Changes

[[Image:Helium_atom_QM.svg.png|right]]

[[Image:Helium_atom_QM.svg.png|right]]

In [[chemistry]] and [[physics]], an '''atom''' ([[Greek language|Greek]] ''ἄτομος'' or ''átomos'' meaning "indivisible") is the smallest particle still characterizing a [[chemical element]]."Atom" in IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/A00493.html]. Accessed March 2007.

In [[chemistry]] and [[physics]], an '''atom''' ([[Greek language|Greek]] ''ἄτομος'' or ''átomos'' meaning "indivisible") is the smallest particle still characterizing a [[chemical element]]."Atom" in IUPAC Compendium of Chemical Terminology, Electronic version, [http://goldbook.iupac.org/A00493.html].  

An atom consists of a dense [[atomic nucleus]] of positively-charged [[proton]]s and electrically-neutral [[neutrons]], surrounded by a much larger [[electron cloud]] consisting of negatively-charged [[electron]]. An atom is electrically neutral if it has the same number of protons as electrons. The number of protons in an atom defines the [[chemical element]] to which it belongs, while the number of neutrons determines the [[isotope]] of the element.

An atom consists of a dense [[atomic nucleus]] of positively-charged [[proton]]s and electrically-neutral [[neutrons]], surrounded by a much larger [[electron cloud]] consisting of negatively-charged [[electron]]. An atom is electrically neutral if it has the same number of protons as electrons. The number of protons in an atom defines the [[chemical element]] to which it belongs, while the number of neutrons determines the [[isotope]] of the element.

==History==

==History==

The idea that matter is composed of discrete units and can not be divided into any arbitrarily small quantities has been around for thousands Of years.

The idea that matter is composed of discrete units and can not be divided into any arbitrarily small quantities has been around for thousands Of years.