Ionic bonding and electron transfer | Stretch and challenge

An explanation often given for ionic bonding is that atoms lose or gain electrons to achieve full outer shells. This activity evaluates that as a complete explanation.

How to use this resource

This resource is made up of a student sheet and lesson slides presenting a problem, four opinions and a table for evaluating the opinions as explanations. The teacher guidance and lesson slides provide a discussion of the answers.

Learners read through ‘The problem’ and then the four opinions expressed (A, B, C and D). Learners decide whether they agree with all, or any of, each of the opinions and then what their own response to the information is.

• When to use? Use as an extension activity after a lesson on ionic bonding. Learners should already have prior knowledge of atomic structure, ionic bonding and the formation of ions.
• Group size? Ask learners to work in groups of two or three.
• Topics: Ionic bonding, electrostatic attraction, and energetic stability.
• How long? 10–15 minutes

The problem

We sometimes explain the formation of ions in terms of atoms transferring electrons in order to gain a full outer shell of electrons (noble gas electron configurations). But, for the process:

Mg(g) + O(g) → Mg²⁺(g) + O^2-(g)
ΔH = +2846 kJ mol^-1

The process is extremely endothermic!

It requires a large investment of energy to transfer the electrons from Mg to O.

The opinions

A: ”We know by experience that magnesium burns exothermically, so electron transfer cannot be the only thing going on.”

B: ”The bonding in magnesium oxide must not be ionic – it must be covalent.”

C: “Ionic bonding occurs because oppositely changed ions attract each other. Energy is given out when the ions come together in a lattice.”

D: ”When you react magnesium and oxygen you do not actually have separate atoms. The magnesium is in a giant lattice and the oxygen is in molecules.”

Discussion of answers

Person A is making a sensible point. The information given suggests that the process of electron transfer on its own is strongly endothermic. Since burning magnesium is exothermic, it must involve more than electron transfer.

Person B has abandoned the ionic model too readily. There is good evidence for the existence of ions – e.g. the conductivity of solutions and molten salts. Since magnesium is a metal and oxygen a non-metal, the bonding is ionic.

Person D is correct that the data are about isolated atoms, but the real reaction is between solid magnesium and oxygen molecules. However, this observation only gets us so far. You can form the isolated atoms of magnesium and oxygen from the solid and gas by investing the energy to break all the bonds. Breaking the bonds will be endothermic, so we have not explained why the process of burning is exothermic and indeed what drives the magnesium to react with the oxygen.

Person C has made a crucial point. The exothermic part of the whole process comes from the coming together of oppositely charged ions into a giant lattice. Opposite charges have potential energy when they are held apart which is converted to heat when they move closer. The mutual attraction of oppositely charged ions is the driving force behind ionic bonding.