Among elements 1–18, which element or elements have the smallest effective nuclear charge if we use the equation zeff

The answer to this question would be: hydrogen

The effective nuclear charge is the net positive charge that applied to an electron of a polyelectronic atom. It was written with the formula of Zeff= Z-S where Zeff means the effective nuclear charge, Z is the nuclear charge and S is the average number of the electron between the electron and nucleus. 

As the atomic number increases, the Z is increased too. That is why Zeff tends to increase with higher atomic number. Hydrogen with smallest atomic number would have smallest Zeff

No multiple choices?