FeS2 is a reactant. When a reactant is added to a reaction which has already reached the equilibrium, more product is produced, so that the equilibrium is reestablished again
Neon (Ne) is the noble gas right before silicon (Si).
Then right after neon is the 3s subshell. It has two electrons and is full.
After 3s comes the 3p subshell, and silicon only has two electrons in the 3p subshell (you can just count the electrons in each subshell on your periodic table).
