Standard Enthalpy of Formation Question

[Sydney Silverman 2C]

Hi! During the last lecture, Dr. Lavelle said that the standard enthalpy of formation for elements in their most stable was zero. I don't have a great chemistry background, and I was a bit confused about this. How do we know if the element is in their most stable form? The example given was 02 being 0 in the combustion of methane. Is there a pattern, or something I could look for to know that the enthalpy of formation will be 0? Thanks!

[Suzy Matinyan]

Hi!

So, I think for me, I would use the O2 as an example of what to look for. So, diatomic elements should be considered in their most stable form. (Ex. H2, N2, Cl2, F2). Also C (graphite) that Lavelle mentioned in class. And probably S8 and P4 occur the most naturally in those states. It's just thinking about the individual elements in their natural state. For this class, I'm sure the main focus is going to be on the diatomic elements and carbon.
But, I think the best way to know if its in its simplest state and therefore delta H of formation is 0, is just to notice that if it existed, then the problem would have needed to tell us so, and therefore if it's not there, it's probably 0.