Complex sentence: 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).
Split into simpler sentences: 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).