2020 P1 Q30 - Deducing Density and Melting Point of Vanadium

Let's take a look at 2020 A Levels H2 Chemistry Paper 1 Question 30.

2020 p1 q30 question

We are required to determine which row shows the density and melting point of vanadium.

Let's locate these 2 metals in the Periodic Table and consider their atomic radii too.

2020 p1 q30 Sr and V info

Strontium is a Group 2 metal in Period 5, while vanadium is a transition metal in Period 4.

Since they are in different periods we have to be careful in comparing their properties.

In syllabus the default comparison is between group 2 calcium and transition metals in Period 4.

2020 p1 q30 compare density Ca and V

Transition metal atoms have bigger mass number and smaller size hence more atoms can be packed per unit volume.

This means transition metals should have higher density as compared to group 2 metals like calcium.

Since strontium is in a different period, we want to make sure this comparison is still valid.

2020 p1 q30 compare density Sr and V

Strontium mass number 87.6, atomic radius 0.215 nm
Vanadium mass number 50.9, atomic radius 0.135 nm

Notice vanadium has a smaller mass number and smaller size than strontium.

This means we cannot use the comparison between calcium and transition metals in period 4 to deduce their densities directly.

Hence let's estimate density via this equation:

density = mass / volume = mass / radius3

Note the accuracy of the equation is not important as long as we apply the same equation to both strontium and vanadium for fair comparison.

density of strontium = 8814
density of vanadium = 20687

Hence vanadium should have a higher density than strontium.

Next let's compare their melting points which is much more straightforward.

2020 p1 q30 compare melting point Sr and V

Transition metals can delocalise their 4s and 3d electrons hence more positively charged cations will attract a bigger sea of delocalised electrons.

This means transition metals will have stronger metallic bonds and higher melting points compared to group 2 metals.

Hence vanadium should have a higher melting point than strontium.

We can now look through the options and determine the answer is D.

2020 p1 q30 answer

Topic: Transition Elements, Inorganic Chemistry, A Level Chemistry, Singapore

Back to list of questions for 2020 A Level H2 Chemistry Paper 1.

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