What causes synthesis reactions?
Synthesis reactions occur when simpler chemical substances (elements or compounds) combine to form more complex substances.
This occurs to increase the stability of the particles involved. Here's an example of a synthesis reaction:
As an element, sodium has a single valence electron which makes sodium atoms highly unstable. This makes sodium an extremely reactive element. When NaCl forms, the bond type between Na and Cl is ionic. Sodium atoms lose one electron (which leads to increased stability) and the chlorine atoms gain one electron (also stabilizes the particle). The positively charged sodium ions are attracted to the negatively charged chloride ions.
This video discusses the different types of chemical reactions and how you can predict products of reactions if provided only information about the reactants.
Hope this helps!