Spectroscopists associated transitions involving energy levels with different values with different groups of lines in the line spectra of the alkali metals. The line groups were called sharp, principal, diffuse, and fundamental. When the angular momentum quantum number was used to describe and explain these groups of lines, s became an abbreviation for = 0, p meant = 1, d meant = 2, and f meant = 3. For consistency, higher values of the angular momentum quantum numbers are designated alphabetically (g means = 4, h means = 5, and so on).
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