I have read that the trend for conductivity in groups 1 and 2 is a decrease down the groups (generally). Apparently this is because the atoms of each element get larger, so while the number of electrons per atom stays the same, the number of electrons per unit volume decreases. But the amount of shielding between the outer electrons and the nucleus increases, so surely the outer electrons become easier to donate to the delocalised sea, and will experience less attraction to the positive ions around them. Which I would have thought made them more mobile and therefore made the metal a better conductor. Obviously I'm wrong but can someone please explain why?