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

(N/A) In multielectron atoms,the outermost electrons are shielded or screened from the nucleus by the inner electrons. This is known as the shielding or screening effect.
Due to the shielding effect,the valence electron is held less tightly by the nucleus because of the intervening core electrons.
For example: The $2s$ electron in lithium $(Li)$ is shielded from the nucleus by the inner core of $1s$ electrons. As a result,the valence electron experiences a net positive charge (effective nuclear charge) which is less than the actual nuclear charge of $+3$.
Group: Shielding effect increases down a group as the number of inner shells increases. If inner orbitals are completely filled,the shielding effect is significant.
Period: In a period,the number of inner shells remains constant while the nuclear charge increases. Consequently,the shielding effect is less dominant compared to the increase in nuclear charge. Regarding orbital penetration,$2s$ electrons are closer to the nucleus than $2p$ electrons,meaning $2s$ electrons experience more effective nuclear charge and less shielding compared to $2p$ electrons.