What do you look at to tell the reactivity of an atom?

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

Which theory was contradicted by experiments with the photoelectric effect? light spreads out after it passes through a small opening. as soon as light strikes metal, electrons will be ejected. visible light, regardless of color, will cause the ejection of electrons when striking metal. the kinetic energy of ejected electrons depends on the frequency of light that strikes the metal.

Find the mass, in grams, of 5.00*10^23 molecules of f2