Electronic Configuration

Every alkali metal ends its electron configuration with a single s-electron sitting outside a completely filled noble-gas core — the general form is $ns^1$.

• Li: $1s^2\,2s^1$
• Na: $1s^2\,2s^2\,2p^6\,3s^1$
• K: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,4s^1$
• Rb: $1s^2\,2s^2\,2p^6\,3s^2\,3p^6\,3d^{10}\,4s^2\,4p^6\,5s^1$
• Cs: $[Xe]\,6s^1$
• Fr: $[Rn]\,7s^1$

This lone valence electron is far from the nucleus and well shielded by the inner shells, so it is very weakly held. That single fact underlies essentially every property discussed in this chapter: it is why these metals are the most electropositive elements known, why they overwhelmingly form the $M^+$ ion by losing this one electron, and why — being so reactive …