This site is intended to give you an overview of the way in which atoms are arranged in metals, and ionic crystals. It will cover close packed structures, simple cubics and will briefly look at some of the defects that may occur in the lattices. For the full range of topics cover please browse the menu bar to the left of the page.
Before looking at the structure and packing of atoms it is important to consider what an atom actually is, and how we will model atoms for the purpose of looking at atomic planes and packing. An atom has no overall charge, though the nucleus has a positive charge and the surrounding electrons have a negative charge to counter the positive of the nucleus. If electrons are displaced a net charge is observed and an ion is formed. Bonds between atoms arise due to electro static interactions, the opposite charges attract
What are metals and ionic crystals?
They are simply a regular array of ions,or atoms, between
which the bonding in sufficiently strong to hold the atoms or ions in this order.
The bonding in metals relies upon the attraction between the metal cations and the delocalised electrons.
Similarly in an ionic solid the bonding is between a metal cation, but rather than the delocalised electrons holding a negative charge,
the negative charge is localised to non metal anion, for example Cl-. The larger the charge of the cation in a
metal the stonger the bonding is, since the is a
larger charge to attract the electrons and also there are more delocalised electrons. The more strongly a cation attracts the electrons, the closer
together they pack. This can result in close packing, where approximately 75% of the space is occupies. In other metal the attraction is far weaker which results in
'looser' packing, and so less of the space is occupied. calculation of the occupied volumes will be explored in each section.
Key assumptions:
- Ions are solid spheres that have constant volume
- Ions are touching adjacent ions where possible
- The charge of a ion is considered as a point charge
In the key ideas section an overviews of Atomic planes, fractional coordinates and projection diagrams are given. This section is likely to be of use to those who are new to this topic.
