If an element has two or more atoms with the same number of protons but different number of neutrons it is called what

The enthalpy of formation of water is -285.8 kj/mol. what can be inferred from this statement?

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.

8. what happens to matter during a chemical reaction