Question

how to write chemical equation and tell me the valency of few main elements

Answer 1

valency of hydrogen =1
helium=0
lithium=1
berlium=2
boron=3
carbon=4
etc.

Answer 2

good way to think about a chemical reaction is the process of baking cookies. You mix the ingredients together (flour, butter, salt, sugar, and eggs), bake it, and see that it changes into something new: cookies! In chemistry terms the equation is the recipe, the ingredients are "reactants," and the cookies are "products." All chemical equations look something like "A + B →C (+ D...)," in which each letter variable is an element or a molecule (a collection of atoms held together by chemical bonds). The arrow represents the reaction or change taking place. Some equations may have a double-headed arrow (↔), which indicates that the reaction can proceed either forward or backward. To write the equations there are a number of important naming rules that you need to know.

Scientists believe that about 25 of the known elements are essential to life. Just four of these – carbon (C), oxygen (O), hydrogen (H) and nitrogen (N) – make up about 96% of the humanbody.