LON-CAPA Filled Shells

The Filled Shell Rule

Introduction

Anyone who has taken an introductory chemistry course is familiar with the the concept of noble gases. Helium, neon, argon, and krypton, are very stable, non-reactive gases. They are sometimes called inert gases. The low reactivity-high stability of these gases seems to be associated with their electron configurations, specifically the electron configurations of their valence shells. Table 1 presents the electron configurations of helium, neon, argon, and krypton with the valence shell electron configurations indicated in red.

Table 1

Electron Configurations of the Noble Gases

Gas	Atomic Number	Electron Configuration
He	2	1s²
Ne	10	1s²2s²2p⁶
Ar	18	1s²2s²2p⁶3s²3p⁶
Kr	36	1s²2s²2p⁶3s²3p⁶3d¹⁰4s²4p⁶

Comparison of the valence shell electron configurations of neon, argon, and krypton makes it easy to see how chemists deduced the octet rule, which states that atoms having 8 electrons in their valence shells are unusually stable. In the case of helium, analogy leads to the duet rule. Taken together the duet rule and the octet rule may be called the filled shell rules.

Currently there is no sound theoretical explanation of these rules. They are summary statements of empirical observations. As such they provide chemists with guidelines that help them orient their thinking about molecular structure. Note the emphasis on molecular structure; the filled shell rules apply to atoms within molecules as well as to isolated atoms.