State the trend in electronegativity for the elements of group 17 and explain why this trend occurs.

Electronegativity decreases from fluorine to astatine in group 17. This is because fluorine has the smallest number of electron shells in the group, which means there is less of a shielding effect than there is for the other elements. As the nuclear charge on fluorine is less well shielded is has a higher affinity for electron pairs and hence a higher electronegativity. Going down the group increases the number of electron shells present, which screens the nuclear charge of the element more effectively and reduces electronegativity.