# Energy needed for an Ionic Bond

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## Main Question or Discussion Point

Hi, so I've been learning about the energy transfers for an ionic bond, but my understanding breaks down at what seems to be the last hurdle. I will write what I know so far.

So we need some energy that is listed as the ionisation energy to remove electrons from one atom then we need to put the removed electrons into another using some energy called the electron affinity for that element. So we now have two ions oppositely charged now bonded, but for this to occur in nature we need the new product to have a lower energy otherwise it wouldn't occur naturally but so far needs energy. From what I've read this energy is given in surplus from the electrostatic attraction between the two ions and it's where I fall down. How does the attraction of these two ions provide energy to ionise and add electrons?

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TeethWhitener
Gold Member
Look up "Born-Haber cycle." This Wikipedia article has a good diagram for the example of lithium fluoride:
https://en.wikipedia.org/wiki/Born–Haber_cycle

Edit: the "electrostatic attraction" you refer to corresponds to the lattice energy in the diagram. This lattice energy can be quite large.

Avardia
Henryk
Gold Member
this, actually, is a very interesting question. In summary, the energy of forming an ionic solid is the algebraic sum of the three components: ionization energy of the donor atom (usually, alkali metal, that energy is positive), plus electron affinity of the acceptor atom (usually halogen, the affinity has to be taken as negative) plus electrostatic energy of the attraction between the positive and negative ions in the crystal. The thing is, the ionization energy plus the electron affinity ends up to be a positive value. So, without the third term, there should be no ionic solid. And there is a catch: in a large solid ionic crystal, there is approximately infinite number of atoms of each polarity and to get the total binding energy, you have to calculate and essentially an infinite sum of positive and negative terms ( attraction between negative and positive ions and repulsion between the ions of the same polarity). The thing is, depending on the order of summation, you get a different answer !!.

TeethWhitener
Gold Member
this, actually, is a very interesting question. In summary, the energy of forming an ionic solid is the algebraic sum of the three components: ionization energy of the donor atom (usually, alkali metal, that energy is positive), plus electron affinity of the acceptor atom (usually halogen, the affinity has to be taken as negative) plus electrostatic energy of the attraction between the positive and negative ions in the crystal. The thing is, the ionization energy plus the electron affinity ends up to be a positive value. So, without the third term, there should be no ionic solid. And there is a catch: in a large solid ionic crystal, there is approximately infinite number of atoms of each polarity and to get the total binding energy, you have to calculate and essentially an infinite sum of positive and negative terms ( attraction between negative and positive ions and repulsion between the ions of the same polarity). The thing is, depending on the order of summation, you get a different answer !!.
No, the energies in question are generally given per mole of substance, and are very much finite quantities.

Henryk
Gold Member
No, the energies in question are generally given per mole of substance, and are very much finite quantities
Yes, I know they are. What I said is that to get the correct number, you have to sum up negative and positive terms of the electrostatic energy in the correct sequence.

TeethWhitener
Gold Member
Yes, I know they are. What I said is that to get the correct number, you have to sum up negative and positive terms of the electrostatic energy in the correct sequence.

PeroK
The question of the chemical reactions can be simplified in the following way.

In the ionic reactions, (an atom gives electrons, and the other one accepts these electrons)

The atom that gives electrons also needs to give energy to have in balance of energy. And the atom that accepts electrons, also needs to receive energy to have also in balance of energy.

Then with the ionic reactions atoms interchange electrons and also interchange energy.

When they have enough energy, the reaction is produced. If they don’t get the enough energy, the reaction fails.

For that reason it is good the existence of energy around the chemical reaction, for this reaction can be successful.

In ionic reactions, atoms remain together for the charges’ attraction.

Henryk
Gold Member