A2 chemistry questions - lattice energies

Okay a table of lattice energies.

Question: Why are there no values for Francium in the first table?
My answer: Because the francium ion has a large ionic radius and a low charge density therefore low polarising power so ionic francium solids decompose with difficulty? Am I on the right lines? :confused:

I think edexcel defines the lattice energy as the energy change when 1 mole of ionic solid is formed from its constituent ions, e.g. A⁺ + B^- => AB_(s) which confuses me even more! :frown:

Question: What is meant by the charge density of an ion?
My answer: Describes the concentration/amount of charge per unit volume of an ion. E.g. an ion with a high charge density has a high amount of charge per unit volume, due to the high charge of the ion and the small ionic radius of the ion.

Reply 1

charco

Study Forum Helper

Widowmaker

I think edexcel defines the lattice energy as the energy change when 1 mole of ionic solid is formed from its constituent ions, e.g. A⁺ + B^- => AB_(s) which confuses me even more! :frown:

Francium is a radioactive element with a very short half-life that only occurs in miniscule quantities as part of radioactive decay series.
Compounds are not made with it but if they were able to be made (without it decaying) then they would be ionic and the element itself would be the most reactive group 1 element.
It's pretty much the same story for astatine - radioactive and extremely short-lived so that compounds cannot be made - effectively it doesn't exist except as a fleeting physical phenomenon

Lattice energy is defined in two different ways according to whether you wish to break the lattice or create it!
1. The energy required to remove the ions from 1 mole of a crystalline substance from their positions in the lattice to infinite separation.(in this case the energy change is positive i.e. endothermic)
2. The energy released when 1 mole of a crystal lattice is formed from its constituent gaseous ions at infinite separation. (in this case the energy change is negative i.e. exothermic)

Your idea of charge density is spot on! Charge density is the electrical equivalent of normal density
density = mass/volume
charge density = charge/volume

thus: high charge and low volume = high charge density eg aluminium ions Al3+