The principal quantum number, denoted as $n$, indicates the main energy level or shell occupied by an electron in an atom. It is a positive integer that determines the size and energy of the orbital.
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The principal quantum number $n$ can take any positive integer value (1, 2, 3,...).
As $n$ increases, the electron's average distance from the nucleus also increases, meaning higher energy levels are farther from the nucleus.
The maximum number of electrons that can occupy a given energy level is given by $2n^2$.
Each principal quantum number corresponds to a specific shell: K (for $n=1$), L (for $n=2$), M (for $n=3$), etc.
Higher principal quantum numbers indicate orbitals with larger sizes and higher energies.