With every electron that gets added, the atomic number increases by one because a proton also gets added.Īfter 1s orbital completely filled, the third electron goes into 2s making the electronic configuration 1s 2 2s 1 of the lithium atom, which is directly below hydrogen in the periodic table. This makes the electronic configuration 1s 2.
This is followed by the second electron it also enters 1s orbital since the s orbital can hold two electrons.
Thus, the first electron enters 1s orbital making the electronic configuration of the hydrogen atom, i.e., 1s 1. The order of the filling starts with 1s orbital, which is the smallest and lowest energy orbital. l = 0 corresponds to the s orbital, l = 1 corresponds to the p orbital, l = 2 corresponds to the d orbital, and l = 3 corresponds to the f orbital. Aufbau principleĮach type of orbitals in the above diagram is colored the same and are arranged in the ascending order of the principal quantum number ( n) from the top to bottom, for example, 2p, 3p, 4p, 5p, 6p… From the left to right, the orbitals are arranged according to the azimuthal quantum number ( l). The order of the filling is presented in the diagram below. ExplanationĪs the aufbau principle says lower orbitals are filled first before higher orbitals. Hence, it is also called as the building-up principle. The etymology of aufbauĪs the rule says the building up of orbitals takes place with the increasing energy of orbitals. The word aufbau ( auf-: up and bau: building) is a german word meaning building up or construction. The non-German reader may think aufbau is the name of a scientist, but it is not. It states that a maximum of two electrons are put into orbitals in the order of increasing orbital energy: the lowest-energy orbitals are filled before electrons are placed in higher-energy orbitals. The IUPAC defines the aufbau principle as: A rule for building up the electronic configuration of atoms and molecules. In other words, electrons will occupy the lowest-energy orbital first before filling higher-energy orbitals.Īccording to the aufbau principle, the order of the filling of orbitals is 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p… The aufbau principle states: In the ground state of an atom, atomic orbitals are filled by electrons in the order of their increasing energies.
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