For the valence shell of $Na$,screening is provided by:

Which of the following statements is correct about the "shielding effect"?
$1$. It is effective when the orbitals in the inner shells are completely filled.
$2$. Inner shells do not show any effect.
$3$. With an increase of shielding down the group,ionization energy increases.
$4$. With increasing nuclear charge,the shielding effect increases.

Among the following,the energy of $2s$ orbital is lowest in

Explain the shielding effect with an example. Also,explain the $2s-2p$ approach in a group and a period.

The unpaired electrons in $Al$ and $Si$ are present in the $3p$ orbital. Which electrons will experience a higher effective nuclear charge from the nucleus?

For which of the following is $Z_{eff}$ equal to $Z$?