On reaction with $Mg$,very dilute nitric acid produces

$BeCl_2 + LiAlH_4 \to X + LiCl + AlCl_3$
Total number of $3c-4e^\Theta$ bonds present in the monomer form of compound $(X)$ is:

What is the sum of neutrons and protons in the heaviest isotope of hydrogen?

Given below are two statements: one is labelled as Assertion $A$ and the other is labelled as Reason $R$.
Assertion $A$ : Hydrogen is the most abundant element in the Universe,but it is not the most abundant gas in the troposphere.
Reason $R$ : Hydrogen is the lightest element.
In the light of the above statements,choose the correct answer from the options given below:

State whether the following statements are True or False:
$(1)$ The metal and non-metal elements of $d$-block react with hydrogen to give molecular hydrides.
$(2)$ At normal temperature,dihydrogen reacts violently with dioxygen.

Isotope$(s)$ of hydrogen which emits low energy $\beta^{-}$ particle with $t_{1/2}$ value $> 12 \ years$ is/are :