Question

What is meant by an effective nuclear charge?

Answer 1

According to Coulomb's law, the attraction of an electron to the nucleus depends only on the charge of an electron (+Z), the charge of the electron (-1) and the distance between them. The amount of positive charge of the nucleus experienced by any individual electron is an effective nuclear charge.

Complete step by step solution:
The effective nuclear charge means net positive charge experienced by an electron in a polyelectronic atom.
There are two factors responsible for increasing the effective nuclear charge, which is electrons and protons.
The effective nuclear charge, ${{Z}_{eff}}=Z-S$. this value gives information about the charge of an electron. Where Z is the atomic number and S is the shielding constant.
For example, consider a neutral neon atom (Ne), a sodium cation $N{{a}^{+}}$, fluoride anion ${{F}^{-}}$, let us find out which of these have more experience and more effective nuclear charge!
Neon has 10 electrons (Z=10). The valence electrons are 8 because the valence shell is 2.
The number of non-valence electrons = total electrons-valence electrons = 10-8= 2 electrons
${{Z}_{eff}}(Ne)=10-2=+8$
Fluorine has 9 electrons (Z=9), but fluoride ion gained one electron, so a total of 10 electrons
The number of non valence electrons = total electrons-valence electrons = 10-8= 2 electrons
${{Z}_{eff}}({{F}^{-}})=9-2=+7$
Sodium has 11 electrons (Z=11). But sodium ion loses one electron and total electrons 10
The number of non valence electrons = total electrons-valence electrons = 10-8= 2 electrons
${{Z}_{eff}}(N{{a}^{+}})=11-2=+9$
Even though all three examples have 10 electrons but the effective nuclear charge changed because each has a different atomic number and the sodium cation has the largest effective nuclear charge.

Note: In periods effective nuclear charge increases on moving left to right. This is an important factor for the periodic properties of elements. As the effective nuclear charge increases in periods then atomic radii, electron affinity, and electronegativity increase.