What does the part of the equation to the left of the arrow represent in a chemical formula

In a chemical formula, the part to the left of the arrow represents the reactants. The reactants are all the chemical elements that will react with each others to make the products (which will be on the right part of the arrow).

Example:
H2 + O2 --> H2O

H2 and O2 are the reactants (on the left part of the arrow) and H2O is the product (on the right side of the arrow)

Hope this helps! :)

Crust,mantle,core,earth

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.