Please help with 10.I am confused between K and H. Answer says K because zeff more, energy is less. But there is no electron in 2s of H. So shouldn't energy be zero here.

# Doubt in periodic property

Formula based go with atomic structure

E_n = -kz^2

K is taken constant becoz n=2 for all

Well if energy in hydrogen 2s is zero in potassium it’s a negative And highest in magnitude thus potassium 2s will have lowest energy.

The formula you are using is applicable only for hydrogen like atoms. Here we will use

E(n) =-K(Zeff)^2 but here for all Zeff=Z-2

So your formula will also give right result but it cannot be used here.