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Aufbau Principle - Definition, Example, Features, Configuration, FAQs

Aufbau Principle - Definition, Example, Features, Configuration, FAQs

Edited By Team Careers360 | Updated on Jun 18, 2022 03:35 PM IST

Explain aufbau principle.

Aufbau principle definition: Structures can be created in the sequence of atomic numbers to determine the electron configuration for every given atom. Beginning with hydrogen and progressing through the periodic table's periods, one proton is added to the nucleus and one electron to the proper subshell at a time until all of the elements' electron configurations are given. The aufbau principle gets its name from the German word aufbau principle, which means "to build up." According to the Pauli exclusion principle, each new electron occupies the lowest energy subshell available, subject to the constraints imposed by the allowable quantum numbers. Only after lower-energy subshells have been filled to capacity do electrons enter higher-energy subshells.

File:Aufbau Principle.png

NCERT Chemistry Notes :

aufbau principle diagram

Consider the electron arrangement of carbon, an atomic number six element. The 1s and 2s orbitals each have four electrons. The 2p subshell is occupied by the remaining two electrons. There is the option of pairing the electrons in one of the 2p orbitals or leaving them unpaired in two different but degenerate p orbitals. The orbitals are filled according to Hund's rule: for an atom with electrons in a set of degenerate orbitals, the lowest-energy configuration is the one with the most unpaired electrons. Thus, the n, l, and ms quantum numbers of the two electrons in the carbon 2p orbitals are identical, but their ml quantum numbers differ (in accordance with the Pauli exclusion principle). With an electron configuration of 1s22s22p2, the orbital diagram for carbon is:

File:Orbital diagram carbon - Hund's Rule.svg

According to Hund's rule, nitrogen (atomic number 7) covers the 1s and 2s subshells and has one electron in each of the three 2p orbitals. The spins of these three electrons are unpaired. In any of the two 2p orbitals (the electrons have opposite spins), oxygen (atomic number 8) has a pair of electrons and a single electron in the other two. Fluorine (atomic number 9) contains only one unpaired electron in its 2p orbital. The noble gas neon (atomic number 10) has all of the electrons coupled, and all of the orbitals in the n = 1 and n = 2 shells are filled.

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Aufbau Principle's Salient Features

  1. According to the Aufbau principle, electrons inhabit the lowest-energy orbitals first. This means that electrons can only enter higher-energy orbitals after lower-energy orbitals have been entirely filled.

  2. The (n+l) rule may be used to establish the sequence in which the energy of orbitals grows, with the sum of the primary and azimuthal quantum numbers determining the orbital energy level.

  3. Lower orbital energies correlate to lower (n+l) values. When two orbitals have the same (n+l) values, the orbital with the lower n value is said to have less energy.

  4. The orbitals are filled with electrons in the following order: 1s, 2s, 2p, 3s, 3p, 4s, 3d, 4p, 5s, 4d, 5p, 6s, 4f, 5d, 6p, 7s, 5f, 6d, 7p, and so on.

Aufbau Principle Exceptions

Chromium's electron configuration is [Ar]3d54s1 not [Ar]3d44s2 (as suggested by the Aufbau principle). This is due to a number of variables, including half-filled subshells enhanced stability and the comparatively small energy gap between the 3d and 4s subshells.

Lower electron-electron repulsions in the orbitals of half-filled subshells increase stability through reducing electron-electron repulsions. Similarly, totally filled subshells improve the atom's stability. As a result, some atoms' electron configurations defy the Aufbau principle depending on the energy gap between the orbitals. Copper, for example, is an exception to this rule, having an electrical configuration that corresponds to [Ar]3d104s1 . The stability given by a totally filled 3d subshell explains this.

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Electronic Configuration using Aufbau principle

Electron configuration of Sulphur

  1. Sulphur has an atomic number of 16, indicating that it has a total of 16 electrons.

  2. According to the Aufbau principle, two of these electrons are in the 1s subshell, eight are in the 2s and 2p subshells, and the rest are dispersed between the 3s and 3p subshells.

  3. As a result, Sulphur’s electron configuration can be expressed as 1s22s22p63s23p4.

Electron configuration of Nitrogen

  • Nitrogen is a seven-electron element (since its atomic number is 7).

  • The 1s, 2s, and 2p orbitals are occupied with electrons.

  • Nitrogen's electron configuration is written as 1s22s22p3.

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Frequently Asked Question (FAQs)

1. What are the Aufbau principle's exceptions?

As atoms benefit from half-filled and fully filled d and f subshells, the d and f block elements don't always follow the Aufbau rule.

2. Define nuclear shell model?

The nuclear shell model, a variant of the Aufbau rule, is used to predict the configuration of protons and neutrons in an atom.

3. What does the l denotes in the n+l rule?

The quantum numbers "n" and "l" in the (n + l) rule are used to specify the state of a specific electron orbital in an atom. Here, n denotes the primary quantum number, which is connected to the orbital's size, and l denotes the angular momentum quantum number, which is related to the orbital's form.

4. State Aufbau rule.

It states that the orbitals of an atom are filled in order of their increasing energies in the ground state. That is, electrons fill lower energy orbitals first, then higher energy orbitals.

5. State Hund’s Rule.

It states that before any orbital in a subshell is doubly occupied, it is singly occupied with one electron, and all electrons in singly occupied orbitals have the same spin.

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