Does the polarity of the binary halogen acids increase or decrease down the group? I have one argument for decrease based solely on electronegativity considerations. I have one argument for increase based on orbital overlap and electron density. Taking electron density in an orbital to be non-uniform and decreasing as we get further from the nucleus, it follows that as the halogens get bigger, there is less electron density at the fringes - or where the hydrogen would overlap. This, combined with the relatively greater electronegativity of the halogens (expect iodine) relative to hydrogen, gives the bond ionic character. Which one is correct?