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Electron Configuration Explanation, Quantum Numbers Types and Examples


Electron configuration can simply be defined as the method in which the electrons are distributed in their atomic orbitals, shells and subshells in an atom. This is generally used to represent the distribution of electrons in the ground state of an atom; however, sometimes, it also indicates the electronic distribution of an ionized atom. Electron configuration is the underlying cause for many of the physical and chemical properties of elements. The valence shell electrons of an atom confer these unique characteristics on the element.

What are Electrons, Orbitals, Shells and Subshells?
All the elements present in the modern periodic table are composed of atoms, which in turn are composed of three sub-atomic particles- protons, neutrons and electrons. Amongst these, electrons are the negatively charged particles that revolve around the nucleus of an atom in fixed paths called shells. Subshells imply the pathway followed by electrons in these shells. And, lastly, the orbital is the space within an atom where the probability of finding an electron is as high as 95%. 

There are four different sets of orbitals, namely s, p, d and f, each having different shapes. Based on the atomic number and the period in which the element lies, there are different energy such as:

Example: 1s and 2s
Here, s stands for the s-orbital, whereas the numbers 1 and 2 signify the energy level. So, even though they have the same orbital, they will be occupying different spaces around the nucleus because of the difference in energy levels. For better correlation of elements in the periodic table and the orbitals, the orbitals are represented by certain blocks on the periodic table. Alkali and Alkaline Earth Metals: s-block (Groups 1 and 2) Main Elements: p-block (Groups 13 to 18) Transition Elements: d-block (Groups 3 to 12) Lanthanide and actinide series: f-block elements How to write the electronic configuration of an atom? There are some terminologies one needs to understand before actually determining the electronic configuration of an atom.

You may also want to learn about iron electron configuration.

1. Principal Quantum Number (n)

The principal quantum number (n) is the shell number of an atom and is used to calculate the number of electrons in each shell. These shells around the nucleus are numbered starting from 1 to n. n=1 is nearest to the nucleus, and the distance from the nucleus will increase with the increasing shell number. The formula for determining the maximum electron holding capacity of a shell is 2n 2 , where n stands for the principal quantum number. Shells are also named K, L, M and N, corresponding to shell numbers 1,2,3 and 4. Example: M shell, n=3, so the maximum number of electrons occupied in the shell will be 2n 2 = 2 × 3 2 = 18


2. Angular Momentum Quantum Number (l)

Angular Momentum Quantum Number or Azimuthal quantum number helps determine the type of orbital the electron is residing in. Based on the principal quantum number, the value of the azimuthal quantum number can vary from zero to (n-1). This number also helps determine the number of electrons present in each subshell by using the formula: 2× (2l+ 1). So, for orbitals s, p, d, f that have l= 0, 1, 2, and 3 respectively, the number of electrons present in these will be 2, 6, 10 and 14, respectively.

You may also want to understand electronic configuration definition.

Example: If n=3, then l = 0, 1, 2 and the names of subshells will be 3s, 3p and 3d.
Note: The value of the azimuthal quantum number is always less than the principal quantum number. Magnetic Quantum Number (m1): It is used to determine the orientation of the orbital. Apart from these quantum numbers, the electron configuration is dependent on Aufbau principle, Pauli’s Exclusion principle and Hund’s rule.

Steps to Write the Electronic Configuration: We will understand how to determine the electron configuration of an atom by using the example of Sodium Na11= 1s2 2s2 2p6 3s1

  • The atomic number of sodium is 11. So, no of electrons in a sodium atom will be 11.
  • These 11 electrons are present around the nucleus in specific orbits.
  • The electronic configuration or the filling of the orbital is governed by Aufbau’s principle. According to Aufbau’s principle, the electrons are filled in the given order: 1s, 2s, 2p, 3s, 3p, 4s, 4p, 5s, 4d, 5p, 6s, 5d, 6p, 7s, 5f, 6d, 7p…
  • As the capacity of each s, p, d, f orbital is 2, 6, 10 and 14, respectively, the electrons of the sodium atom will fill the orbitals based on the capacity of the orbital and by following Aufbau’s principle.
  • Firstly, the lowermost energy shell will be filled with 2 electrons= 1s 2 , followed by 2 electrons entering the 2s orbital.
  • The p-orbitals have a maximum capacity of 6 electrons, which will be completely filled in the case of sodium.
  • However, the last electron of sodium will only partially fill the 3s orbital.
  • This valence shell electron helps in the reaction of sodium with chlorine to form sodium chloride salt.
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