The existence of "triads" of elements was a big clue that eventually led to the discovery of the periodic table. A triad is a group of three elements for which the middle element has properties that are the average of the properties of the first and third elements. Some triads include: $(\mathrm{O}, \mathrm{S}, \mathrm{Se}) ;(\mathrm{Cl}, \mathrm{Br}, \mathrm{I}) ;$ and $(\mathrm{Li}, \mathrm{Na}, \mathrm{K}) .$ Verify that
the atomic weight of the middle element in each triad is approximately the average of the other two. Explain why the melting points of the compounds formed by combining each element with hydrogen (e.g., $\mathrm{HCl}$, HBr, and HI) might be expected to show the same trend.