What is Atomic orbital?
[[File:Electron orbitals.svg|thumb|420px|The diagram to the left shows the orbitals in order of increasing energy. To the right are examples of orbital shapes.]] '''Atomic orbitals''' are the places surrounding the [[Nucleus (physics)|nucleus]] of an atom where the [[electrons]] are most likely to be at any given time. It is a [[Function (mathematics)|mathematical function]] that describes the wave-like behavior of either one electron or a pair of electrons in an [[atom]].<ref>{{cite book|first1= Milton|last1= Orchin|first2=Roger S.|last2=Macomber|first3=Allan|last3=Pinhas|first4=R. Marshall|last4= Wilson|year=2005|url=http://media.wiley.com/product_data/excerpt/81/04716802/0471680281.pdf|title= Atomic Orbital Theory}}</ref> The word 'orbital' is used because it was thought that electrons behaved [[Bohr model|similarly to the solar system]], where the nucleus is like the [[sun]] and the [[electron]]s [[orbit]] like the [[planet]]s. The number of atomic orbitals in an [[chemical element|element]] is defined by the [[Periodic table period|period]] the element is in. [[Electrons]] move between orbitals depending on how much energy they have and how many other electrons there are. In [[atomic theory]] and [[quantum mechanics]], an atomic orbital is a quantum number. Each such orbital can be occupied by one or two electrons. The way orbitals are arranged is related to the [[electron configuration]]s of atoms. They were derived from descriptions provided by early spectroscopists of certain [[alkali metal]] [[Spectral line|spectroscopic lines]] as being '''s'''harp, '''p'''rincipal, '''d'''iffuse, and '''f'''undamental.<ref>{{cite book|first=David|last=Griffiths|year=1995|title=Introduction to Quantum Mechanics|url=https://archive.org/details/introductiontoqu00grif_848|pages=[https://archive.org/details/introductiontoqu00grif_848/page/n203 190]–191|publisher=Prentice Hall|isbn=978-0-13-124405-4}}</ref><ref>{{cite book|first=Ira|last=Levine|year=2000|title=Quantum Chemistry|url=https://archive.org/details/quantumche