Pauli's exclusion principle states that no two electrons in an atom can have the same set of four quantum numbers. In other words, an orbital can hold a maximum of two electrons, and these two electrons must have opposite spins.
When writing electronic configurations, you can use Pauli's exclusion principle in the following ways:
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Orbital Filling: Each electron will occupy the lowest available energy orbitals first. This is known as the Aufbau principle.
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Maximizing Spin: When filling orbitals of the same energy (degenerate orbitals), such as the three p orbitals in a given energy level, you should first place one electron in each orbital with parallel spins (up spin) before pairing them (adding the second electron with opposite spin). This follows Hund's rule.
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Notation: When writing the electronic configuration, you indicate the number of electrons in an orbital using superscripts. If an orbital has two electrons, they will be represented with the same subscript but opposite spins (not typically shown explicitly in configurations). For example:
- For helium (He, atomic number 2): \(1s^2\) — two electrons in the 1s orbital.
- For nitrogen (N, atomic number 7): \(1s^2 2s^2 2p^3\) — the 2p orbital has three electrons with parallel spins in three separate orbitals before any pairing occurs.
By following Pauli's exclusion principle, you ensure that the electronic configuration correctly reflects how electrons occupy orbitals in an atom.
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