2021 P1 Q4 - Deducing Group of Elements using Successive IE

Let's take a look at 2021 A Levels H2 Chemistry Paper 1 Question 4.

2021 p1 q4 question

We need to determine which element has the highest first ionisation energy given the 7th to 10th IE of 4 consecutive elements.

When identifying the group of an element from successive IE, we always look out for a significant increase in IE as it corresponds to a change in principal quantum shell.

We can then deduce the number of valence electrons and determine its Group.

2021 p1 q4 big jump in IE

Notice there is a big jump in 7th to 8th IE for W.

Since W to Z are consecutive elements, there is also a big jump in 8th to 9th IE for X, and 9th to 10th IE for Y.

We just need to focus on W and deduce which group it is from.

2021 p1 q4 deduce group for W

Since the 8th IE for W is significantly higher than its 7th IE, its 8th electron must be from a filled inner shell, and W has 7 valence electrons.

Hence W is from group 17.

Since the elements are consecutive, X must be from Group 18, Y from Group 1 and Z from Group 2.

2021 p1 q4 deduce group for all elements

We can now compare the first IE to decide which element has the highest first IE.

2021 p1 q4 deduce highest IE

In general, effective nuclear charge increases across the Period since proton number and nuclear charge increase while shielding effect stays constant when electrons are added to the same shell.

Hence X will have a higher first IE than W.

Comparing X and Y (and Z) down the group, Y (and Z) will have one more shell so valence electrons are much farther away from the nucleus and there is greater shielding effect due to additional filled inner shell.

Hence effective nuclear charge and first IE for Y (and Z) will be lower than X.

Therefore the element with the highest first IE should be X (option B).

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

Back to list of questions for 2021 A Level H2 Chemistry Paper 1.

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