Where Does the Energy for Ionic Bonding Come From?

In summary, the transfer of electrons in ionic bonding involves a metal giving up its outermost electron to gain the electron configuration of a noble gas, which is a stable state. While there is energy required to remove the electron and force an additional electron to be accepted by a non-metal, the overall energy is reduced due to the reduced screening of electrons in the same shell and the sawtooth-like dependence of ionization energy and electron affinity. The process comes to an end when the valence shell is empty or completely full. The stability of a full outer shell does not necessarily drive the reaction, but rather the continuous reduction of energy.
  • #1
resurgance2001
197
9
I am sure this question will have been asked elsewhere, so please forgive me if it has.

My question involves the transfer of electrons in ionic bonding. We are told that a metal will give up its outer most electron because by doing so it will gain the electron configuration of a noble gas which is stable. This is the simple explanation which is given at GCSE and I think A level also. However, my question is really, why? It still requires energy to remove the electron (1st ionisation energy) and it further requires energy (no?) to force an additonal electron to be accepted by a halogen for example. So where is this energy coming from? Does the 'stability' of a full outer shell involve energy in some way? Thanks
 
Chemistry news on Phys.org
  • #2
The point is that electrons in the same shell are not very efficient in screening each other from the charge of the nucleus. For an alkali metal, this is of little relevance as it only has one electron in the valence shell, so it will always see about 1 nuclear charge. On the other extreme in halogens, an electron feels attracted by more than 1 nuclear charge unit. Hence electrons in non-metals are much more strongly bound than in metals.
 
  • #3
You have to consider all the energies involved in the reaction of (say) Na(s) and Cl2(g) to give NaCl(s). Have you come across the Born-Haber cycle?
 
  • #4
Yes - I have done questions about the Born-Haber cycle and I do appreciate the arguments about screening.

I still struggling though to understand from the point of view of energy why the electron is actually transferred say from the sodium atom to the chlorine atom. Because what we are 'told' (apart form the screening argument) is that the atom 'wants' to gain the electron configuration of a noble gas. This seems to be implying that the stability of a noble gas configuration by itself is a driving force the reaction to take place. This actually came from a student of mine who, having heard the standard arguments that are given in all the textbooks, just said she wasn't satisfied and didn't get it. Then I realized that I had not really thought about it myself and had just accepted without questioning the explanation that atoms 'want' to gain a noble gas configuration. Is there, for example, some intrinsic lowering of potential energy when the out shell is full? Or does entropy come into play here is some way. Or is it (I suspect it might be) some quantum mechanical effect at play here. What is so special about having a full outer shell? There must be energy involved in this somehow.
 
  • #5
resurgance2001 said:
it further requires energy (no?) to force an additonal electron to be accepted by a halogen for example.

No, the halogens and most other non-metals have positive electron affinities, meaning they release energy when they accept an additional electron: https://en.wikipedia.org/wiki/Electron_affinity

The electron affinity of the halogens is comparable to the first ionization energy of sodium and the other alkali metals.
 
  • Like
Likes baldbrain
  • #6
Ok - that makes more sense now. Thanks
 
  • #7
No, the noble gas configuration is not an attraction point per se. The point is rather that both electron affinity and ionisation energy show a saw tooth like dependence when plotted as a function of the number of the number of electrons. Ionisation energy is lowest for a valence shell containing just one electron and highest for a noble gas shell, while electron affinity is highest for a noble gas and lowest for an alkali metal. Hence an atom with less than a half filled shell can continuously reduce energy by giving electrons to atoms with a more than half filled shell. The process comes to an end when the valence shell is empty, as further ionisation would require to remove electrons from a completely filled shell, which requires much more energy. On the other hand, an atom with more than half filled shell can take up further electrons with only a modest increase of energy as due to the reduced screening, the additional electrons will not see a neutral atom but some unscreened nuclear charge. Again, this will come to an end when a shell is completely full and a electrons would have to enter a new shell.
 
  • Like
Likes baldbrain

Related to Where Does the Energy for Ionic Bonding Come From?

What is ionic bonding?

Ionic bonding is a type of chemical bonding that occurs between a metal and a non-metal. It involves the transfer of electrons from one atom to another, resulting in the formation of ions that are held together by electrostatic forces.

How does ionic bonding differ from covalent bonding?

In ionic bonding, electrons are transferred from one atom to another, while in covalent bonding, electrons are shared between atoms. Ionic bonds are generally stronger and involve the formation of ions, while covalent bonds are weaker and involve the sharing of electrons.

What are some examples of substances that exhibit ionic bonding?

Examples of substances that exhibit ionic bonding include sodium chloride (table salt), magnesium oxide, calcium carbonate, and potassium iodide.

What are the properties of ionic compounds?

Ionic compounds tend to have high melting and boiling points, are soluble in water, and conduct electricity when dissolved in water or in molten form. They also tend to be brittle and have a crystalline structure.

What are some real-life applications of ionic bonding?

Ionic bonding is used in many industrial processes, such as the production of fertilizers, soaps, and batteries. It is also important in biological systems, as many essential minerals and vitamins are ionic compounds. Additionally, ionic bonding is used in the production of ceramics and as a method for water purification.

Similar threads

  • Chemistry
Replies
3
Views
2K
Replies
7
Views
4K
Replies
11
Views
2K
  • Atomic and Condensed Matter
Replies
8
Views
2K
  • Biology and Chemistry Homework Help
Replies
1
Views
927
  • Chemistry
Replies
3
Views
3K
Replies
18
Views
22K
  • Quantum Physics
Replies
21
Views
1K
Back
Top