nuclear chemistry, and the chemistry of the f-block elements.
That would surely be an accomplishment on par with deciphering the structure of the atom, discovering the expansion of the universe or winning 10 NBA titles in 21 years.
In 1925-26, the future Nobel prize-winner Max Born presented two series of lectures at the Massachusetts Institute of Technology: one on the structure of the atom, the other on the lattice theory of rigid bodies.
But until the details of the internal structure of the atom were discovered, and the nature of the changes that gave rise to radioactivity were worked out, scientists could only refer to this new energy source as atomic energy.
Chapters 5-8 present the electronic structure of the atom, infer properties of the periodic table, and apply this to mineral properties, particularly silicates.

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