GCSE - Exam Technique: Explaining Reactivity Trends

When answering exam questions on reactivity trends within a group, you must address the following points if you want to maximise your marks:

• How the number of shells changes down the group 
• How the number of shells affects the atomic radius and shielding
• How atomic radius and shielding affects the attraction between the nucleus and outer shell electrons
• How the changing force of attraction affects how easily electrons are lost or gained. 

As you go down group 1, the reactivity of the elements increases; Li < Na < K. In order to explain this properly, we need to hit all of the points above. As an example, I will use an exam question taken from the aQA exam question bank, Exambuilder and created a model answer:

(ii)     Potassium is more reactive than sodium.

Explain why, in terms of electronic structure.

               Potassium has more shells than fluorine. This makes its atomic                         radius greater and also the effect of shielding increases, reducing                       the attraction between the nucleus and the outer shell. This                               makes the electron in the outer shell of potassium easier to                               remove than in sodium. 

As you descend group 7, the elements get less reactive F > Cl > Br > I. Again, it is vitally important to hit those key points again - remember, you're trying to show the examiner all that you know! Here's another question from AQA Exambuilder, along with a model answer to illustrate this point:

(c)   Explain, in terms of electrons, why fluorine is the most reactive element in Group 7.

Of all the group 7 elements, fluorine has the fewest number of shells, which reduces the atomic radius and the effect of shielding. This means that in fluorine, the force of attraction between the nucleus and the outer shell is greatest, so it is easier for fluorine to gain an eighth electron to fill its outer shel than it is for the rest of group 7. 

It's really important to do practice paper questions as part of your revision - I cannot stress that enough! Your exam technique is a skill that will develop over time, but practicing writing answers and reviewing them with the mark scheme will make a big difference to your marks because you'll build an idea of what the examiner is looking for in certain types of questions. If you ask your teacher for exam questions, they swill likely be more than happy to supply you with some!