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The '''Aufbau principle''' states that, hypothetically, [[electron]]s orbiting one or more [[atom]]s fill the lowest available [[energy level]]s before filling higher levels (e.g., 1s before 2s). In this way, the electrons of an [[atom]], [[molecule]], or [[ion]] harmonize into the most stable [[electron configuration]] possible.
''[[wikt:Aufbau|Aufbau]]'' is a German noun that means "construction". The Aufbau principle is sometimes called the '''building-up principle''' or the '''Aufbau rule'''.
The details of this "building-up" tendency are described mathematically by [[atomic orbital]] functions. Electron behavior is elaborated by other principles of [[atomic physics]], such as [[Hund's rule]] and the [[Pauli exclusion principle]]. Hund's rule asserts that even if [[Degenerate orbitals|multiple orbitals of the same energy]] are available, electrons fill unoccupied orbitals first, before reusing orbitals occupied by other electrons. But, according to the Pauli exclusion principle, in order for electrons to occupy the same orbital, they must have different [[Spin (physics)|spin]]s (-1/2 and 1/2).
A version of the Aufbau principle known as the [[nuclear shell model]] is used to predict the configuration of [[proton]]s and [[neutron]]s in an [[atomic nucleus]].<ref>{{cite book | last1 = Cottingham | first1 = W. N. | last2 = Greenwood | first2 = D. A. | title = An introduction to nuclear physics | publisher = Cambridge University Press | date = 1986 | ISBN = 0 521 31960 9 | chapter = Chapter 5: Ground state properties of nuclei: the shell model }}</ref>
==Madelung energy ordering rule==
[[File:Klechkovski rule.svg|thumb|upright=1.5|Order in which orbitals are arranged by increasing energy according to the Madelung rule. Each diagonal red arrow corresponds to a different value of {{nowrap|''n + ℓ''.}}]]
The order in which these orbitals are filled is given by the ''n + ℓ rule'', also known as the '''Madelung rule''' (after [[Erwin Madelung]]), or the '''Janet rule''' or the '''Klechkowski rule''' (after [[Charles Janet]] or [[Vsevolod Klechkovsky]] in some, mostly French and Russian-speaking, countries), or the '''diagonal rule'''.<ref>{{cite web | url = http://www.wyzant.com/resources/lessons/science/chemistry/electron_configuration | title = Electron Configuration | publisher = [[WyzAnt]] }}</ref> Orbitals with a lower ''n + ℓ'' value are filled before those with higher ''n + ℓ'' values. In this context, ''n'' represents the [[principal quantum number]] and ''ℓ'' the [[azimuthal quantum number]]; the values ''ℓ'' = 0, 1, 2, 3 correspond to the ''s'', ''p'', ''d'', and ''f'' labels, respectively.
The rule is based on the total number of nodes in the atomic orbital, ''n + ℓ'', which is related to the energy.<ref>{{cite book |last1 = Weinhold | first1 = Frank | last2 = Landis | first2 = Clark R. |title=Valency and bonding: A Natural Bond Orbital Donor-Acceptor Perspective |location=Cambridge |publisher=Cambridge University Press | date = 2005 | pages = 715–716 |isbn=0-521-83128-8}}</ref> In the case of equal ''n + ℓ'' values, the orbital with a lower ''n'' value is filled first. The fact that most of the ground state configurations of neutral atoms fill orbitals following this ''n + ℓ, n'' pattern was obtained experimentally, by reference to the spectroscopic characteristics of the elements.<ref>{{cite journal |last=Scerri |first=Eric R. |title=How Good is the Quantum Mechanical Explanation of the Periodic System? |journal=[[Journal of Chemical Education|J. Chem. Ed.]] |volume=75 |issue=11 |pages=1384–85 |date=1998 |url= http://www.chem.ucla.edu/dept/Faculty/scerri/pdf/How_Good_is.pdf |doi=10.1021/ed075p1384 |bibcode=1998JChEd..75.1384S }}</ref>
The Madelung energy ordering rule applies only to neutral atoms in their ground state, and even in that case, there are several elements for which it predicts configurations that differ from those determined experimentally.<ref>{{cite journal |last=Meek |first=Terry L. | last2 = Allen | first2 = Leland C. |title=Configuration irregularities: deviations from the Madelung rule and inversion of orbital energy levels |journal=[[Chemical Physics Letters|Chem. Phys. Lett.]] |volume=362 |issue=5–6 |pages=362–64 |doi=10.1016/S0009-2614(02)00919-3 |date=2002 |bibcode=2002CPL...362..362M }}</ref> [[Copper]], [[chromium]], and [[palladium]] are common examples of this property. According to the Madelung rule, the 4s orbital (''n + ℓ'' = 4 + 0 = 4) is occupied before the 3d orbital (''n + ℓ'' = 3 + 2 = 5). The rule then predicts the configuration of <sub>29</sub>Cu to be 1s<sup>2</sup>2s<sup>2</sup>2p<sup>6</sup>3s<sup>2</sup> 3p<sup>6</sup>4s<sup>2</sup>3d<sup>9</sup>, abbreviated [Ar]4s<sup>2</sup>3d<sup>9</sup> where [Ar] denotes the configuration of Ar (the preceding noble gas). However the experimental electronic configuration of the copper atom is [Ar]4s<sup>1</sup>3d<sup>10</sup>. By filling the 3d orbital, copper can be in a lower energy state. Similarly, chromium takes the electronic configuration of [Ar]4s<sup>1</sup>3d<sup>5</sup> instead of [Ar]4s<sup>2</sup>3d<sup>4</sup>. In this case, chromium has a half-full 3d shell. For palladium, the Madelung rule predicts [Kr]5s<sup>2</sup>4d<sup>8</sup>, but the experimental configuration [Kr]4d<sup>10</sup> differs in the placement of two electrons.
==History==
===The Aufbau principle in the new quantum theory===
[[File:Sommerfeld ellipses.svg|thumb|In the [[old quantum theory]], orbits with low angular momentum (''s''- and ''p''-orbitals) get closer to the nucleus.]]
The principle takes its name from the German, ''Aufbauprinzip'', "building-up principle", rather than being named for a scientist. In fact, it was formulated by [[Niels Bohr]] and [[Wolfgang Pauli]] in the early 1920s, and states that:
{{cquote|The orbitals of lower energy are filled in first with the electrons and only then the orbitals of high energy are filled.}}
This was an early application of [[quantum mechanics]] to the properties of [[electron]]s, and explained chemical properties in [[physics|physical]] terms. Each added electron is subject to the electric field created by the positive charge of the [[atomic nucleus]] ''and'' the negative charge of other electrons that are bound to the nucleus. Although in hydrogen there is no energy difference between orbitals with the same principal quantum number ''n'', this is not true for the outer electrons of other atoms.
In the [[old quantum theory]] prior to [[quantum mechanics]], electrons were supposed to occupy classical elliptical orbits. The orbits with the highest angular momentum are 'circular orbits' outside the inner electrons, but orbits with low angular momentum (''s''- and ''p''-orbitals) have high [[orbital eccentricity]], so that they get closer to the nucleus and feel on average a less strongly screened nuclear charge.
===The ''n + ℓ'' energy ordering rule===
A periodic table in which each row corresponds to one value of ''n + ℓ'' was suggested by [[Charles Janet]] in 1927. In 1936, the German physicist [[Erwin Madelung]] proposed his empirical rules for the order of filling atomic subshells, based on knowledge of atomic ground states determined by the analysis of atomic spectra, and most English-language sources therefore refer to the Madelung rule. Madelung may have been aware of this pattern as early as 1926.<ref>{{cite journal |title= The Order of Electron Shells in Ionized Atoms |last1= Goudsmit |first1= S. A. |last2= Richards |first2= Paul I. |journal= [[Proceedings of the National Academy of Sciences of the United States of America|Proc. Natl. Acad. Sci.]] |pages= 664–671 (with correction on p 906) |volume= 51 | issue= 4 |date= 1964 |url= http://www.pnas.org/content/51/4/664.full.pdf |bibcode = 1964PNAS...51..664G |doi = 10.1073/pnas.51.4.664 }}</ref> In 1962 the Russian agricultural chemist [[V.M. Klechkovsky|V.M. Klechkowski]] proposed the first theoretical explanation for the importance of the sum ''n + ℓ'', based on the statistical [[Thomas–Fermi model]] of the atom.<ref>{{cite journal | title = Theoretical justification of Madelung's rule | journal = [[Journal of Chemical Education|J. Chem. Ed.]] | url = http://jchemed.chem.wisc.edu/Journal/Issues/1979/Nov/jceSubscriber/JCE1979p0714.pdf | last = Wong | first = D. Pan | date = 1979 | issue = 11 | pages = 714–718 | volume = 56 | doi = 10.1021/ed056p714|bibcode = 1979JChEd..56..714W }}</ref> Many French- and Russian-language sources therefore refer to the Klechkowski rule.
In recent years some authors have challenged the validity of Madelung's rule in predicting the order of filling of atomic orbitals. For example, it has been claimed, not for the first time, that in the case of the scandium atom a 3d orbital is occupied 'before' the occupation of the 4s orbital. In addition to there being ample experimental evidence to support this view, it makes the explanation of the order of ionization of electrons in this and other transition metals far more intelligible, given that 4s electrons are invariably preferentially ionized.<ref>{{cite journal | title = The Trouble With the Aufbau Principle | journal = [[Education in Chemistry]] | url = http://www.rsc.org/eic/2013/11/aufbau-electron-configuration | last = Scerri | first = Eric | date = 2013 | issue = 11 | pages = 24–26 | volume = 50 }}</ref>
==See also==
* [[Electron configuration]]
* [[Valence electrons]]
* [[Wiswesser's rule]]
==References==
<references/>
==Further reading==
* [http://www.iun.edu/~cpanhd/C101webnotes/modern-atomic-theory/images/energy-levels.jpg Image: Understanding order of shell filling]
* [[Jan C. A. Boeyens|Boeyens, J. C. A.]]: ''Chemistry from First Principles''. Berlin: Springer Science 2008, ISBN 978-1-4020-8546-8
*{{cite journal | doi = 10.1007/s10698-005-2141-y | last = Ostrovsky | first = V.N. | journal = Foundations of Chemistry | volume = 7 | issue = 3 | pages = 235–39 | title = On Recent Discussion Concerning Quantum Justification of the Periodic Table of the Elements | url = http://www.springerlink.com/content/p2rqg32684034736/fulltext.pdf | year = 2005}} [http://www.springerlink.com/content/p2rqg32684034736/fulltext.pdf?page=1 Abstract].
*{{cite journal | title = On the dynamical symmetry of the periodic table. II. Modified Demkov-Ostrovsky atomic model | first = Y. | last = Kitagawara | year = 1984 | journal = J. Phys. B: At. Mol. Phys. | volume = 17 | issue = 21 | pages = 4251–59 | doi = 10.1088/0022-3700/17/21/013 | url = http://www.iop.org/EJ/article/0022-3700/17/21/013/jbv17i21p4251.pdf | author2 = Barut, A.O.|bibcode = 1984JPhB...17.4251K }}
*{{cite journal | title = The Trouble with the Aufbau Principle | first = E.R. | last = Scerri | year = 2013 | journal = Education in Chemistry | issue = November | pages = 24–26 | url = http://www.rsc.org/eic/2013/11/aufbau-electron-configuration }}
*{{cite journal | title = Transition Metals and the Aufbau Principle | first = L. G. | last = Vanquickenborne | year = 1994 | journal = Journal of Chemical Education | volume = 71 | issue = 6 | pages = 469–471 | url = http://depa.fquim.unam.mx/amyd/archivero/4svs3d_26264.pdf |bibcode = 1994JChEd..71..469V |doi = 10.1021/ed071p469 }}
==External links==
*[http://chemed.chem.purdue.edu/genchem/topicreview/bp/ch6/quantum.html#aufbau Electron Configurations, the Aufbau Principle, Degenerate Orbitals, and Hund's Rule] from [[Purdue University]]
[[Category:Electron states]]
[[Category:Foundational quantum physics]]
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== Pengertian ==
Aufbau berarti membangun. Menurut prinsip Aufbau ini elektron di dalam suatu atom akan berada dalam kondisi yang stabil bila mempunyai energi yang rendah, sedangkan elektron-elektron akan berada pada orbital-orbital yang bergabung membentuk subkulit<ref name=":0">{{cite|last=Chang|first=R.|year=2007|title=Kimia Dasar (terj)|publisher=Erlangga|location=Jakarta}}</ref>. Jadi, elektron mempunyai kecenderungan akan menempati subkulit yang tingkat energinya rendah.<ref name=":1">{{cite|last=Keenan|year=1990|title=General Chemistry|publisher=Elsiever|location=Boston}}</ref>
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