With the changes in time, scientists re-examine their understanding of the atom and one such scientist was Erwin Schrödinger.
In this new model, it was proposed that the electrons could also behave like waves (wave-particle duality). In order to determine the distribution of the electrons in an atom, this new model showed the nucleus of the atom surrounded by clouds of electron densities. As the electrons moved at fast rates around the nucleus, it was hard to pinpoint the exact location of the electron which gave rise to the famous Heisenberg Uncertainty Principle (video 1).
The new atom on the block
*JC/ Yr5-6 Syllabus
Under the atomic theory and quantum mechanics, the clouds of spaces showed the probabilities where the electrons can be found and accounts for the wave-like behaviour seen. These clouds are referred to as electron orbitals. With each energy level (or traditionally called electron shell), there is a number of orbitals. There are 4 types of orbitals (s, p, d and f). For your syllabus, since we focus on the 1st 20 elements (the 1st 4 periods), let us see the orbitals at each energy level.
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Energy level 1: 1s
Energy level 2: 2s 2p
Energy level 3: 3s 3p 3d
Energy level 4: 4s 4p 4d 4f
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In addition, there is an order in which each orbital is filled and you can see the 3 rules in video 2.
The three rules are (1) Auf Bau Principle, (2) Pauli Exclusion Principle and (3) Hund's rule.
This shows us the scientific endeavour where we are constantly changing our ways of thinking and, with more scientific investigation and in light of new evidence, create new models.