Questions on the Halogens

Group (VII) (The Halogens)

1.      Explain the trend in atomic size in descending the group.

Each time the group is descended by one, a new quantum level is added. Therefore, the distance from the nuclear charge to the outer electrons increases and so does the screening effect, therefore, the attraction on the outer electrons is reduced and the atoms get bigger.

2.      What trend in electron affinities would you predict?

As the atomic size (distance from outer electrons to the nuclear charge) and the screening effect increases going down the group, the attraction on an added electron from the nuclear charge will decrease. Therefore, I would expect the electron affinity to decrease going down the group.

3.      Based on number 2, what trends in reactivity would you expect? Give reasons.

The halogens have seven electrons in their outer shell and so react by obtaining one more (either by sharing in covalent bonding or gaining one in ionic bonding). One of the factors influencing their reactivity will be their ability to gain an electron. This will be greatest when they are smallest with the least screening  (greatest attraction). Therefore, the reactivity should decrease going down the group or increase going up the group.

4.      The actual values for the electron affinity are; F_(g)   -328 (KJ/mol), Cl_(g)   

-349, Br_(g) -324 and I_(g) -245

What is surprising is that Cl has a higher value (more heat released) than F. This is because that F is such a small atom that the electrons in the outer quantum level are closely packed. This causes there to be more repulsion for an incoming electron and therefore, the overall heat energy released is actually less than for Cl. However, F is still more reactive than Cl and this is because in a reaction other things are occurring, such as: Atomisation and hydration energy (if its in a solution). Flourine has the lowest atomization energy and the greatest hydration energy. This compensates for the slightly lower electron affinity value. Therefore, overall fluorine is the most reactive of the halogens.

You may find the atomisation data for fluorine and chlorine surprising as they are both gases at room temperature. Therefore, atomisation involves breaking the bond between the atoms. Look at the following link which includes bond energy data, and all should be clear to you.

Chem Guide Group 7 properties