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Going beyond what the note covers, this statement refers to the difference between the theoretical number of sublevels possible in quantum mechanics and the actual sublevels occupied by ground-state electrons in known elements.

1. Theoretical Sublevels ($n$ values):
   In quantum mechanics, the principal quantum number $n$ defines a main energy level. Each energy level theoretically contains $n$ sublevels:

• Level $n = 1$ has $1$ sublevel: $s$

• Level $n = 2$ has $2$ sublevels: $s, p$

• Level $n = 3$ has $3$ sublevels: $s, p, d$

• Level $n = 4$ has $4$ sublevels: $s, p, d, f$

• Level $n = 5$ has $5$ sublevels: $s, p, d, f, g$

• Level $n = 6$ has $6$ sublevels: $s, p, d, f, g, h$

• Level $n = 7$ has $7$ sublevels: $s, p, d, f, g, h, i$

1. Why Only $s, p, d, f$ Are Used:
   Across all $118$ known elements in the periodic table (up to Oganesson, $Z = 118$), electrons in their ground state fill available sublevels according to the Aufbau principle (from lowest energy to highest). The heaviest known element, Oganesson, completes the filling of the $7p$ sublevel ($7p^6$).

Even though higher shells like $n = 5$, $n = 6$, and $n = 7$ mathematically possess room for additional sublevels ($g$, $$h$