From the survey of my weekly JC1 A-Level H2 Chemistry Tuition Classes, i realised that many students are confused about the proper way of filling orbitals with electrons, when they first hear it from their teacher / lecturer in Junior Colleges (JCs) lectures as well as tutorials.
This concept is very important and will determine if you can master Advanced Chemistry i.e. A-Level H2 Chemistry in future.
Let’s take a look.
The electronic configurations of an element describes how the electrons of its atoms are being arranged in their shells, sub-shells and orbitals.
In an atom, the orbitals are filled with electrons in the order of increasing energy level (starting from 1s orbital) by following 3 rules:
A) Aufbau Principle
The added electron will always occupy the orbital with the lowest energy first.
B) Pauli Exclusion Principle
Each orbital can hold a maximum of 2 electrons of the opposite spins.
C) Hund’s Rule of Multiplicity
When filling up a sub-shell, each orbital must be occupied singly (and the electrons spins must be the same) before they are occupied in pairs.
It is also important to note that the paired electron must be placed in the opposite spin because this way it will make the paired electrons to be stable. The magnetic attraction which results from their opposite spins will be able to counter-balance the electrical repulsion which results from their identical charges.
Another point to note is that the energy level of 4s orbital (unfilled state) is lower in energy as compared to the 3d orbitals. Therefore during the filling of orbitals, electrons will occupy the 4s orbital first, fill it up, before filling the 3d orbitals. [Look forward to the next blog post where i will share with you a strategy to remember the Order of Filling of Orbitals]
However, during the removal of electrons to form positive ions (cations), do note that the 4s electrons are also removed first. This is because once the 3d orbitals are occupied by electrons, these orbitals (filled state) will repel the 4s orbital to a slightly higher energy level.
Hope you find the above discussion useful towards your learning journey of Advanced Chemistry.
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