Next, Group 17 elements (also called halogens) are p-block elements with a characteristic outer-shell configuration of ns²np⁵. Here are some of the trends related to Group 17 elements.
We will not be covering previously mentioned trends such as melting point, boiling point, electronegativity and ionisation energy as the explanations for these trends can be found in previous chapters.
An alternative method to prove decreasing reactivity will require the use of displacement reactions. This happens where a more reactive/oxidizing halogen will displace a less reactive/oxidising halogen from its compound. The table below illustrates the workings of a displacement reaction:
Apart from the properties of halogens, questions related to the properties of halogen halides can be tested as well. A commonly tested property is thermal stability. Firstly, let's take a look at Worked Example 2 to understand the proper presentation format for thermal stability.
Worked example 2
Thermal stability of Group 17 hydrides decreases down the group. Explain why that is so, quoting evidence from the data booklet.
Down the group, the size of halogen increases. The valence orbital used for bonding becomes larger and more diffused. There is less effective orbital overlap between the 1s orbital of hydrogen and the valence p orbital of the halogen. Hence, H-X bond strength decreases down the group as shown by the decrease in bond energy from +431 KJ mol⁻¹ in H-Cl to +299 KJ mol⁻¹ in H-I. Since H-X bond is more easily broken down the group, thermal stability of the halogen halide decreases.
Although no longer in the syllabus, it is good to know that the acidity of Group 17 hydrides increases down the group.
Note: the higher the K value, the more acidic the solution
Worked example 3
Explain why the acidity of Group 17 hydrides increases down the group
To dissolve in water, the H-X bond must be broken. Thus the acidity of the HX depends on the H-X bond strength. The weaker the H-X bond, the greater the ease of breaking the H-X bond in order to produce H⁺ hence the more acidic the HX in aqueous solution. Since bond energy decreases in the order H-F >> H-Cl >> H-Br >> H-I, acid strength thus increases in the order H-I >> H-Br >> H-Cl >> H-F
Note: the highlighted portions in both Worked Example 2 and 3 indicate the important marking points when answering questions related to thermal stability and acidity.