#seanchuachemistry #h2chemistry #alevelchemistry In an atom, the orbitals (a region of space around the nucleus in which the probability of finding a particular electron is the greatest) are filled in order of increasing energy (starting from 1s orbital) according to the following rules:
1) Aufbau Principle: The added electron will always occupy the orbital with the lowest energy first
2) Pauli Exclusion Principle: Each orbital can hold a maximum of two electrons of opposite spins
3) Hund's Rule of Multiplicity: When filling a sub-shell, each orbital must be occupied singly (keeping electron spins the same) before they are occupied in pairs
Note that electrons occupy the 4s orbital (lower energy) first before filling the 3d orbitals. Once the 3d orbitals are occupied by electron(s), these repel the 4s orbital to a slightly higher energy level and must be presented correctly when you write down your electronic configuration for atoms.
Two Key Exceptions:
a) Electronic configuration 4s1 3d5 is more stable than 4s2 3d4 because a half-filled 3d sub-shells are more stable e.g. Cr
b) Electronic configuration 4s1 3d10 is more stable than 4s2 3d9 because a completely filled 3d sub-shell is more stable e.g. Cu
Do look at my other KZitem videos regarding Atomic Structure to have a better understanding of Atoms, Atomic Structure, Orbitals and Electronic Configurations.
You can also visit my Advanced Chemistry Blog at www.ALevelH2Chemistry.com to look at more questions, notes and discussion of Singapore's JC A-Level H2 Chemistry.
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