According to slaters rule order of effective nuclear charge for last electron in case of Li, Na and K.

Zeff = (Z-σ)( I ) Li(Z=3)...1s22s1σ for lithium = 2(0.85)=1.7Zeff = (3-1.7) = 1.3( II ) Na(z=1)...1s22s22p63s1σ = 2(1)+8(0.85)=8.8Zeff = (11-8.8) = 2.2( III ) K(z=19)...1s2 2s2 2p63s2 3p6 4s1σ = 10(1)+8(0.85)=16.8Zeff = (19-16.8) = 2.2