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Principal quantum number

quantum number of an atomic orbital. It is usually written by the letter n. It can have values 1, 2, ...

• The atomic orbitals with n=1 are said to be in the 'K'-shell
• The atomic orbitals with n=2 are said to be in the 'L'-shell
• The atomic orbitals with n=3 are said to be in the 'M'-shell
• The atomic orbitals with n=4 are said to be in the 'N'-shell