Metallic Bonding Information Sheet Template (Page 2 of 3) in pdf

Sodium has the electronic structure 1s

. When sodium atoms come together, the electron in the 3s atomic orbital of one

sodium atom shares space with the corresponding electron on a neighboring atom to form a molecular orbital - in much the same sort

of way that a covalent bond is formed.

The difference, however, is that each sodium atom is being touched by eight other sodium atoms - and the sharing occurs between the

central atom and the 3s orbitals on all of the eight other atoms. And each of these eight is in turn being touched by eight sodium atoms,

which in turn are touched by eight atoms - and so on and so on, until you have taken in all the atoms in that lump of sodium.

All of the 3s orbitals on all of the atoms overlap to give a vast number of molecular orbitals which extend over the whole piece of metal.

There have to be huge numbers of molecular orbitals, of course, because any orbital can only hold two electrons.

The electrons can move freely within these molecular orbitals, and so each electron becomes detached from its parent atom. The

electrons are said to be delocalized. The metal is held together by the strong forces of attraction between the positive nuclei and the

delocalized electrons.

This is sometimes described as "an array of positive ions in a sea of electrons". If you are going to use this view, beware! Is a metal

made up of atoms or ions? It is made of atoms. Each positive center in the diagram represents all the rest of the atom apart from the

outer electron, but that electron hasn't been lost - it may no longer have an attachment to a particular atom, but it's still there in the

structure. Sodium metal is therefore written as Na - not Na

Metallic bonding in magnesium

If you work through the same argument with magnesium, you end up with stronger bonds and so a higher melting point.

Solution

Magnesium has the outer electronic structure 3s

. Both of these electrons become delocalized, so the "sea" has twice the electron

density as it does in sodium. The remaining "ions" also have twice the charge (if you are going to use this particular view of the metal

bond) and so there will be more attraction between "ions" and "sea".

More realistically, each magnesium atom has 12 protons in the nucleus compared with sodium's 11. In both cases, the nucleus is

screened from the delocalized electrons by the same number of inner electrons - the 10 electrons in the 1s

orbitals.

That means that there will be a net pull from the magnesium nucleus of 2+, but only 1+ from the sodium nucleus.

Metallic Bonding Information Sheet Template Page 2