Which of the following elements forms a covalent electron-deficient hydride?

The total number of isotopes of hydrogen and the number of radioactive isotopes among them,respectively,are:

Hydrogen acts as a reducing agent and thus resembles

Action of water or dilute mineral acids on metals can give

Identify the reactions in which $H_2$ is liberated? $(i)$ $Zn + NaOH_{(aq)} \longrightarrow$ $(ii)$ $HCOOH \xrightarrow[\text{Conc. } H_2SO_4]{373 \ K}$ $(iii)$ $CH_{4(g)} + H_2O_{(g)} \xrightarrow[Ni]{1270 \ K}$ $(iv)$ $Zn + H^+_{(aq)} \longrightarrow$ $(v)$ $C_{(s)} + H_2O_{(g)} \xrightarrow{1270 \ K}$

Non-stoichiometric hydrides are formed by