Understand the principal quantum number.

Question

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The notation "3S" in chemistry refers to a specific electron orbital.

Understand the principal quantum number.
The number '3' in '3S' represents the principal quantum number ( $n$ ). This number indicates the main energy level or electron shell where the orbital is located. In this case, $n=3$ , meaning the orbital is in the third energy level.
Understand the azimuthal quantum number.
The letter 'S' in '3S' represents the azimuthal quantum number ( $l$ ). This number defines the shape of the orbital. For an 'S' orbital, $l=0$ , which means it has a spherical shape.
Combine the meanings.
Therefore, '3S' describes an electron orbital that is in the third principal energy level ( $n=3$ ) and has a spherical shape (s-orbital, $l=0$ ). An s-orbital can hold a maximum of two electrons.

The meaning of '3S' is:
$\boxed{\text{An s-orbital in the third principal energy level.}}$

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