In both cases, the nucleus is screened from the delocalised electrons by the same number of inner electrons - the 10 electrons in the 1s 2 2s 2 2p 6 orbitals. More realistically, each magnesium atom has 12 protons in the nucleus compared with sodium's 11. The remaining "ions" also have twice the charge and so there will be more attraction between "ions" and "sea". Both of these electrons become delocalised, so the "sea" has twice the electron density as it does in sodium. Magnesium has the outer electronic structure 3s 2. If you work through the same argument with magnesium, you end up with stronger bonds and so a higher melting point. Sodium metal is therefore written as Na - not Na +. Is a metal made up of atoms or ions? It is made of atoms.Įach positive centre 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. The metal is held together by the strong forces of attraction between the delocalised electrons and the positive ions.īeware if you are going to use the term "an array of positive ions in a sea of electrons"! Metallic bonding is often described as an array of positive ions in a sea of electrons.
The rest of each atom (the nucleus and the inner electrons) is essentially a sodium ion, Na +.
So each atom's outer electrons are involved in this delocalisation or sea of electrons.
This means that they are no longer attached to a particular atom or pair of atoms, but can be thought of as moving freely around in the whole structure. The outer electrons have become delocalised over the whole metal structure. There have to be huge numbers of molecular orbitals, of course, because any orbital can only hold two electrons. 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.Īll 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. 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. When sodium atoms come together, the electron in the 3s atomic orbital of one sodium atom shares space with the corresponding electron on a neighbouring atom to form a molecular orbital - in much the same sort of way that a covalent bond is formed. Sodium has the electronic structure 1s 22s 22p 63s 1. Even a metal like sodium (melting point 97.8☌) melts at a considerably higher temperature than the element (neon) which precedes it in the Periodic Table. Metals tend to have high melting points and boiling points suggesting strong bonds between the atoms. It explains how the metallic bond arises and why its strength varies from metal to metal. This page introduces the bonding in metals.