Question

Why the effective nuclear charge (Zeff) of the elements in same group of periodic table are the same? Thank you?

Answer 1

They all have the same number of valence electrons.

Explanation:

The way to calculate effective nuclear charge is with this formula:

`Z_"eff"="number of protons"-"core electrons"`

The number of protons will simply be the atomic number and the core electrons are all of the electrons EXCEPT the valence electrons (`"core electrons"="total electrons"-"valence electrons"`)

Thus, `Na` will have an effective nuclear charge of 1 because it has 11 protons, 1 valence electron, and 10 core electrons:

`Z_"eff"="11"-"10"=1`

If we go down the alkali metal family, every element will have 1 valence electron, and thus will all have an effective nuclear charge of 1.

`Rb`, for example, will look like this:

`Z_"eff"=37-36=1`

Hope this helped!