Definition of Penultimate Shell

The penultimate shell in an atom is the second-to-last electron shell. It's the shell immediately before the outermost or valence shell. For example:

• In sodium (Na), the penultimate shell is the second shell (2s² 2p⁶)
• In chlorine (Cl), it's the third shell (3s² 3p⁵)

Understanding the penultimate shell is important for predicting chemical behavior and electron configurations.