In chemistry, the electronic configuration of atoms is the arrangement of electrons in specific orbitals of an atom or a molecule. The first example of electronic configuration was provided in Bohr’s model of an atom. The electronic configuration involves the placement of different electrons in different shells and subshells of an atom. Though Niel Bohr had discovered the concept of electronic configuration, many other principles from different scientists are used to determine the electronic configuration of an atom, like Pauli’s exclusion principle, Aufbau principle, etc. Every element of the periodic table has its unique electronic configuration with a few exceptions.

History

• Thomson’s model of the atom developed in 1904 – also known as the plum pudding model –  stated that an atom is a sphere in which positive and negative charges are embedded
• Then came Rutherford’s model which stated that in an atom, the positive charge is present at the centre; most of the space around the positive charge is empty, and electrons move around the nucleus in circular orbits.
• In 1913, Bohr proposed his atomic model which states that electrons are present in a fixed orbit around the nucleus
• He also stated the number of electrons that can be accommodated in a shell can be calculated by the formula 2n2, where n is the principal quantum number. For example, the first shell can accommodate 2 electrons, while the second shell can accommodate 8 electrons
• The quantum mechanical description of an atom followed, and it described the electronic configuration in terms of quantum numbers. This description in terms of quantum numbers constitutes the electronic configuration of an atom

Quantum Numbers

• Principal quantum number (n) – It determines the principal electron shell and the distance between the electrons and the nucleus. The n can be any positive integer from 1 onwards
•  Azimuthal quantum number (l) – Azimuthal Quantum Number characterises  the shape and angular distribution of an orbital. The value of l specifies the s, p, d, f subshells, each having a unique shape. l ranges from ‘0’ to ‘n-1’
•  Magnetic quantum number (ml) – It determines the number and orientation of orbitals within a subshell. Its value of ml ranges from  ‘-l’ to ‘+l’
•  Electron spin quantum number (ms) – It is the fourth quantum number and states that electrons have a spin which is denoted by an arrow. An arrow in the downwards direction  is -1/2 and an arrow in the upwards direction  is +1/2. It is independent of other quantum numbers. ms= +1/2 or -1/2

Quantum Numbers and the Electronic Configuration of Atoms

• Electrons possess a negative charge and occupy the orbitals present around the atom’s nucleus
• The principal quantum number, denoted by n, determines the maximum number of electrons accommodated in a shell
• The 2n2 rule is used to determine the maximum number of electrons that can occupy an orbital
• The K shell can have a maximum of 2 electrons, L shell a maximum of 8 electrons, M shell a maximum of 18 electrons, and N shell a maximum of 32 electrons
• Azimuthal quantum number, denoted by l, determines the subshell of an electron
• The electrons can occupy four different orbitals: s orbital, p orbital, d orbital, and f orbital. All the orbitals have different shapes and orientations in the atom

Principles Used in the Electronic Configuration of Atoms

Different principles are used to determine the electronic configuration of an atom.

• Pauli’s exclusion principle states that 2 electrons in an orbital cannot have the same four quantum numbers. In other words, the 2 electrons in an orbital will have opposite spins
• Hund’s rule states that every orbital of a subshell is first singly occupied by an electron and then doubly occupied to achieve full configuration
• Aufbau’s principle states that electrons first fill the lower energy levels and then higher energy levels

Examples of electronic configuration of a few elements of the periodic table

• Lithium – contains 3 electrons. The electronic configuration is 1s2, 2s1
• Boron – contains 5 electrons. The electronic configuration is 1s2, 2s2, 2p1

• Chlorine – contains 17 electrons. The electronic configuration is 1s2, 2s2, 2p6, 3s2, 3p5
• Calcium – contains 20 electrons. The electronic configuration is 1s2, 2s2, 2p6, 3s2, 3p6, 4s2

Exceptions of the electronic configuration

• Chromium

Number of electrons – 24 electrons

Electronic configuration –  1s2, 2s2, 2p6, 3s2, 3p6, 3d5, 4s1

• Copper

Number of electrons – 29

Electronic configuration –  1s2, 2s2, 2p6, 3s2, 3p6, 3d10, 4s1

The exceptions in the electronic configuration of atoms occur due to the extra stability of half-filled and full-filled orbitals. All electrons want to achieve a stable state; therefore, they prefer half-filled and full-filled orbitals. 

The Need for Electronic Configuration of Atoms

• The electronic configuration of atoms tells us about the chemical behaviour of the elements
• It informs us about the location of an element in terms of where it would be placed in the modern periodic table
• It explains the stability of elements of the periodic table
• It tells us about the unreactive nature of the noble gases of the periodic table
• It also tells us about the arrangement of electrons in the orbitals, their shape, size, and structure

Conclusion  

It can be concluded that electronic configuration is a process of arrangement of electrons in the orbitals of an atom. There are many principles based on which the electrons are filled in an orbital. Bohr’s atom model was the first theory to accept the idea of the electronic configuration of atoms. Though the theory was incompatible with wave-particle duality, it was a stepping stone to the quantum mechanical description of an atom. This theory was successful in providing the electronic configuration of elements of the periodic table.