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!

GCSE - Models of the Atom

In Ancient Greece, it was first believe that everything was composed of four 'elements'; earth, wind, fire and water. However, a rare few a were prepared to think differently. Enter the Greek philosopher Demokritos, who believed that everything was in fact made up from small pieces indivisible matter called atoms, taking their name from the Greek word 'atomos', which means 'uncuttable'.

For a long time, nobody was able to provide any reasonable suggestion of what atoms may be made from, until the English Physicist J J Thompson. In 1897 he proposed the plum pudding model of the atom, which was sensible for its time.

The plum pudding model appeared plausible as a completely solid atom would explain why they couldn't be squashed. It also included positive and negative parts, which could cancel each other out, making an atom neutral overall, as they were known to be.

However, in 1905 Ernest Rutherford performed an experiment that would force scientists to think differently about atoms. In his experiment, he fired alpha particles (positively charged) at a thin leaf of gold foil. According to the plum pudding model, the alpha particles should have passed straight through the atoms with little or no deflection, but that was not the case.

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While most of the alpha particles did pass through, some were deflected by more than 4°, indicating a concentration of positive charge in an atom, later named the nucleus. This experiment was repeated over 100,000 times, with the same outcome each time and the plum pudding model simply did not account for the experimental results. For this reason, Rutherford proposed the nuclear model of the atom. 

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The nuclear model was later revised by Niels Bohr. who gave us the Bohr model of the atom that we still use today. 

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And so concludes our history of atomic structure. The key points to remember in an exam situation are:

• The plum pudding model is made of randomly arranged negative electrons embedded inside a positive pudding. 
• The gold leaf provided data that the plum pudding model could not explain which prompted the development of an alternative model.
• The data from the gold leaf experiment was thought to be reliable as over 100,000 readings were taken.