• When halogen atoms gain an electron during reactions, they form -1 ions called halide ions
• We can use electronic configuration to explain the trends in chemical reactivity down group 7

The atoms of the elements of group 7 all have 7 electrons in their outer shell

• Reactivity of group 7 non-metals?decreases?as you go down the group
  • As you go down group 7, the number of shells of electrons?increases, the same as with all other groups

   

• However, halogen atoms form negative ions when they?gain an electron?to obtain a full outer shell
  • This means that the increased distance from the outer shell to the nucleus as you go down a group makes the halogens become?less reactive

   

• Fluorine is the smallest halogen, which means its outermost shell is the?closest?to the positive nucleus of all the halogen
  • Therefore, the ability to?attract an electron?is strongest in fluorine making it the most reactive
  • As you move down the group, the forces of?attraction?between the nucleus and the outermost shell?decreases
  • This makes it?harder?for the atoms to gain electrons as you descend the group
  • Therefore, the halogens are less reactive the further down the group you go