Answer: To calculate for the enthalpy of formation, it is the sum of all values of products - sum of all values of reactants.
Explanation:
I’m not sure what chemical reaction you are referencing to, but I attached an image below to help you see what I’m saying.
1s2 2s2 2p6 3s2 3p6 = 18e- so it should be Argon (Ar)
The most abundant element in the earths crust is oxygen
Its Phosphorus with a negative 3 charge.