Electronic Configuration for First 30 Elements

In this video created by Chemistry Guru, Singapore's leading JC Chemistry tuition centre, we want to learn how to write out the electronic configuration for the first 30 elements using the Quantum Model of Atoms.

Based on the Quantum Model, we know that within each principal quantum shell there are subshells and the number of subshells is related to the principal quantum number.

For principal quantum number n = 1, there is 1 subshell 1s.
For principal quantum number n = 2, there are 2 subshells 2s and 2p.
For principal quantum number n = 3, there are 3 subshells 3s, 3p and 3d.
And so on.

Each subshell has a certain number of orbitals and each orbital can hold up to 2 electrons.

Therefore the maximum number of electrons in a subshell is as shown:

When we fill up the electrons, electrons will be added to a subshell with the lowest energy state first, followed by the next most stable state, and so on.

The order of filling up electrons for an element X is as shown:

Notice we fill up electrons to 4s subshell first followed by 3d subshell as 4s subshell is more stable when empty.

When the first 36 electrons are filled, the electronic configuration for element X would look like this:

Take note when the electrons are filled, 3d subshell becomes more stable than 4s subshell, hence we write 3d first followed by 4s.

There's a trick to write out the electronic configuration quickly.

Notice that 1s² 2s² 2p⁶ will add up to 10 electrons.

3s² 3p⁶ and 4s² will also add up to 10 electrons.

3d subshell can also hold up to 10 electrons.

So we can add the electrons by sets of 10 to quickly determine the electronic configuration.

In this video we also discuss some examples, namely:

₁₁Na, ₂₆Fe, ₂₆Fe²⁺, ₂₄Cr and ₂₉Cu

Try writing out the electronic configuration for these examples and check out the video for the solutions!

Topic: Atomic Structure, Physical Chemistry, A Level Chemistry, Singapore

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