Metallic Bond Formation: Explained

[scott_alexsk]

How are metallic bonds formed? Does it have anything to do with the merging of d orbitals to obtain a half full shell configuration or is it something else? Thank you for your time.
-Scott

 

[armandowww]

I think that metals are not linked up by means of atomic orbitals. Sodium and copper are metals as well, but their locations on priodic table is different.
More important is if last electronic states (not core ones) are spread throughout the lattice. It occurs when nuclei got only core electrons. Then no dependence will emerge on whatever atomic orbital types are involved.
Correct my language errors.
alhoa

 

[Astronuc]

http://en.wikipedia.org/wiki/Metallic_bond

Metallic bonds involve the outermost (valence) electrons which form a conduction band in metal. The transitional metals' bonds do involve the d-electrons.

Transition elements tend to have high tensile strength, density and melting and boiling points. As with many properties of transition metals, this is due to d orbital electrons' ability to delocalise within the metal lattice. In metallic substances, the more electrons shared between nuclei, the stronger the metal.

http://en.wikipedia.org/wiki/Transition_metals#Electronic_configuration

I believe in Lanthanides and Actinides, the f-electrons participate in bonds.

Of course, I cannot vouch for the accuracy of Wikipedia, and one should use other references for corroboration.

Hopefully Zapperz and others will comment.

 

[scott_alexsk]

Thank you for replying Aldriono and Astronuc. I really don't know much about metallic bonds except for that they conduct electricity. Do ionic bonds regularly conduct electricity?
-Scott

 

[armandowww]

no they don't

 

[photon79]

metallic bonds can be explained on the basis of free electron theory in which +ve ions are immersed in a sea of free electrons.

Thank you for replying Aldriono and Astronuc. I really don't know much about metallic bonds except for that they conduct electricity. Do ionic bonds regularly conduct electricity?
-Scott

In ionic bonds complete transfer of electron is there forming +ve and -ve ions, so no more free electrons and no more conductivity.

 

[Gokul43201]

One way to simplistically think about things is the molecular-orbital approach. When you overlap a pair of orbitals of equal energy you create two new energy levels (above and below the original energy) called bonding and antibonding orbitals. By extension, one could surmise that an overlap between several identical orbital leads to the formation of bonding and antibonding bands (aka the valence and conduction bands). The nature of these bands depends on, among other things, the density of electrons that are involved in the bonding and the interatomic spacings. As you increase the electron density and the interatomic spacing, you decrease the gap between the two bands. For certain meterials, the two bands overlap, and these are what are good conductors. The reason they are good conductors is that there are now available empty states just above the highest occupied level (as opposed to the case where there is a gap between the lower occupied band and the higher unoccupied band).

It is these "good conducors" that we call metals. What we now have is that the valence electrons (the d-electrons) that are responsible for bonding can easily move about the body of the metal if they have empty states at nearby (within thermal energies) energies. This gives rise to the so called "Fermi sea" or Fermi gas of electrons that permeate the entire crystal and reduce the potential energy of the positive ions making up the lattice. So, the metallic "bond" is - to use an analogy - the low pressure effect created by the free electrons that keeps the positive ions happy.

 

[photon79]

One way to simplistically think about things is the molecular-orbital approach. When you overlap a pair of orbitals of equal energy you create two new energy levels (above and below the original energy) called bonding and antibonding orbitals.

But this is true only in the case of covalent bond which is purely chemical by the sense that electrons are shared between two or more nuclie. Also bonding and anti-bonding orbital concept is seen only in the case of covalent bond but not in metallic.
In metallic bonds there are no two atoms from which two orbitals can be overlaped, all are ions which lost electrons. So I don't think here is a chnce of interference of atomic orbitals.
In Metallic bond electrons are under the influence of the periodic potential which later gives rise to energy gaps involving Bragg's reflection and making the electrons bound to certain regions paving the way to distinction between semiconductors, insulators and metals(conductors).

 

[scott_alexsk]

Gokul, so what kind of bonds do the atoms form per say, being sigma bonds or pi bonds? So do the atoms dissociate their electrons to obtain a half filled orbital configuration, a nobel gas configuration, or both in some cases? Correct me if I am wrong but these bands, control whether or not an element has metal or nonmetal characteristics. Does the location of these bands cause the metalalloy steps (in the periodic table), since over time the conduction band exceeds the valence band? Tell me if I am completely wrong because I would not be surprized if I was.

 

[Gokul43201]

But this is true only in the case of covalent bond which is purely chemical by the sense that electrons are shared between two or more nuclie. Also bonding and anti-bonding orbital concept is seen only in the case of covalent bond but not in metallic.

First off, I said what followed was a simplistic approach to form an intuition for what's happening. That said, the above approach does accurately convey some of the key elements of the picture. While it's not a complete picture (for instance, so far, none of the features resulting from having a discrete translational symmetry are explained), it is certainly incorrect to say that this applies only to covalent bonds between a small number of atoms.

For more on this, see :
http://www.mrl.ucsb.edu/~seshadri/2005_218/class08_covalent1.pdf
http://www.chembio.uoguelph.ca/educmat/chm729/band/concept.htm
http://www.seas.upenn.edu/~chem101/sschem/conduction.html

 

[Gokul43201]

Gokul, so what kind of bonds do the atoms form per say, being sigma bonds or pi bonds?

Neither. The metallic "bond" is not really a bond in the way that you are used to thinking of covalent or ionic bonds.

So do the atoms dissociate their electrons to obtain a half filled orbital configuration, a nobel gas configuration, or both in some cases?

Again, the answer is "neither". You no longer have a concept of a bunch of valence electrons that are localized within a small region of space (known as a bond). The valence electrons are completely delocalized over the entire chunk of the metal. The metallic "bond" is nothing but this sea of electrons.

Correct me if I am wrong but these bands, control whether or not an element has metal or nonmetal characteristics. Does the location of these bands cause the metalalloy steps (in the periodic table), since over time the conduction band exceeds the valence band? Tell me if I am completely wrong because I would not be surprized if I was.

This is correct. But I would use the word "overlap" rather than "exceed".

 

[scott_alexsk]

What is the difference between the valence band and the conductive band? Is it just that the valence band contains electrons and the conductive empty space? Just to clarify.

So the conductive band overlapping the valence band causes the atoms at the metalliod steps to have a low electronegativity. Also for nonmetals in all cases does the valenece band overlap the conductive band?

So these metals are not forming bonds, they are delocalizing electrons and deriving their strenght from the delocalization (like benzene).

It is just that I have a hard time understanding how these metals can delocalize their electrons in the first place. I mean these atoms have a very low electronegativity, so they would never gain electrons. Are they just letting all of them go, but holding onto some of them because of their postive charge.

What happens to the orbitals? I would think the entire sample would be just one big orbital. Am I incorrect in thinking this?

Gokul I really appreciate you taking the time to discuss this topic with me,
-Scott

Edit: Is this Band Theory?

 

[photon79]

What is the difference between the valence band and the conductive band? Is it just that the valence band contains electrons and the conductive empty space? Just to clarify.

conduction band is NOT EMPTY. There are carriers(electrons in general) which are mobile.

[/QUOTE]. Also for nonmetals in all cases does the valenece band overlap the conductive band?[/QUOTE]

For metals valance and conducton bands are merged i.e for conductors. (note that som times distinction between semiconductors, conductors and insulators is made based on valence and conduction band differences)


[/QUOTE]What happens to the orbitals? I would think the entire sample would be just one big orbital. Am I incorrect in thinking this? [/QUOTE]

When we speak of metals (as far as I now) orbital concept doesn't work, more over we don't need it there. Here we can think electrons as if they form a classical gas.
When you think of an atom or chemical bonds or charge distributions you can speak of orbitals(note in all the above cases electrons are somewhat localised,,,somewhat static) , when you think of a metal(or crystal), density of states etc you don't need to worry about orbitals(this is like considering more number of electrons ,,,somewhat dynamic)

[/QUOTE] Is this Band Theory?[/QUOTE]

There is lot more in band theory.

 

[scott_alexsk]

Well I assumed if the conduction band in nonmetals was not overlapping the valence band, it would be empty, since the conduction band is of a higher energy level, and would not be filled (as easily, at least to my understanding). When I asked if this was Band Theory I was not implying that this is the only concept, but whether or not this is part of it. Now I am a little confused about the formation of conduction bands and valence bands. Are they only created when one has overlapping orbitals? It seems like it is otherwise. Thank you for taking the time to respond Photon79.
-Scott

Edit: Well I suppose that the description of electrons in a metal are like a gas, only pretains to a single aspect, since when one is trying to find the absorbtion spectrum you assume that the compound forms one big orbital.

 

[photon79]

Now I am a little confused about the formation of conduction bands and valence bands. Are they only created when one has overlapping orbitals? It seems like it is otherwise.

Electrons which have more energy(K.E) will form CB and electrons with lower energy will create VB.
CB is nothing but space where electrons are more mobile hence conductive and in VB they are somewhat bound.
I suggest some readings on solidstate physics from Richard Bube(Electrons in solids) or Kittel or Aschroft & Mermin.
For Molecular orbital concept hope Gokul will spend some time.