What is the difference between an element and a compound in terms of these models?

What does x represent in the formula for the compound xcl4?

What is the number of moles of chemical units represented by 9.03x10^24? and how do i show work? (dumb it down )

16. in a reaction that has reached equilibrium, a. the forward and reverse reactions are occurring at the same rate. b. the reactants and products are in equal concentrations. c. the forward reaction has gone further than the reverse reaction. d. there are equal numbers of atoms on both sides of the equation. e. a, b, and d are correct.