What is the vertical and horizontal trend in Binary Acid Strength? (1) HX
Going right, the acid strength increases because the electronegativity increases. The increase makes the central atom want to pull more electrons toward itself, giving the H a partial positive charge.
Going down a group, the acid strength increases NOT because of electronegativity, but because of the increase of the increase in bond length. As the central atom gets bigger, so does the H-X bond making it easier for the bond to break.