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

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

Element H have the smallest effective nuclear charge Further explanation The effective nuclear charge is the nuclear charge which is influenced by the shielding effect of electrons

The effective nuclear charge (Zeff) can be formulated as:

with

Z = nuclear charge = atomic number

S = shield constant
Shielding constants are constants that are generated from the core attraction of electrons in the inner shell
Rules for calculating shield constants-S (Slater’s Rules)
1. Electrons in orbitals are grouped according to the shell / 1s / 2s, 2p / 3s, 3p / 3d / 4s, 4p / 4d / 4f / 5s, 5p / 5d / 5f /
2. The electron to the right of the specified electron, shield constants = 0
3. The electron to the left of the specified electron, (for the ns, np orbitals), the value of the shield constant: * Electrons in the same shell shielding effects (same ns and np orbitals): 0.35
* Electrons in the deeper shell (n-1, in the lower shell one energy level), shielding effects = 0.85
* Electrons in the shell deeper (left of n-1, in the shell two energy levels and lower) shielding effects = 1.00
4. For d and f orbitals, electrons in the same group, shielding effects: 0.35, while the electrons to the left (in all lower energy levels), shielding effects: 1.00 Elements with atomic numbers 1-18 lie in periods 1 through period 3
If we want to calculate the smallest effective nuclear charge, then we determine the electron configuration of each element
Logically, with the smallest atomic number, the H atom will have the smallest Z eff
Configuration of H element
H: 1s¹ Z = 1
S = 0
Z eff = Z – S = 1-0 = 1
So the valueof Z eff = Z