Why did the Quantum Mechanical Model replace the Bohr atomic model?
1 Answer
The main reason is because it offers explanatory as well as descriptive and predictive power.
Explanation:
The Bohr model shows that there are 2 electrons in the innermost shell, then 8, 18 and so on, but it doesn't have an explanation for why that is the case. It has descriptive power - it describes what we see - and predictive power - it correctly predicts new results.
Ideally, though, a scientific theory also has explanatory power - it explains the reason for the things is describes and predicts.
Quantum theory uses the Pauli exclusion principle to explain that each orbital can contain a maximum of two electrons, which must have opposite spins.
It shows that there is an s-orbital only in the inner shell, for 2 electrons total.
In the second shell there is an s-orbital (2 electrons) and three p-orbitals (3x2=6 electrons) for a total of 8 electrons.
In the third shell there is an s-orbital, three p-orbitals and 5 d-orbitals, each with 2 electrons, for a total of 18 electrons.
In the fourth shell we add 7 f-orbitals.
This is why the quantum theory is more powerful than Bohr's theory: it's explanatory power.
