Phosphorus pentachloride gas is formed by the reaction of solid phosphorus and chlorine gas . write a balanced chemical equation for this reaction.

Chlorine gas has the the chemical formula Cl2
Solid phosphorus has the chemical formula P4
Phosphorous pentachloride has the chemical formula PCl5

Now the UNBALANCED reaction will be as follows:
P4 + Cl2 > PCl5

We need to balance the equation:
1- We have 4 solid phosphorous atoms as reactants, therefore, the product should have 4 phosphorus atoms as well.
     P4 + Cl2 > 4PCl5
2- Now, we have 20 chlorine atoms in the products while only 2 atoms of chlorine as reactants. Therefore, we need to multiply Cl2 by 10 to balance the equation as follows:
    P4 + 10Cl2 > 4PCl5

The final balanced equation is as follows:
P4 + 10Cl2 > 4PCl5