Which are isotopes? an atom that has an atomic number of 20 and a mass number of 42 is an isotope of an atom that has?

Which statement about hf is true? it is zero for any compound in its standard state. it is positive when the bonds of the product store more energy than those of the reactants. it is negative when a compound forms from elements in their standard states. it is zero for any element that is in the liquid state.