The number of the following pairs of elements which exhibit a diagonal relationship is:
$1. B \text{ and } Si$
$2. Be \text{ and } Al$
$3. Li \text{ and } Mg$
$4. Al \text{ and } S$

Assign true $(T)$ or false $(F)$ for the following statements and select the correct option for your answer:
$(I)$ $I.P.$ of $O_{(g)}$ is less than $I.P.$ of $O^{-}_{(g)}$
$(II)$ $I.P.$ of $Ne_{(g)}$ is greater than $I.P.$ of $Ne^{+}_{(g)}$
$(III)$ $E.A.$ of $O^{+}_{(g)}$ is greater than $E.A.$ of $O_{(g)}$
$(IV)$ $I.P.$ of $N_{(g)}$ is greater than $I.P.$ of $N^{+}_{(g)}$

Given below are two statements :
Statement $I$ : $K > Mg > Al > B$ is the correct order in terms of metallic character.
Statement $II$ : Atomic radius is always greater than the ionic radius for any element.
In the light of the above statements, choose the correct answer from the options given below

Assertion : Best diagonal relationship is shown between $Be$ and $Al$.
Reason : Ionization energy of $Be$ is almost the same as that of $Al$.

State whether the following statements are true or false:
$(i)$ Higher the ionization enthalpy,lower the screening effect.
$(ii)$ The ionization enthalpy of $Be$ is higher than that of $B$.
$(iii)$ The shielding effect increases as we move from left to right in a period.
$(iv)$ The increasing order of the first ionization energy is $B < Be < O < N$.

Which of the following set of properties generally decreases along a period?