The atom is a basic unit of matter that consists of a dense central nucleus surrounded by a cloud of negatively charged electrons. The atomic nucleus contains a mix of positively charged protons and electrically neutral neutrons (except in the case of hydrogen-1, which is the only stable nuclide with no neutrons). The electrons of an atom are bound to the nucleus by the electromagnetic force. Likewise, a group of atoms can remain bound to each other by chemical bonds based on the same force, forming a molecule. An atom containing an equal number of protons and electrons is electrically neutral, otherwise it is positively or negatively charged and is known as an ion. An atom is classified according to the number of protons and neutrons in its nucleus: the number of protons determines the chemical element, and the number of neutrons determines the isotope of the element.

Chemical atoms, which in science now carry the simple name of "atom," are minuscule objects with diameters of a few tenths of a nanometer and tiny masses proportional to the volume implied by these dimensions. Atoms can only be observed individually using special instruments such as the scanning tunneling microscope. Over 99.94% of an atom's mass is concentrated in the nucleus, with protons and neutrons having roughly equal mass. Each element has at least one isotope with an unstable nucleus that can undergo radioactive decay. This can result in a transmutation that changes the number of protons or neutrons in a nucleus. Electrons that are bound to atoms possess a set of stable energy levels, or orbitals, and can undergo transitions between them by absorbing or emitting photons that match the energy differences between the levels. The electrons determine the chemical properties of an element, and strongly influence an atom's magnetic properties. The principles of quantum mechanics have been successfully used to model the observed properties of the atom.

Read more about Atom:  Etymology, Identification, Origin and Current State

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... The radical mechanism starts with abstraction of the hydrogen atom from the substrate to form QH (the rate determining step), hydroxyl bridged compound Q and the free alkyl radical ... As the radical approaches, the H atom of the alkane leave the coplanar tricoordinate O environment and bends upward to create a tetrahedral. 2e- reduction process of bridging the O1 atom to give a water molecule, followed by elimination of the alcohol and regeneration of the enzyme ...
Atom - Origin and Current State - Rare and Theoretical Forms
... The most likely candidate for a stable superheavy atom, unbihexium, has 126 protons and 184 neutrons ... although the baryogenesis theories may offer an explanation.) As a result, no antimatter atoms have been discovered in nature ... Other exotic atoms have been created by replacing one of the protons, neutrons or electrons with other particles that have the same charge ...

Famous quotes containing the word atom:

    All you of Earth are idiots!... First was your firecracker, a harmless explosive. Then your hand grenade. They begin to kill your own people a few at a time. Then the bomb. Then a larger bomb, many people are killed at one time. Then your scientists stumbled upon the atom bomb—split the atom. Then the hydrogen bomb, where you actually explode the air itself.
    Edward D. Wood, Jr. (1922–1978)

    Idealism sees the world in God. It beholds the whole circle of persons and things, of actions and events, of country and religion, not as painfully accumulated, atom after atom, act after act, in an aged creeping Past, but as one vast picture, which God paints on the instant eternity, for the contemplation of the soul.
    Ralph Waldo Emerson (1803–1882)

    The unleashed power of the atom has changed everything save our modes of thinking and we thus drift toward unparalleled catastrophe.
    Albert Einstein (1879–1955)