Question

Some elements and their valencies are given:

Elements	Valency
Ba	2
Cl	1
Zn	2
O	2

Write the chemical formula of barium chloride.
Write the chemical formula of zinc oxide.
The chemical formula of calcium oxide is CaO. What is the valency of Ca?

Answer 1

a) The symbol of the barium element is Ba and chlorine is Cl. The valency of the barium atom is ‘2’. The valency of the chlorine atom is ‘1’. After criss-crossing the symbol and valency, we obtain the chemical formula of barium chloride as ${\mathrm{BaCl}}_2$.

b) The symbol of the zinc element is Zn and oxygen is O. The valency of the zinc atom is ‘2’. The valency of the oxygen atom is ‘2’.
After criss-crossing the symbol and valency, we obtain the chemical formula of zinc oxide as ${\mathrm{Zn}}_2{O}_2$.
This means 2 atoms of zinc react with 2 atoms of oxygen, i.e. 1 zinc atom react with 1 oxygen atom. Hence the simplified chemical formula would be ZnO.

c) We know that the valency of oxygen is 2. Let us assume the valency of calcium to be x.
This means, that if we criss-cross the symbol and valency, we would get the formula as ${\mathrm{Ca}}_2{O}_x$. We have been given the chemical formula CaO. This means 1 atom of calcium reacts with one atom of oxygen. Therefore 2 atoms of calcium would react with 2 atoms of oxygen. Hence the value of x would be 2.