Final answer:
Binary acidity increases as you move down the periodic table due to the decrease in bond strength between hydrogen and the larger atoms in lower periods.
Step-by-step explanation:
As you move down the periodic table, binary acidity increases. This is because the acid strength of binary compounds of hydrogen with nonmetals increases as the H‑A bond strength decreases. A weaker H‑A bond means the compound can more easily dissociate into H+ and A‑, thus increasing its acidity. For example, for the halogens in group 17, the order of increasing acidity is HF < HCl < HBr < HI, demonstrating that acidity increases as we go down the column. The larger size of atoms like Iodine compared to Fluorine means a weaker bond to Hydrogen (H), and thus a stronger acid.
Increasing electronegativity also affects acidity as we go across a row in the periodic table. However, the question specifically asks about the trend when moving down the periodic table, where polarity plays less of a role than bond strength. Lastly, the acid strengths of binary hydrides increase for both trends: as we move down a column and from left to right across a row of the periodic table.