# What exactly is lattice energy?

I was going through my chemistry textbook (Chemistry, 10th Ed. by Raymond Chang) when I encountered this explanation of lattice energy.

9.3 Lattice Energy of Ionic Compounds

We can predict which elements are likely to form ionic compounds based on ionization energy and electron affinity, but how do we evaluate the stability of on ionic compound? Ionization energy and electron affinity are defined for processes occurring in the gas phase, but at 1 atm and 25 °C all ionic compounds are solids. The solid state is a very different environment because each cation in a solid is surrounded by a specific number of anions, and vice versa. Thus, the overall stability of a solid ionic compound depends on the interactions of all these ions and not merely on the interaction of a single cation with a single anion. A quantitative measure of the stability of any ionic solid is its lattice energy, defined as the energy required to completely separate one mole of a solid ionic compound into gaseous ions (see Section 6.7).

In this text, it seems like the writer is implying that for an ion pair in the gaseous state, ionisation energy (IE) and electron affinity (EA) can be used to measure it's stability. But even an ion pair in the gaseous state is held by an electrostatic force that gives it more stability, so more energy will be needed to separate the ion pair and that energy would be more than IE and EA. What does the writer mean, and what is the correct definition of lattice energy?

• "Gas phase" is supposed to mean "zero contact, infinite distance between ions" in this context. – Karl Aug 5 '20 at 12:36
• In other words, that sentence about the gas phase and 1 atm/25°C is utter bullshit. Can you please give the book title and author? – Karl Aug 5 '20 at 19:52
• It begins by explaining that the relative EA and IE of two atoms can be used to estimate whether they form an ionic compound. The EA and IE are often combined to compute an electronegativity parameter, which is compared between atoms. In this case the question is about the nature of the bond between the atoms, not about the stability of the resulting solid. – Buck Thorn Aug 6 '20 at 9:27
• @Karl chemistry, tenth edition by Raymond chang – Taofeek Aug 6 '20 at 13:29
• The last sentence is just as wrong. The lattice energy is the one needed to separate the ions to infinite distance. That´s not the same as vaporising them. The difference won´t be large, but Mr. Chang is still lying to students for no good reason. Perhaps he thought they were too stupid to grasp the idea of a theoretical value, or was trying to be practical, whatever. Schoolbook authors often do that, and its also disgusting there, but a university professor? – Karl Aug 6 '20 at 15:57

Notice that the text says nothing explicit about ion pairs.

IE + EA relate to free gaseous unbound ions. Like:

$$\ce{M(g) -> M+(g) + e-(g)}\tag{IE}$$

resp.

$$\ce{X(g) + e-(g) -> X-(g)}\tag{EA}$$

The lattice energy (LE) is the energy released by forming solid ionic compounds from free ions:

$$\ce{ M+(g) + X-(g) -> MX(s)}\tag{LE}$$

Gaseous ion pairs have already released a part of the lattice energy.

Note that for ion pairs, the pairing energy may be smaller than the sum of IE+EA, especially for big ions, so forming pairs may not be favoured.

• The writer says "IE and EA are defined for processes occuring in the gas phase" what processes is the writer talking about? – Taofeek Aug 5 '20 at 11:04
• See the update. – Poutnik Aug 5 '20 at 11:05
• Thanks, but the whole text seems to be incoherent to me. I'd appreciate it if you could just explain to me the exact message the writer is trying to convey in that text. I'm sorry for any inconvenience. One of the major things confusing me is that the writer said "IE and EA are defined for processes occuring in the gas phase, but at 1 atm and 25c all ionic compounds are solids" this statement seems to imply that because the solid state is different, IE and EA can no longer be used to measure stability. – Taofeek Aug 5 '20 at 11:43
• I would rather mark it as comprehension issue than text incoherence. I recommend you to recollect knowledge about gaseous and solid states and definitions of terms. – Poutnik Aug 5 '20 at 12:01
• Can you give a brief summary of the text, so as to round it up – Taofeek Aug 5 '20 at 12:43