# The alkali metals from salt- like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following orders ?

$\begin {array} {1 1} (1)\;LiH > NaH>KH>RbH>CsH & \quad (2)\;CsH>RbH>KH>NaH>LiH \\ (3)\;KH>NaH>LiH>CsR>RbH & \quad (4)\;NaH>LiH>KH>RbH>CsH \end {array}$