It is the reaction in which two or more simple substance react to give one or more complex product.
For example:
Mg + Cl₂ → MgCl₂
when magnesium combine with chlorine it form magnesium chloride.
Magnesium is alkaline earth metal and present in group two. Magnesium has two valance electrons. Magnesium atom loses its two electron to complete the octet. Chlorine is halogen element and have seven valance electrons. Chlorine needed one electron to complete the octet. When it combine with magnesium atom it accept the two electrons from magnesium atom that's why two chlorine atoms are combine with one magnesium atom to make the compound neutral.
