Question

Rank the following elements by effective nuclear charge, Zeff, for a valence electron.F LI Be B N

Answer 1

Answer: F>N>B>Be>Li

Explanation: Electrons are attracted towards the nucleus due to the positive charge present in the nucleus and negative charge on the electrons. This nucleus charge for all the electrons present in different shells of an atom is not same. Inner electrons decreases the nuclear charge felt by the outer shell electrons and its known as shielding effect.

Effective nuclear charge is calculated using the formula:

$Z_e_f_f=Z-S$

where, $Z_e_f_f$ stands for effective nuclear charge, Z is the atomic number and S is the shielding constant value which is calculated using Slater's rule.

As per Slater's rule, the contribution by the electrons in nth shell is 0.35 and

the contribution by the electrons present in n-1 shell is 0.85 and for rest of the shells like n-2, n-3 etc it is 1.00. It is also 1.00 for the n-1 shell electrons that are present in d and s.

Electron configurations for all the elements we have are:

Li = $1s^22s^1$

Be = $1s^22s^2$

B = $1s^22s^22p^1$

N = $1s^22s^22p^3$

F = $1s^22s^22p^5$

The number of inner shell electrons is equal for all these elements so for the valence electrons the shielding constant would almost be same.

So, effective nuclear charge will be greater for the element to which the atomic number is higher.

The order of atomic numbers is F>N>B>Be>Li . So, the highest to lowest order of effective nuclear charge will also be F>N>B>Be>Li .

Answer 2

1.The effective nuclear charge (Zeff or Z*) is the net positive charge experienced by an electron in a multi-electron atom. The term “effective” is used because the shielding effect of negatively charged electrons prevents higher orbital electrons from experiencing the full nuclear charge.

Equation:  Zeff = Z – S

2.where Z is the number of protons  

3. S is the number of electrons between the nucleus and the electron in question.

Decreasing order  effective nuclear charge:

F>N>B>Be>Li