Trends in Z_eff down a Group of the Periodic Table (Alkali Metals)

Experimental Observation of the Atomic Properties Neutral Alkali Atoms tells us that the effective nuclear charge increases with Z. The effective sheilding of the inner shell becomes less effective as the atom becomes more complex. The covalent radius of the alkali atoms is not really a covalent radius at all and poorly describes the electron radius.

Atom	Electron Config	IP1 (eV)	'Covalent' Radius (A)	Zeff (Radius)	Zeff (IP)
Li	[He]2s	5..39	1.52	[2.09]	1.26
Na	[Ne]3s	5.14	1.86	[3.84]	1.84
K	[Ar]4s	4.34	2.27	[5.59]	2.26
Rb	[Kr]5s	4.18	2.48	[8.00]	2.77
Cs	[Xe]6s	3.89	2.65	[10.8]	3.21
Fr	[Rn]7s	-	-	-	-

The following formula were used