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Aufbau Principle

Theterm ‘Aufbau’ came from a German word "Aufbauen" i.e."to build". Hence there is a set of rules available forwriting the electron configurations of atoms. This set of rules iscalled as AufbauRule.You must know that in writing the electronic configuration of anatom, the electron must be filled in a certain manner from lowerenergy level to higher energy level.

Let’sdiscuss about Aufbauprinciple.It’s basically difficult to write AufbauPrincipledefinition as it is a set of four basic rules as given below;


Basic Rules


  • All electrons havedifferent quantum number or no two electrons in the atom can havesame four quantum numbers nlm,and s.

  • During filling ofelectrons in orbitals; first lower energy orbitals than higherenergy orbitals occupy in atom.

  • The pairing ofelectron cannot happen until the same energy level orbitals areunoccupied.

  • If two orbitals havesame (n+l)values, than orbital with lower ‘n’value will occupy first than other one.


For example out of 2pand 3s orbitals; 2p will occupy first because of n=2compare to 3s for which n=3.Although both orbitals have same (n+l) value that is 4 yet havedifferent ‘n’ value. By using the given set of rules; we canwrite the orbitals in increasing order of energy that is as givenbelow;

1s, 2s, 2p, 3s, 3p,4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, 8s

Wecan draw a simple AufbauDiagram.First make a column and write ‘s’ orbitals from 1 to 8. Then drawa second column and write 'p' orbitals starting at n=2.

Nextin third column write’d’ orbitals starting at n=3and after that 4f and 5f in a final column. Now read the chart byrunning the diagonals starting from 1s. The arrows show the path ofelectron filling direction. Remember;

  • s orbitals can havemaximum 2 electrons

  • p orbitals can havemaximum 6 electrons

  • d orbitals can havemaximum 10 electrons

  • f orbitals can havemaximum 14 electrons

Let’s take anexample to write the electronic configuration of element by usingthese set of rules. Take Nitrogen atom with 7 electrons and sevenprotons. Start with the first orbital that is 1s. You can fill 2electrons in 1s, now five electrons left. They will occupy in 2nd shell, that has 2s and 2p. 2s orbital can hold two electrons andfinal three electrons will move to 2p which can occupy six electrons.But we have three electrons left so instead of paring the electrons,we have to place them un-paired, one at each position. Hence theelectronic configuration of nitrogen becomes; 1s2 2s2 2p3.