Question

How do I complete and balance this synthesis reaction equation: __Ca + __ O2 -->

Answer 1

Since it's a synthesis reaction, there can be only one product, so it has to be the ionic compound formed when calcium loses 2 electrons to form a +2, and each oxygen gains 2 valence electrons to form a -2 ion.
\[Ca + O{_2} \rightarrow CaO\]
Balancing from left to right gets you
\[2Ca + 1O{_2} \rightarrow 2CaO\]