The group 1 alkali metals and the group 2 , alkaline earth metals , form arsenides with isolated arsenic atoms which can be considered to be the As 3 − anion , for example heating arsenic powder with excess sodium gives sodium arsenide ( Na 3 As ) .
The group 1 alkali metals and the group 2 , alkaline earth metals , form arsenides with isolated arsenic atoms . 
 They form upon heating arsenic powder with excess sodium gives sodium arsenide ( Na 3 As ) .