2019 P1 Q3 - Identify Elements via Ionisation Energy Trends

Let's take a look at question 3 for 2019 A Levels H2 Chemistry Paper 1.

2019 p1 q3 question

We are required to identify tellurium out of the rest of the elements given.

1. Identify Group 15 and 16 Elements

First we need to figure out the ionisation energy trends that is given in the question.

By writing out the electronic configuration, we can deduce the subshell from which the electrons are removed.

For Group 15 elements, the first 3 electrons are removed from np subshell, and the fourth electron is removed from ns subshell.

For Group 16 elements, the first 4 electrons are all removed from np subshell.

2019 p1 q3 compare group 15 n 16 elements

So for Group 15 elements, there is a change in subshell from 3rd to 4th ionisation.

Electrons in ns subshell are more stable as compared to np subshell and hence require significantly more energy to remove.

Therefore we will expect a significant jump in third to fourth IE for Group 15 elements.

2019 p1 q3 identify group 16 elements

There is a significant jump from third IE to fourth IE for elements A and C.

Hence A and C are from group 15, and B and D are from group 16.

Remember we are interested in identifying tellurium which is in group 16, so we will eliminate options A and C.

2. Identify Tellurium

Comparing tellurium and selenium, Te is further down the group.

This means Te has more inner principal quantum shells and has greater shielding effect.

The valence electrons are also further away from the nucleus, so it requires less energy to remove electrons from Te.

Therefore we will expect Te to have a lower second, third and fourth IE as compared to Se.

2019 p1 q3 identify tellurium

Comparing the IE trends of B and D, we can clearly see that B has lower second, third and fourth ionisation energies than D.

Hence B will be tellurium, and the answer to this question will be option B.

Topic: Atomic Structure, Physical Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2019 A Level H2 Chemistry Paper 1

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