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Article ID: 10
Last updated: 08 Jul, 2013
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All matter is composed atoms.  Atoms are an ordered collection of various subatomic particles.  The central nucleus contains the protons and neutrons, around this are the all important electrons.

The simple schematic diagram to the right shows this simple model of a helium atom.     

The nucleus contains two protons and two neutrons, and two electrons that exist in an orbital (s orbital) around the nucleus.

Important key aspects of atomic structure:

·         Atoms have a nucleus at their core and are surrounded by electrons, e-

·         The nucleus is comprised of protons, p+  and neutrons, n0

·         Protons are positively charged, +1 unit

·         Neutrons have no charge

·         Electrons are much smaller and negatively charged, -1 unit

·         The number of protons in the nucleus of an atom is given by a unique atomic number, Z  which defines the type of atom

·         The total number of protons and neutrons in the nucleus is given by the mass number, A

·         A neutral atom must have the same number of electrons surrounding the nucleus as there are protons in the nucleus, so Z also defines the number of electrons in a neutral atom.

·         In a collection of neutral atoms, some may have nuclei with different numbers of neutrons. These are known as isotopes of that element.


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Department of Chemistry & Chemical Biology