What is the nuclear charge in the periodic table?
The nuclear charge (Z) is defined as the total charge in the nucleus due to the presence of positive charge protons.
As we move through the periodic table, the atomic number continuously increases due to the increase in the number of protons in the nucleus, thus the nuclear charge also increases.
Effective Nuclear Charge ( Zeff):
Effective Nuclear Charge is defined as” the net positive charge experienced by an electron in a poly electronic atom”.
The term “effective “is used due to the presence of negatively charged electrons in the inner shells prevents the nuclear effect to be fully experienced by the valence shell electrons( due to the shielding effect).
Effective Nuclear Charge Calculation:
In a poly electronic atom, the valence electrons are simultaneously attracted to the positively charged nucleus and repelled by the negatively charged electrons.
The effective nuclear charge calculated for such an electron is given by the following equation:
Zeff = Z- S
Z = total proton numbers in the nucleus of an atom
S= shielding effect
Difference between Nuclear charge & Effective Nuclear charge:
Nuclear charge is the total electronic charge of the nucleus of an atom (due to the presence of the total number of protons in the nucleus) whereas effective nuclear charge is the attraction of the nucleus positive charge acting on the valence electrons.
Remember that the effective nuclear charge is always less than the total number of protons present in the nucleus due to the inner electron repulsion experienced by the valence electrons (shielding effect).
Effective Nuclear Charge Trend:
Effective nuclear charge increases across the period as the shielding effect remains constant.
Greater Effective Nuclear Charge = Greater attraction between nucleus and valence electrons = Small atomic size
Let’s take an example of period 3 elements to clearly understand the concept of effective nuclear charge.
As you can see in the table that when we move across the period from left to right, sodium (Na) has only one valence electron , Mg has 2 valence electrons , Al has 3 , Si has 4 , P has 5 , S has 6 ,Cl has 7 while argon(Ar) has eight valence electrons.
Moving across the period, the atomic number increases due to the increase of the number of protons in the nucleus. Therefore, nuclear charge of period 3 elements also increases.
Also on the addition of proton numbers in the nucleus, the electrons feel greater nuclear attraction because the inner shell electron number remains constant. Therefore, experience the same inner electron repulsion and increase nuclear attraction across the period.
Greater nuclear attraction on valence electrons pull the valence electrons closer to the nucleus and thus the atom gets smaller.
Down the group, the nuclear charge increase due to the increase in the proton numbers whereas the effective nuclear charge decreases due to the addition of electron shells the inner electron repulsion increases.
Which atom has greater nuclear charge and effective nuclear charge, sodium or chlorine?
As sodium atom having 11 protons and chlorine atom having 17 protons so due to greater number of protons chlorine having a greater nuclear charge.
Also effective nuclear charge of chlorine is greater than sodium because both atoms having same number of inner shell electrons experience the same inner electron repulsion.